Journal archives for April 2022

April 01, 2022

Even marsupials living fast and dying young do not match the pace of life of comparable eutherians, part 1

@chuditch @josefnr @torhek @ethan241 @sea-kangaroo @isaacclarey @chewitt1 @ericwilliams @yayemaster @bsmit @tfrench @chiara_paniccia @apgarm @batworker @patrickhaffner @ken_j_allison @alanmuchlinski @asemerdj @mpmoskwik @guillaumeamirault @enricotosto96 @camilojotage @julianbiol @jorgebrito @sulloar @josev_ge @diegoalmendras @azambolli @douglasriverside

(First, please see https://en.wikipedia.org/wiki/Basal_metabolic_rate. To give readers some idea of what the numbers mean: the average adult human body, when at rest, uses energy at about the same rate - viz. about 80 Watts - as the average laptop computer, when in operation.)

Marsupials are extremely diverse morphologically and ecologically (https://en.wikipedia.org/wiki/Marsupial). However, they tend to lag behind comparable eutherians in pace of life (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5079228/#:~:text=Marsupials%20generally%20have%20a%20low,of%20equivalently%2Dsized%20placental%20mammals.).

For example, the house mouse (Mus musculus, https://en.wikipedia.org/wiki/House_mouse) and 'marsupial mice' (Sminthopsis, https://en.wikipedia.org/wiki/Dunnart) both have body mass about 20 grams. However, the house mouse, when at rest, uses energy at 0.271 Watts, whereas the corresponding value for 'marsupial mice' is only 0.126 Watts - less than half as much.

We can define pace of life as the overall total rate at which the body of a given species uses energy, relative to body mass. Species with the fastest pace of life have maximal rates of metabolism (resting and active), growth, and reproduction per unit mass (https://www.nature.com/articles/s41559-019-0938-7 and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3140270/).

We would expect that, all else being equal, species with fast pace of life also live only briefly before senescing. This is in keeping with the idea that 'what burns bright tends not to burn for long'.

A familiar example is the house mouse, in which individuals live for only about two years even if they evade mishap. This species - consistent with the rich resources available to it and the predation it tends to attract - not only metabolises and breeds rapidly but is 'hardwired' to have limited life span (https://en.wikipedia.org/wiki/Maximum_life_span).

If this inverse relationship between pace of life and life span holds true for mammals in general, then it seems reasonable to expect those species renowned for the shortness of their life span to have have particularly rapid metabolism.

Does this apply to marsupials?

Particularly abrupt senescence has been studied in certain genera of marsupials such as Antechinus (https://en.wikipedia.org/wiki/Antechinus) and Phascogale (https://en.wikipedia.org/wiki/Phascogale). This constitutes semelparity (https://en.wikipedia.org/wiki/Semelparity_and_iteroparity).

Males make such intense efforts to reproduce that they seem virtually to commit physiological suicide (https://www.nationalgeographic.com/science/article/why-a-little-mammal-has-so-much-sex-that-it-disintegrates). Their first year of sexual activity is also their last. In Antechinus, most female individuals die after raising only a single litter.

However, see the following information. The values show that, even in the case of these small-bodied, insectivorous marsupials, metabolism is slower than in various comparable mammals - particularly shrews (Soricidae) - living on continents other than Australia.

Values for body mass (in grams) and basal metabolic rate (in Watts) are given by Savage et al. 2004 (https://besjournals.onlinelibrary.wiley.com/doi/10.1111/j.0269-8463.2004.00856.x ). To make sense of any comparisons, one must keep body mass as similar as possible.

MARSUPIALS

Dasyuromorphia: Dasyuridae:

Antechinus flavipes 46.5 g, 0.252 Watts
https://en.wikipedia.org/wiki/Yellow-footed_antechinus

Antechinus stuartii 25.0 g, 0.189 Watts
https://en.wikipedia.org/wiki/Brown_antechinus

Antechinus swainsonii 66.9 g, 0.351 Watts
https://en.wikipedia.org/wiki/Dusky_antechinus

(In general, Antechinus spp. have body mass about 50 grams, life span 2 years in females and 1 year in males, neonatal body mass 0.016 g, weaning at 3-4 months old, sexual maturity at about 10 months, gestation period 1 month, time spent by offspring in the pouch 37-178 days, number of offspring per birth 6-12. Eight species in this genus are semelparous.)

Phascogale tapoatafa 154 g, 0.694 Watts
https://en.wikipedia.org/wiki/Brush-tailed_phascogale
(Phascogale tapoatafa is the largest-bodied mammal on Earth in which males senesce at the end of their first breeding season. Females may survive to breed a second time. Information for this species: body mass female mean 145 g, male mean 199 g; breeding seasonal; gestation period about 30 days; number of mammae 8; number of offspring per birth more than 8; time from birth to weaning about 40 days. Phascogale calura is similar to P. tapoatafa in being semelparous.)

Didelphimorphia: Didelphidae:

Marmosa robinsoni 122 g, 0.547 Watts
https://en.wikipedia.org/wiki/Robinson%27s_mouse_opossum

Monodelphis brevicaudata 91.5 g, 0.366 Watts
https://en.wikipedia.org/wiki/Northern_red-sided_opossum

Monodelphis domestica 104 g, 0.335 Watts
https://en.wikipedia.org/wiki/Gray_short-tailed_opossum

EUTHERIANS

Eulipotyphla: Erinaceidae:

Hylomys suillus 58 g, 0.335 Watts
https://en.wikipedia.org/wiki/Short-tailed_gymnure

Eulipotyphla: Soricidae:

Blarina brevicaudata 21 g, 0.344 Watts
https://en.wikipedia.org/wiki/Northern_short-tailed_shrew

Crocidura flavescens 33.2 g, 0.248 Watts
https://en.wikipedia.org/wiki/Greater_red_musk_shrew

Crocidura olivieri 38.6 g, 0.323 Watts
https://en.wikipedia.org/wiki/African_giant_shrew

Suncus murinus 39.7 g, 0.403 Watts
https://en.wikipedia.org/wiki/Asian_house_shrew
(body mass 30-80 and up to 170 grams, life span 1.5-2.5 years, weaning at age 17-20 days, sexual maturity at 45-60 days, gestation period 27-31 days, offspring per birth 1-8, neonatal body mass 2-2.5 g)

Afrosoricidae: Tenrecidae:

Echinops telfairii 116.4 g, 0.750 Watts
https://en.wikipedia.org/wiki/Lesser_hedgehog_tenrec

Hemicentetes semispinosus 116.4 g, 0.380 Watts
https://en.wikipedia.org/wiki/Lowland_streaked_tenrec

Limnogale mergulus 77.7 g, 0.355 Watts
https://en.wikipedia.org/wiki/Web-footed_tenrec

Nesogale dobsoni 44.6 g, 0.315 Watts
https://en.wikipedia.org/wiki/Dobson%27s_shrew_tenrec

Nesogale talazaci 44.0 g, 0.243 Watts
https://en.wikipedia.org/wiki/Talazac%27s_shrew_tenrec

Macroscelidea: Macroscelididae:

Elephantulus brachyrhynchus 49.9 g, 0.303 Watts
https://en.wikipedia.org/wiki/Short-snouted_elephant_shrew

Elephantulus intufi 46.5 g, 0.290 Watts
https://en.wikipedia.org/wiki/Bushveld_elephant_shrew

Elephantulus myurus 63.0 g, 0.387 Watts
https://en.wikipedia.org/wiki/Eastern_rock_elephant_shrew

Galegeeska rufescens 53.0 g, 0.317 Watts
https://en.wikipedia.org/wiki/Rufous_elephant_shrew

Macroscelides proboscideus 39.0 g, 0.292 Watts
https://en.wikipedia.org/wiki/Macroscelides_proboscideus

Petrosaltator rozeti 49.0 g, 0.288 Watts
https://en.wikipedia.org/wiki/North_African_elephant_shrew

The values given above show that the eutherians - with the partial exception of Tenrecidae - exceed the like-size marsupials in metabolic rates.

(The exception of certain species of Tenrecidae is understandable because these insectivores are somewhat analogous to marsupials in living on an isolated landmass with limited predation, namely Madagascar.)

The number of offspring per birth is greater in Antechinus than in shrews of similar body mass, as one would expect from the sizes of the neonates. (Neonatal body mass is 140-fold less in Antechinus than in Suncus murinus, the largest-bodied of all shrews, despite their similar body masses in adulthood.)

However, this does not mean that Antechinus has a fast pace of life.

Suncus murinus (https://en.wikipedia.org/wiki/Asian_house_shrew) grows faster than Antechinus at all stages: gestation, suckling, and development to full body-size. This shrew grows from conception to sexual maturity four-fold faster than does Antechinus of similar body mass: about 82 days vs about 335 days.

Perusal of the values in Savage et al. (2004), beyond the species chosen above, confirms that shrews in general metabolise more rapidly than do equally small-bodied marsupials. Please consider the following.

  • Marsupials of body mass about 10 grams have basal metabolic rate less than 0.1: to be precise, 0.06-0.07. Shrews of similar body mass have basal metabolic rate 0.19 (Blarina), 0.12-0.15 (Crocidura), more than 0.25 (Neomys), or 0.30 (Sorex). These values all exceed 0.1, and several of them are 0.2 or more.
  • Marsupials of body mass 20-25 grams have basal metabolic rate 0.13-0.19. No shrew of the same body mass is available, but even shrews of body mass 15-20 grams have basal metabolic rate 0.13-0.33.
  • Crocidura spp. (33 g, 0.25; 39 g, 0.32) and Suncus sp. (40 g, 0.40) are unusually large-bodied shrews, and thus best-suited to comparison with Antechinus sp. (46.5 g, 0.25) and Pseudantechinus sp. (https://en.wikipedia.org/wiki/False_antechinus, 43 g, 0.15). This shows that, at body mass about 45 g, the marsupials have basal metabolic rate about 0.2, which is only about half of the value for the shrews. A shrew of body mass less than 35 g attains the basal metabolic rate of Antechinus (or close relative) of body mass about 45 g.
  • Marsupials in the genera Ningaui and Planigale have body mass 7-12 grams and basal metabolic rate 0.06-0.09. By contrast in shrews, Crocidura, which has similar body mass, has basal metabolic rate 0.11-0.17. For example, compare Crocidura suaveolens (6.9 g, 0.112) with Planigale tenuirostris (7.1 g, 0.063); this is about a two-fold difference. Or compare Ningaui yvonnae (11.6 g, 0.088) with Crocidura spp. (about 12 g, 0.155); this is a difference of up to two-fold.

These values show that shrews, when at rest, metabolise at least twice as rapidly as do like-size marsupials (including genera without the extremely abrupt senescence seen in Antechinus).

Also note that, although pregnancy in Antechinus is abbreviated as in all marsupials, its gestation period is no shorter than that of Suncus murinus: about 1 month in both cases. This is presumably owing to the slow growth of the embryo before birth in the marsupial. Antechinus weans its offspring about 4.5 months after their conception, whereas the corresponding value for Suncus murinus is a only about 47 days - a difference of nearly three-fold.

In summary:

Certain marsupials are remarkable in having specialised on abrupt turnover of generations - to the point of semelparity. However, this is no indication of fast pace of life compared with like-size, insectivorous eutherians occurring on a continent with an intense predatory regime, such as Africa. Although Antechinus and Phascogale are specialised for a short life span, they do not break the generalisation that marsupials are slower in their pace of life than eutherians comparable in body mass.

Slow pace of life is indeed a remarkably consistent aspect of the biology of marsupials, but far less widely known than their remarkably small body size at birth.

to be continued...

Posted on April 01, 2022 22:46 by milewski milewski | 7 comments | Leave a comment

April 04, 2022

Even marsupials living fast and dying young do not match the pace of life of comparable eutherians, part 2

David Macdonald has partly explained semelparity as follows:
"Semelparity only occurs in predictable, highly seasonable environments, and it has its 'raison d'etre' in the excrutiatingly slow reproduction rate of small marsupials...Because the mother may be suckling as many as ten young (in Antechinus stuartii), her metabolic rate in late lactation (weaning is not before 14 weeks old) can be ten to twelve times the basal rate - a mammalian record. Female reproduction is therefore timed to ensure that late lactation coincides with the period of maximum availability of the insect and spider prey on which the species feeds, which falls in late spring or early summer."

This approach may also aid our interpretation of the only marsupial occurring in the United States and Canada, namely Didelphis virginiana (https://en.wikipedia.org/wiki/Virginia_opossum).

Didelphis virginiana is well-known for its limited life span and correspondingly large number of offspring per birth. It can live to a maximum of five years. By contrast, Felis catus (https://en.wikipedia.org/wiki/Cat) can live to a maximum of 34 years. The normal lifespans of adults in the two species respectively are about three years and about 15 years. The apparent fecundity of D. virginiana seems consistent with its early senescence, and suggests a fast pace of life.

However, the marsupial metabolises slowly compared with like-size eutherians (https://lafeber.com/vet/basic-information-sheet-virginia-opossum/). This difference is partly indicated by body temperatures: only 35 degrees Celsius in D. virginiana vs 38.5 degrees Celsius in F. catus.

Taking the broadest possible view, how can we explain the paradox that the mammals that burn themselves out the fastest are marsupials? I offer the following rationale.

In marsupials, metabolic limitation means limitation in rates of growth. Hence no marsupial can match the rates of growth of the fastest-growing eutherians, such as lagomorphs and ruminants.

Certain marsupials compensate for this by having a large number of offspring per birth, which is particularly possible for them because all marsupials by definition have extremely small neonates.

The combination of large litters with a rapid growth by marsupial standards allow some marsupials to rival the overall reproductive rates of fecund rodents such as rats - even though they do not rival eutherians metabolically.

So why do the males of some of these marsupials senesce after one or two years of life, whereas like-size rodents and other fast-growing mammals usually continue in good health until killed by predators? Because:

  • maximum rates of growth in marsupials may only be possible where a disproportionate share of the available resources is allocated to breeding females, and
  • the niches involved are adaptive to seasonal/episodic conditions, in terms of both rainfall and wildfire.
Posted on April 04, 2022 04:49 by milewski milewski | 0 comments | Leave a comment

April 06, 2022

Variation in braininess among marsupials

@chuditch @josefnr @torhek @ethan241 @sea-kangaroo @isaacclarey @chewitt1 @ericwilliams @yayemaster @bsmit @tfrench @chiara_paniccia @apgarm @batworker @patrickhaffner @ken_j_allison @alanmuchlinski @asemerdj @mpmoskwik @guillaumeamirault @enricotosto96 @camilojotage @julianbiol @jorgebrito @sulloar @josev_ge @diegoalmendras @azambolli @douglasriverside

Marsupials (https://en.wikipedia.org/wiki/Marsupial) are generally less brainy than eutherian (https://en.wikipedia.org/wiki/Eutheria) mammals at body sizes approximately exceeding those of Rattus (https://search.informit.org/doi/10.3316/INFORMIT.585794741422012 and https://en.wikipedia.org/wiki/Brain_size and https://www.karger.com/Article/Fulltext/377666 and https://onlinelibrary.wiley.com/doi/10.1002/bies.201100013).

As an extreme example, the Virginia opossum (https://en.wikipedia.org/wiki/Virginia_opossum) is 5-fold less brainy than any cat (https://en.wikipedia.org/wiki/Felidae) of similar body mass. The brain volumes respectively are Didelphis virginiana 25 units of volume vs Felis catus 125 units of volume.

Dactylopsila (https://en.wikipedia.org/wiki/Striped_possum) is the brainiest marsupial, but falls short of cats - which themselves are about average for eutherians and only a little brainier than the general mammalian average.

Nonetheless, within marsupials there is 5-fold variation in braininess. The purpose of this series of Posts is to rank the various marsupials in order of braininess.

My sources are:

On the scale of braininess used by the first reference above,

This means that:

DATA

The following shows marsupials in order of decreasing braininess according to Nelson and Stephan (1982). A score of 100 equates approximately to Rattus (https://en.wikipedia.org/wiki/Rattus) among eutherians.

Scoring more than 180

Scoring 150-180

Scoring 140-170

Scoring 130-140

Scoring 110-130

Scoring 90-110 (approximately equivalent to Rattus among eutherians)

Scoring 80-90

Scoring less than 80

DISCUSSION

The common brush-tailed possum (Trichosurus vulpecula, https://en.wikipedia.org/wiki/Common_brushtail_possum) and the ring-tailed lemur (Lemur catta, https://en.wikipedia.org/wiki/Ring-tailed_lemur) both have body mass about 1.7 kilograms. However, the former (a phalangerid marsupial) is half as brainy as the latter (a prosimian primate). Their brains weigh 11 grams and 22 grams respectively.

The following mammals all weigh about 200 grams (slightly more massive than Rattus), but differ in braininess in decreasing order as follows:

As in the comparison of possum with lemur above, a petaurid marsupial has a smaller brain than that of an ecologically comparable, like-size prosimian primate.

The numbat (Myrmecobius fasciatus, https://en.wikipedia.org/wiki/Numbat) of southern Australia is comparable with a sengi (Rhynchocyon, https://en.wikipedia.org/wiki/Rhynchocyon) of Africa. However, the former scores lower (149) in braininess than the latter (180).

Along similar lines: kangaroos are less brainy than ruminants, and the thylacine is less brainy than canids despite its superficial evolutionary convergence. However, the relative braininess of potoroos, bettongs, and rat-kangaroos (e.g. https://en.wikipedia.org/wiki/Musky_rat-kangaroo) is puzzling, because in other respects these seem as primitive as other marsupials.

In general, many marsupials of diverse diets (burramyids, most dasyurids, peramelids, pseudocheirids, phalangerids) are about as brainy as the eutherians formerly called 'Insectivora' (https://en.wikipedia.org/wiki/Insectivora). The latter likewise range considerably in braininess, from 47 to 180.

The Virginia opossum is not only the most northerly of all marsupials but also one of the least brainy - which seems paradoxical given its success in the relatively intense predatory regime of North America.

Posted on April 06, 2022 05:03 by milewski milewski | 16 comments | Leave a comment

April 08, 2022

Why does the biggest communal nest of any bird occur only in one southern African semi-desert? part 1

@tonyrebelo Hi Tony, does this rationale make sense to you? With thanks from Antoni

The communal nest of Philetairus socius (https://en.wikipedia.org/wiki/Sociable_weaver and http://www.oiseaux-birds.com/card-sociable-weaver.html and https://www.nwf.org/Magazines/National-Wildlife/2002/Oh-What-a-Nest and https://phys.org/news/2019-07-massive-sociable-weavers-house-species.html) of southern Africa is the largest on Earth.

This species has no counterparts on other continents or even in ecologically similar parts of the same continent such as the Sahel or northeastern Africa.

Which combination of ecological factors has produced this adaptation?

Why does such a large construction (https://www.inaturalist.org/observations/47986970 and https://www.inaturalist.org/observations/37159904 and https://www.inaturalist.org/observations/11064462 and https://www.youtube.com/watch?v=HPdLqL_Tzso and https://thefunambulistdotnet.wordpress.com/2010/12/18/architectures-without-architects-sociable-weavers-nests/) occur in Namibia and Northern Cape province of South Africa (https://en.wikipedia.org/wiki/Sociable_weaver#/media/File:Philetairus_socius_distribution_map.png), as opposed to any other semi-desert with scattered trees?

In the summer heat and winter frost of the dry interiors of all vegetated continents, there are obvious economies of scale in large communal nests, for shelter and thermodynamic homeostasis (https://www.researchgate.net/publication/229635804_The_thermal_significance_of_the_nest_of_the_Sociable_Weaver_Philetairus_socius_Winter_observations and https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-048X.2012.05797.x).

However, it is only in one situation on Earth, and one species of bird, that communal nesting on this scale has arisen. In East Africa, related and comparable birds do occur in the form of genus Pseudonigrita (https://en.wikipedia.org/wiki/Pseudonigrita), but their nests are far less communal and thus far smaller (https://www.inaturalist.org/observations/28021578 and https://www.inaturalist.org/observations/108449462 and https://www.inaturalist.org/observations/47881640 and https://www.inaturalist.org/observations/47120758 and https://www.inaturalist.org/observations/109837557).

One obvious environmental prerequisite is freedom from wildfire. This helps to explain why no comparable nest occurs in, for example, Australia, where wildfire is remarkably widespread under semi-arid climates except on saline/sodic substrates (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x).

However, this is an insufficient explanation because extensive semi-arid areas of the Americas, as well as northeastern Africa, are also free of wildfires.

The availability of suitable forms of grass, such as Stipagrostis (https://en.wikipedia.org/wiki/Stipagrostis), is also a prerequisite. However, such grasses are far more widespread (e.g. https://en.wikipedia.org/wiki/Stipagrostis_uniplumis) than is the sociable weaver.

I suggest that the crucial factor is dietary: although the sociable weaver belongs to a mainly granivorous family, and although it eats grass seeds as one of its staples, it is odd in its dependence on a peculiar kind of termite.

This explanation invokes the perennial availability of a large-bodied, surface-foraging, diurnal hodotermitid: Hodotermes mossambicus (https://en.wikipedia.org/wiki/Hodotermes and https://termites.myspecies.info/content/hodotermes). This insect is arguably crucial to the sociable weaver - which seldom drinks because it lives remotely from surface water - as a source of not only food but also hydration.

The sociable weaver is not specialised enough on social insects to qualify as myrmecophagous (https://en.wikipedia.org/wiki/Myrmecophagy and https://www.tandfonline.com/doi/pdf/10.1080/02541858.1988.11448112).

However, it does rely on eating a species of termite (https://www.tandfonline.com/doi/abs/10.1080/00306525.1973.9639162?journalCode=tost20 and https://www.tandfonline.com/doi/pdf/10.1080/02541858.1988.11448112 and https://ielc.libguides.com/sdzg/factsheets/sociableweaver/diet) that is itself odd in being less a detritivore than a grazer. This insect forms part of a guild of grazers including ruminants and hares, and continues to be active on the ground surface even in the frosty mid-winter, at the driest time of year.

However, the availability of Hodotermes is also insufficient to explain the extreme adaptation of the sociable weaver. This is because this genus of grass-eating termites is abundant also in semi-arid East Africa, where it does not seem to be important in the diets of the closest relatives of the sociable weaver (https://academic.oup.com/auk/article/97/2/213/5188549?login=false).

The difference is that southern Africa has at most one rainy season per year, whereas northeastern Africa reliably has two (https://journals.ametsoc.org/view/journals/clim/28/6/jcli-d-14-00484.1.xml). This makes desiccation a greater risk in the habitat of the sociable weaver than in the habitats of Pseudonigrita spp.

As far as I know, no other species of bird, beyond Africa, relies on a combination of seeds and termites to the degree shown by the sociable weaver.

Although seeds are available in all semi-deserts, Australia and the Americas lack any counterpart for Hodotermes. Because termites contain sufficient water to allow independence from drinking in the dry season, a potential niche has arisen in southern Africa for a sedentary bird such as the sociable weaver. And the means whereby this niche has been made viable is gregariousness in foraging, roosting, and breeding, facilitated by an extremely economical nest-complex, plus a degree of cooperative breeding (https://en.wikipedia.org/wiki/Cooperative_breeding and https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.379.7366&rep=rep1&type=pdf) unusual in Ploceidae.

A closely related and partly sympatric ploceid, namely the white-browed sparrow-weaver (Plocepasser mahali, https://www.inaturalist.org/taxa/14115-Plocepasser-mahali), has a diet remarkably similar to that of the sociable weaver (https://www.tandfonline.com/doi/pdf/10.1080/02541858.1988.11448112). So, why are the nests non-communal in genus Plocepasser?

A possible answer is that the white-browed sparrow-weaver

Furthermore, of the other three species in this genus,

to be continued...

Posted on April 08, 2022 16:38 by milewski milewski | 12 comments | Leave a comment

April 09, 2022

Why does the biggest communal nest of any bird occur only in one southern African semi-desert? part 2

In taking Hodotermes as a staple, the sociable weaver itself belongs to a foraging guild (https://en.wikipedia.org/wiki/Guild_(ecology)) that contains a remarkable assortment of sympatric species:

Availability of termites allows a sedentary life, because these insects are reliable even in drought. However, they have limited food-value, demanding metabolic and reproductive economy in their consumers.

Although the sociable weaver differs from the aardwolf in not specialising on termites, it resembles this hyenid in that Hodotermes tides both species over the lean season of winter, when both water and energy are in short supply.

Posted on April 09, 2022 18:59 by milewski milewski | 1 comment | Leave a comment

April 11, 2022

What was the original relationship between megaherbivores and bambis in south-central African savannas, in terms of biomass?

It seems reasonable to expect that small-bodied organisms tend to be more numerous than large-bodied organisms.

Faunas and communities of hoofed mammals can include species of a remarkably wide range of body sizes, from bambis to megaherbivores.

At the collective level of biomass (https://en.wikipedia.org/wiki/Biomass_(ecology)), which weigh more in a given area under natural conditions: the relatively scarce, large-bodied hoofed mammals or the relatively numerous, small-bodied ones?

One might expect the small-bodied but numerous species collectively to outweigh the large-bodied, scarce species.

However, in African savannas this seems not to be the case among ungulates, under prehistoric conditions. It is the large-bodied species that seem naturally to have the overall competitive advantage. The trouble with large body-size is that it brings vulnerability to extermination by humans.

We have information on something approaching an original situation in what is now Zambia (https://en.wikipedia.org/wiki/Zambia). A survey of the fauna of hoofed mammals (http://www.rhinoresourcecenter.com/pdf_files/124/1245160404.pdf) was performed in 1931-1933 by the accomplished zoologist C R S Pitman (https://en.wikipedia.org/wiki/Charles_Pitman_(game_warden)).

This information has the disadvantage that it was just an educated guess, rather than a rigorous survey by modern standards. Furthermore, Pitman made no attempt to estimate populations for the abundant suids, Phacochoerus africanus and Potamochoerus larvatus.

However, the same information has the advantage that it was acquired before European influence much distorted the faunal communities by hunting and confinement to conservation reserves. For example, Pitman notes a rapid increase of the population of Syncerus caffer at the time as unwelcome; and Diceros bicornis was later completely exterminated from Zambia.

We can assess the ecological importance of body sizes by multiplying the population tally by the average body mass (of females, which are usually the more numerous and metabolically active sex).

Here are the estimates of Pitman (1933) for what is now Zambia, excluding Barotseland. Body mass refers to adult females. Suids are omitted. Species are listed in decreasing order of populations.

Kobus leche kafuensis 75 kg 250,000 and Kobus leche smithemani 150,000 https://en.wikipedia.org/wiki/Lechwe
Sylvicapra grimmia splendidula 15 kg more than 60,000 https://en.wikipedia.org/wiki/Common_duiker
Syncerus caffer 500 kg more than 60,000 https://en.wikipedia.org/wiki/African_buffalo
Hippotragus equinus 255 kg 60,000 https://en.wikipedia.org/wiki/Roan_antelope
Alcelaphus lichtensteini 135 kg 60,000 https://en.wikipedia.org/wiki/Lichtenstein%27s_hartebeest
Aepyceros melampus 45 kg 50,500 https://en.wikipedia.org/wiki/Impala
Taurotragus oryx livingstonei 400 kg 30,000 https://en.wikipedia.org/wiki/Common_eland
Connochaetes taurinus mattosi 200 kg 30,000 and Connochaetes taurinus cooksoni 2,000 https://en.wikipedia.org/wiki/Blue_wildebeest
Equus quagga boehmi and crawshayi 300 kg 30,000 https://en.wikipedia.org/wiki/Plains_zebra
Raphicerus sharpei 10 kg 25,000 https://en.wikipedia.org/wiki/Sharpe%27s_grysbok
Tragelaphus scriptus and sylvaticus 30 kg 25,000 https://en.wikipedia.org/wiki/Cape_bushbuck
Kobus vardoni 70 kg 20,000 https://en.wikipedia.org/wiki/Puku
Kobus defassa 15,000 and Kobus ellipsiprymnus 180 kg 12,000 https://en.wikipedia.org/wiki/Waterbuck
Redunca arundinum 48 kg 14,000 https://en.wikipedia.org/wiki/Southern_reedbuck
Ourebia ourebi 15 kg 12,000 https://en.wikipedia.org/wiki/Oribi
Loxodonta africana 3000 kg 12,000 https://en.wikipedia.org/wiki/African_bush_elephant
Strepsiceros strepsiceros 170 kg 10,000 https://en.wikipedia.org/wiki/Greater_kudu
Hippotragus niger 220 kg 10,000 https://en.wikipedia.org/wiki/Sable_antelope
Oreotragus oreotragus 10 kg 8,000 https://en.wikipedia.org/wiki/Klipspringer
Tragelaphus spekii 40 kg 8,000 https://en.wikipedia.org/wiki/Sitatunga
Damaliscus lunatus superstes 120 kg 6,000 https://en.wikipedia.org/wiki/Common_tsessebe
Hippopotamus amphibius 1500 kg 3,000 https://en.wikipedia.org/wiki/Hippopotamus
Philantomba monticola 5 kg perhaps 3,000 https://en.wikipedia.org/wiki/Blue_duiker
Diceros bicornis 950 kg 1,500 https://en.wikipedia.org/wiki/Black_rhinoceros
Cephalophus sylvicultor 70 kg approximately 1,500 https://en.wikipedia.org/wiki/Yellow-backed_duiker
Giraffa tippleskirchi thornicrofti 850 kg 300 kg https://en.wikipedia.org/wiki/Giraffe

I have multiplied population tally by body mass, to produce values for biomass. Species are listed in decreasing order of biomasses.

Loxodonta africana 36,000,000 kg
Kobus leche 30,000,000 kg
Syncerus caffer 30,000,000 kg
Hippotragus equinus 15,300,000 kg
Taurotragus oryx livingstonei 12,000,000 kg
Equus quagga 9,000,000 kg
Alcelaphus lichtensteini 8,100,000 kg
Connochaetes taurinus 6,400,000 kg
Kobus defassa/ellipsiprymnus 4,860,000 kg
Hippopotamus amphibius 4,500,000 kg
Aepyceros melampus 2,272,500 kg
Hippotragus niger 2,200,000 kg
Strepsiceros strepsiceros 1,700,000 kg
Diceros bicornis 1,425,000 kg
Kobus vardoni 1,400,000 kg
Sylvicapra grimmia 900,000 kg
Tragelaphus sylvaticus/scriptus 750,000 kg
Damaliscus lunatus superstes 720,000 kg
Redunca arundinum 672,000 kg
Tragelaphus spekii 320,000 kg
Giraffa tippelskirchi thornicrofti 255,000 kg
Raphicerus sharpei 250,000 kg
Ourebia ourebi 180,000 kg
Cephalophus sylvicultor 105,000 kg
Oreotragus oreotragus 80,000 kg
Philantomba monticola 15,000 kg

Here are the species-tallies categorised by body size, with biomasses expressed in tonnes:

  • Megaherbivores (more than 850 kg) 36,000 + 4,500 + 1425 + 255 = 42,180 tonnes
  • Largest-bodied ruminants (300-850 kg) 30,000 + 12,000 = 42,000 tonnes
  • Ungulates 75-300 kg 15,300, + 9,000 + 8,100 + 6,400 + 4,860 + 2,200 + 1,700 + 720 = 48,280 tonnes
  • Ruminants 17-75 kg 30,000 + 2,272.5 + 1,400 + 750 + 672 + 320 + 105 = 35,519.5 tonnes (which would possibly have approached 40, 000 tonnes had the two species of suids been included)
  • Bambis (ruminants less than 17 kg) 900 + 250 + 180 + 80 + 15 = 1,425 tonnes

My commentary:

Each of of the five categories of body mass other than bambis approximates 40,000 tonnes of biomass. Megaherbivores consisted mainly of the savanna elephant, the largest-bodied ruminants consisted mainly of the savanna buffalo, ungulates 75-300 kg consisted mainly of the roan antelope and the plains zebra, and ruminants 17-75 kg consisted mainly of lechwes.

By contrast, the category of bambis provided minor biomass. Even if Pitman undercounted bambis - which are diminutive enough to hide easily - by five-fold, their biomass would be only one-fifth of those for any of the other categories.

The savanna elephant and the bush duiker are both widespread in African savannas, with generalised diets; and both were originally widespread in what is now Zambia. However, the estimated biomass of the former was 36,000 tonnes, whereas that of the latter was only 900 tonnes.

This is a difference of 40-fold, suggesting that even if small-bodied species compensate greatly by increasing their populations after the extermination of large-bodied species, it is unlikely that they can emulate the latter ecologically.

Indeed, I would argue that the very existence of niches for bambis in the first place in African savannas is by virtue of the continual rejuvenation of the vegetation by megaherbivores. Once all megaherbivores are exterminated, I would expect bambis to disappear as well owing to their thermodynamic inefficiency. In reality, humans tend to emulate the megaherbivores they have exterminated, by breaking woody plants and disturbing the ground, thus ensuring that the smallest ungulates continue to survive, and possibly with population-densities exceeding those in the original, natural regime.

Posted on April 11, 2022 11:29 by milewski milewski | 0 comments | Leave a comment

April 12, 2022

In being widespread among continents, many genera of grasses defy biogeography

(writing in progress)

Everyone knows that, in general, the various continents have different biotas. In plants as well as in animals, it is unusual - because of the tyranny of distance - for any given genus to occur naturally on several continents.

There are many exceptions to this rule, partly because Asia and North America have been connected for most if the last two million years by a Broad land-Bridge across Beringia. However, plant genera with cosmopolitan distributions beyond the Holarctic tend to have diaspores easily transported over great distances. These tend to be either:

  • windblown, as in e.g. daisies (Asteraceae), or
  • dispersed by birds which eat fleshy fruits and then pass seeds in viable condition.

Because of this pattern of continental distribution, any globally-experienced botanist, familiar with many genera, can generally infer the continent concerned from any local floristic checklist.

Grasses (Poaceae) are, however, anomalous. Their general pattern is that many or most genera occur naturally on several continents.

I first noticed this many years ago while trying to identify which genera of grasses are particularly associated with the fauna of grazers of Africa, which is so rich in mammals including ungulates. I expected that the extreme favourability of Africa for grazers, from termites through rodents and lagomorphs to the hippopotamus and square-lipped rhino, would be based at least partly on a distinctive flora of African grasses at the level of genus.

This turned out to be not the case: almost all of the palatable grasses of Africa belong to genera shared with at least one other continent, and in many cases all three southern continents.

Typical examples of this pattern include Urochloa, Panicum, Cynodon, Pennisetum, Setaria, Cenchrus, Digitaria, Eragrostis, Sporobolus, Brachiaria, etc. Many of these genera contain species particularly favoured by grazers, to the point of near-mutualism in the sense of adaptation to form lawns when repeatedly grazed. Yet at the level of genus they span the continents.

The case of Urochloa mosambicensis (https://www.inaturalist.org/taxa/170075-Urochloa-mosambicensis and https://www.tandfonline.com/doi/abs/10.2989/10220110209485784 and https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.572.3781&rep=rep1&type=pdf and https://www.classicafrica.com/images/customer-files//HensmanMasters.pdf) is as intriguing as any. This species forms stoloniferous lawns associated with the square-lipped rhino in Zululand. In the past, when this species of rhino had been nearly exterminated, much of thus area was dominated by the tussock-grass Themeda triandra. As the population of the square-lipped rhino increased, lawn-forming grasses including U. mosambicensis took over.

Despite this association, Urochloa is not restricted to Africa, occurring naturally also in Madagascar, China, India, and Mexico (https://en.wikipedia.org/wiki/Urochloa).

The cosmopolitan tendency of grass genera, including those most associated with large-bodied grazers, is puzzling because these plants have no known mechanism of dispersal across sea barriers.

(writing in progress)

Posted on April 12, 2022 08:40 by milewski milewski | 4 comments | Leave a comment

April 13, 2022

Interspecific variation in flags as features of adaptive colouration in hares, part 1: Lepus californicus

@biohexx1 @aguilita @marcelo_aranda @clint_perkins @mooseandsquirrel @asemerdj @maxallen @zarek @lefebvremax @saber_animal @tordenvinge @stephen54

Flags are conspicuous features of adaptive colouration in animals. Because of their medium scale relative to the whole figure, they require motion for their activation (see https://www.inaturalist.org/journal/milewski/48447-conspicuous-features-of-colouration-in-giraffes#).

For example, a caudal flag is present in several families and many genera of ruminants: the tail is erected to display striking contrast between dark and pale (https://www.deeranddeerhunting.com/article-index/8-messages-deer-send-hunters-with-their-tails).

The genus Lepus (https://en.wikipedia.org/wiki/Hare) contains about 30 species, most of which - although smaller-bodied than ruminants and with proportionately longer ears - are ecologically similar.

In this series of Posts, I examine the various flags in the various species of Lepus.

LEPUS CALIFORNICUS

I begin with Lepus californicus (https://academic.oup.com/mspecies/article/doi/10.2307/3504151/2600727?login=false) because this is the most frequently photographed species in the Americas.

The following view of L. californicus shows cryptic colouration with no conspicuous features: https://www.inaturalist.org/observations/38108260. However, in certain subspecies, the pale ventral tract extends high enough on the flank to be conspicuous: https://www.inaturalist.org/observations/39856570.

The anterior surface of the erect ear pinnae lack conspicuous colouration (https://www.inaturalist.org/observations/43121294 and https://www.inaturalist.org/observations/31805387 and https://www.inaturalist.org/observations/31519113).

All flags are rather poorly developed considering that both the tail and the ear pinnae are relatively long in this species (https://www.youtube.com/watch?v=01XFPe4un4g).

The colouration on and near the tail may not qualify as a caudal flag. This is because the tail:

The moderately conspicuous posterior surface of the ear pinnae of L. californicus qualifies as an auricular flag, particularly when the figure is fleeing and viewed from behind.

The pale, sheeny posterior bases of the ear pinnae (https://www.inaturalist.org/observations/39597865 and https://www.inaturalist.org/observations/5375556 and https://www.inaturalist.org/observations/2712599) are more conspicuous than the dark tips. However, in most subspecies the nape is not dark enough to offset this pallor.

Please see https://www.researchgate.net/publication/232670045_Ear_Flashing_Behavior_of_Black-tailed_Jackrabbits_Lepus_californicus.

The following show that the posterior surface of the forelegs is whitish whereas that of the hindlegs is relatively dark: https://www.inaturalist.org/observations/201009 and https://www.inaturalist.org/observations/67114783. However, this is possibly owing simply to soiling.

The following further illustrate the features described above.

https://www.inaturalist.org/observations/55648446
https://www.inaturalist.org/observations/63349588
https://www.inaturalist.org/observations/79551929
https://www.inaturalist.org/observations/69034947
https://www.inaturalist.org/observations/79491708
https://www.inaturalist.org/observations/75092188
https://www.inaturalist.org/observations/77646185
https://www.inaturalist.org/observations/91161420
https://www.inaturalist.org/observations/18813811
https://inaturalist.nz/photos/13793036

I have not investigated differences among the 17 subspecies. However, ssp. altamirae in Tamaulipas, Mexico (https://en.wikipedia.org/wiki/Tamaulipas), is different enough to be a different species: https://www.inaturalist.org/observations/42101903 and https://www.inaturalist.org/observations/41326141. Also see https://www.inaturalist.org/observations/37377894 and https://www.inaturalist.org/observations/32795199.

When L. californicus is viewed from a perspective based on ruminants, it gives the impression that it uses the ear pinnae in place of the tail to flag its flight.

In summary, the only flag for which L. californicus qualifies is a pale auricular flag. As caveats, the conspicuous pale is

  • restricted to the posterior surface of the ear pinnae and appears only when the ear pinnae are erect,
  • basal/proximal rather than distal (the darkness of the posterior tips of the ear pinnae thus providing limited contrast),
  • dependent on sheen, and
  • not offset by dark on the nape although it is offset to some degree by the dark on the hindquarters when viewed directly from behind.

to be continued...

Posted on April 13, 2022 13:51 by milewski milewski | 2 comments | Leave a comment

April 14, 2022

Interspecific variation in flags as features of adaptive colouration in hares, part 2: other species of semi-arid North America

@azgulo @panza_rayada @marcelo_aranda @aguilita @juancruzado @maxallen

I continue by turning from Lepus californicus to other, partly sympatric species in North America.

LEPUS CALLOTIS

Lepus callotis (https://academic.oup.com/mspecies/article/doi/10.2307/3504150/2600687?login=false and https://animalia.bio/white-sided-jackrabbit) differs from most subspecies of L. californicus in that:

A dark spot near the tip on the anterior surface of the ear pinna may possibly be prominent enough to qualify for a second auricular flag (https://inaturalist.ala.org.au/observations/5872954 and https://www.inaturalist.org/observations/31344301 and https://www.inaturalist.org/observations/99020814 and scroll in https://blog.nature.org/science/2021/09/13/8-cool-us-mammals-you-havent-seen/).

Lepus callotis is the only member of its genus in which a patch on the lower flank and upper hindleg is conspicuously pale even when the figure is stationary (https://www.inaturalist.org/observations/7572440 and https://www.inaturalist.org/observations/2663664 and https://www.inaturalist.org/observations/98081231 and https://www.inaturalist.org/observations/71549847 and https://www.inaturalist.org/observations/30019714 and https://www.inaturalist.org/observations/26589752 and https://www.inaturalist.org/observations/67100108). This species occurs in bonded pairs, and the pale feature may possibly provide a mechanism of monitoring each other's whereabouts.

The pale feature on the lower flank and upper hindleg qualifies as a haunch flag when expanded, as follows.

In alarm and while fleeing, the pale patch is stretched in a dorsal direction by twitching of the skin to encompass the haunch and the posterior upper flank (https://upload.wikimedia.org/wikipedia/commons/3/34/Lepus_callotis_callotis_408118_2.jpg and https://www.inaturalist.org/observations/2367046 and https://www.inaturalist.org/observations/22923071 and https://www.inaturalist.org/observations/304813). This brings it into particularly conspicuous contrast with the dark upper surface of the tail (https://inaturalist.ala.org.au/observations/105006395 and https://inaturalist.ala.org.au/observations/105756901 and second photo in https://inaturalist.ala.org.au/observations/3926181 and https://inaturalist.ala.org.au/observations/104689188).

An expanded position seems sometimes to remain in rigor mortis (https://www.inaturalist.org/observations/40844441).

Lepus callotis seems to qualify for stotting, based on Best and Henry (1993): "When flushed, L. callotis alternately flashes its white sides while running away from the intruder...Another escape behavior is that of leaping straight upward while extending the hind legs and flashing the white sides. This behavior is seen when the white-sided jackrabbit is startled or alarmed by a predator."

What may not previously have been noticed is that the pale patch can also be contracted when the figure crouches in hiding (https://www.inaturalist.org/observations/30019704 and compare the second and third photos in https://www.inaturalist.org/observations/1391841), presumably by movement of the skin in a ventral direction.

LEPUS ALLENI

Lepus alleni (https://www.researchgate.net/publication/281130018_Evolutionary_History_of_the_Antelope_Jackrabbit_Lepus_alleni and https://www.wildphotosphotography.com/WildPhotos/MAMMALS/Antelope_Jackrabbit.htm) does not possess caudal or auricular flags. Instead, this species is specialised in a haunch flag comparable with that in L. callotis but differing somewhat in its mechanism.

The following show general aspects of L. alleni:

https://www.youtube.com/watch?v=z4IZVwz-q60
https://www.inaturalist.org/observations/107916837
https://www.inaturalist.org/observations/85846524
https://www.inaturalist.org/observations/73705551
https://www.inaturalist.org/observations/36509677
https://www.inaturalist.org/observations/5645606

Note that:

However, the particularly extensive haunch flag can be 'flashed' by twitching of the skin, which exposes the whitish aspect of the fur. The dark on the tail is not extensive enough to add much to this display.

Best and Henry (1993, https://academic.oup.com/mspecies/article/doi/10.2307/3504245/2600676?login=false) state the following, which I have interspersed with illustrations:

"The antelope jackrabbit runs in a nearly horizontal plane for several strides then may make a stride higher than the others. Although not common, this behavior usually takes place in tall grass or when brush obstructs its sight of possible danger (Howell, 1944). As it starts to run, it makes four or five long hops on the hind legs alone, kangaroo fashion, then reverts to the usual mode of locomotion. Occasionally, with ears erect, the kangaroo hops are again displayed, apparently to see or hear possible pursuers (Swarth, 1929)...The leisurely moving, unfrightened individual that is not flashing its white sides (https://www.alamy.com/stock-photo-antelope-jackrabbit-lepus-alleni-oracle-pinal-county-arizona-united-15344934.html and https://www.inaturalist.org/observations/1103230 and https://www.inaturalist.org/observations/36542328 and https://www.alamy.com/stock-photo-antelope-jackrabbit-lepus-alleni-oracle-pinal-county-arizona-united-13554693.html) does not make observation leaps. When the antelope jackrabbit runs away from an observer, a conspicuous white area is displayed on the rump (https://www.alamy.com/stock-photo-antelope-jackrabbit-lepus-alleni-oracle-pinal-county-arizona-united-15374731.html and https://www.inaturalist.org/observations/52604210 and https://www.inaturalist.org/observations/43462880 and https://www.inaturalist.org/observations/5645517 and https://www.researchgate.net/figure/Lepus-alleni-displaying-white-flanks-when-startled_fig1_281130018 and https://www.inaturalist.org/observations/37574457 and https://www.inaturalist.org/observations/1710513 and https://www.inaturalist.org/observations/24287893 and https://www.inaturalist.org/observations/21184853 and http://www.mammalogy.org/lepus-alleni-1346). This area appears to shift each time the animal turns, the white being kept toward the observer, partly by the manner of holding the skin and partly by the zigzag course taken by the running animal. The white is flashed by a set of skin muscles that pulls the skin of the hind quarters over the back and up one side and at the same time everting the hairs, thereby exposing a surprisingly large white area on the left or right rear. Individuals hopping about to feed, or running but not alarmed, as in play or chasing each other, do not show this white (Vorhies and Taylor, 1933)."

The following show the difference, on the haunch, between:

The flared effect seems sometimes to remain in rigor mortis (https://www.inaturalist.org/observations/67467899).

LEPUS TOWNSENDII

Lepus townsendii exceeds all three previous species in its emphasis of caudal and auricular flags. This is because:

What this means is that L. townsendii possesses:

  • a caudal flag, present only in the summer coat, consisting of a tail that is conspicuously pale depite blending with the grey body, and
  • three different auricular flags, viz. darkish anterior surface of the ear pinnae (in winter coat), dark-tipped posterior surface of the ear pinnae (in the winter coat), and pale posterior surface of the ear pinnae (in the summer coat).

The following further illustrate the above features.

https://www.inaturalist.org/observations/75000504
https://www.inaturalist.org/observations/19699841
https://www.inaturalist.org/observations/62082575
https://www.inaturalist.org/observations/58191888
https://www.inaturalist.org/observations/55593440
https://www.inaturalist.org/observations/43268453
https://www.inaturalist.org/observations/42082091
https://www.alamy.com/white-tailed-jackrabbit-running-through-field-in-scenic-saskatchewan-image4196322.html
https://www.alamyimages.fr/photo-image-townsend-blanc-lepus-townsendi-173106409.html

The following shows that the colouration of the anterior surface of the ear pinnae is inconspicuous in the summer coat: https://www.inaturalist.org/observations/59782801. The following shows that the feet can be pale enough, in the summer coat, to be a candidate for a pedal flag: https://www.inaturalist.org/observations/27993641.

Summarising the first four species:

Lepus californicus, L. callotis, L. alleni, and L. townsendii are all ecologically similar, replacing each other (with wide overlap) in various regions of North America under relatively dry climates. However, they are surprisingly different in the incidence and location of flags as part of their adaptive colouration. The poor development of flags in L. californicus is partly explained by its habitat, which contains shrubs that provide cover and allow this species to rely on hiding.

to be continued...

Posted on April 14, 2022 19:58 by milewski milewski | 1 comment | Leave a comment

April 15, 2022

Why has there never been a donkey-size jackrabbit? part 1

The main orders of herbivorous terrestrial mammals show a remarkably wide range of body sizes. Why does this not apply to lagomorphs?

Ruminants and herbivorous rodents range from:

These represent a 500-fold and a 5,000-fold range in body mass respectively.

However, the order Lagomorpha (https://en.wikipedia.org/wiki/Lagomorpha and https://www.frontiersin.org/articles/10.3389/fevo.2021.636402/full#B88) does not conform. All hares, rabbits, and pikas have remained small-bodied, weighing less than 20 kilograms, even during the era of gigantism in the Pleistocene.

The largest-bodied extant lagomorphs are hares (Leporidae: Lepus, body mass up to 7 kilograms). These overlap in body size with the most diminutive of ruminants (e.g. https://en.wikipedia.org/wiki/Dik-dik).

How does the basic nature of lagomorphs help to explain this limitation on body size in a herbivorous order of mammals?

A recent attempt to answer this question (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8252017/#:~:text=We%2C%20therefore%2C%20hypothesized%20that%20the,%2C%20depending%20on%20the%20environment) is hard to understand.

Instead, I suggest that the solution lies in hares being different from ungulates and rodents in their combination of specialisations in

  • digestion,
  • vigilance,
  • posture, and
  • locomotion.

The guts, sense organs, neck, and limbs of hares are all specialised in ways suggesting a limitation on body size.

SPECIALISATION IN DIGESTION

Hares oddly combine repeated digestion with once-off chewing. Would this work at body sizes larger than those of hares?

SPECIALISATION IN VIGILANCE

Hares oddly combine night-eyes with long ears. Would this work at body sizes larger than those of hares?

SPECIALISATION IN POSTURE

Hares oddly combine a short neck with long legs. Would this work at body sizes larger than those of hares?

SPECIALISATION IN LOCOMOTION

Hares oddly combine soft feet with extremely rapid galloping. Would this work at body sizes larger than those of hares?


Having provided this conceptual framework, I will now expand on each of the above oddities.

SPECIALISATION IN DIGESTION

Hares ferment food in their hindgut, not their foregut (see https://allfamousbirthday.com/faqs/which-is-better-foregut-or-hindgut-fermentation/ and https://www.researchgate.net/figure/The-digestive-tract-of-a-rabbit-Numerical-values-are-those-observed-in-a-25-kg-New_fig1_275519577).

Hares thus differ from ruminants, including bambis (diminutive ruminants). Instead of chewing the cud, they eat the least-fibrous portion of their feces (https://en.wikipedia.org/wiki/Cecotrope and https://www.sciencedirect.com/science/article/abs/pii/0301622681900543).

This means that hares have to take the time to chew their food thoroughly in the first place as they forage. By contrast, ruminants can swallow food hastily and then retire to a safe place to chew thoroughly.

Herbivorous mammals have four main options for thorough digestion - which requires fermentation so that microbes can release metabolisable energy from otherwise indigestible fibre.

These are:

Cud-chewing (as in bambis) and eating of feces (as in hares) both achieve thorough digestion. However, they are mutually exclusive, i. e. cannot co-occur in the same animal.

A crucial advantage of rumination is that it combines thorough digestion in the stomach and small intestine with an ability to fill the fermentation-chamber initially with hastily-swallowed food. This is important because the chewing of fibrous food is noisy enough to interfere with auditory vigilance.

Foregut-fermentation with cud-chewing therefore allows ruminants to

  • minimise the time spent foraging, while
  • maximising the time spent lying vigilant while chewing the food thoroughly after retiring to a position of relative safety.

Furthermore: as body size increases, the ability of the mouthparts to select the nutrient-richest, least fibrous food-items decreases. Because the large parts of plants tend to be the most fibrous, large-bodied herbivores must spend much of each 24-hour period foraging and chewing.

Few herbivorous eutherian mammals with body masses 10-50 kg have - as far as we know - ever existed In the last of the four categories above (viz. hindgut-fermentation without systematic eating of feces). This suggests an evolutionary barrier to the increase in body size by hares or other lagomorphs, even if they abandoned the caecotrophy that is characteristic of lagomorphs.

The specialisation of hares in digestion may thus preclude much increase in body size.

SPECIALISATION IN VIGILANCE

Hares lack the visual system - including a long neck - of ungulates, that allows wide scanning by both day and night. Instead, they combine eyes specialised for darkness with specialised hearing, as indicated by the length of the ear pinnae.

Hares are, as far as I know, the only animals combining vertically-oriented pupils with eyes placed so laterally (https://www.inaturalist.org/observations/67587308 and https://pixels.com/featured/big-eyes-brown-hare-lepus-europaeus-wonderfulearth.html and https://www.ephotozine.com/photo/back-view-of-hare-36048706 and https://commons.wikimedia.org/wiki/File:Old_hare_copy.jpg and https://www.alamy.com/stock-photo-brown-hare-lepus-capensis-rear-view-of-single-adult-sitting-in-field-139323674.html) that the field of binocular vision (https://en.wikipedia.org/wiki/Binocular_vision) is minimal.

This may be partly because hares are more strictly nocturnal than bambis. All hares (except for Lepus arcticus in the brief Arctic summer) are active mainly by night.

It is often assumed that bambis are also nocturnal. However, several species are actually active mainly by day (e.g. see https://en.wikipedia.org/wiki/Blue_duiker and https://en.wikipedia.org/wiki/G%C3%BCnther%27s_dik-dik). Furthermore, species venturing into the open (e.g. https://en.wikipedia.org/wiki/Royal_antelope) only by night may nonetheless be active by day under cover of shrubs and trees.

The shape of the pupils in hares is shown in: https://twitter.com/GarrityPete/status/1131838112060104704/photo/4 and https://www.gettyimages.com.au/detail/photo/african-savanna-hare-in-masai-mara-national-reserve-royalty-free-image/520578266?adppopup=true and https://www.alamy.com/stock-photo-single-adult-brown-hare-portrait-close-up-of-eyes-and-face-81611479.html and https://www.gettyimages.com.au/detail/photo/head-shot-of-a-beautiful-brown-hare-lepus-europaeus-royalty-free-image/1193234080?adppopup=true and https://www.agefotostock.com/age/en/details-photo/brown-hare-european-hare-feldhase-lepus-europaeus-lying-resting-in-meadow-relaxed-very-detailed-close-up-wildlife-europe/VX1-3358923 and http://wildlifephotographic.blogspot.com/2014/12/just-hare-neccesities.html and https://www.gettyimages.com.au/detail/photo/scrub-hare-kasane-moremi-game-reserve-botswana-royalty-free-image/999492980?adppopup=true).

The above contrasts with ruminants, which have horizontally-oriented pupils (https://www.gettyimages.com.au/detail/photo/close-up-of-deer-looking-away-royalty-free-image/1083779544?adppopup=true and https://es.123rf.com/photo_107041593_close-up-of-the-eye-of-a-red-deer-cervus-elaphus-.html and https://www.quora.com/Do-deer-and-antelopes-have-different-vision-during-nighttime) and retinas.

The long ear pinnae of hares can be interpreted partly as compensating for the limitations on visual vigilance by day, and the noise of chewing while exposed to danger during foraging by night.

The specialisation of hares in vigilance may thus preclude much increase in body size.

to be continued...
(see https://www.inaturalist.org/journal/milewski/64506-why-has-there-never-been-a-donkey-size-jackrabbit-part-2#)

Posted on April 15, 2022 01:22 by milewski milewski | 28 comments | Leave a comment