An adaptive interpretation of the striping in the numbat (Myrmecobius fasciatus)

Also see https://www.inaturalist.org/journal/milewski/82502-comparison-of-feral-cat-with-native-cat-in-australia-differences-in-pace-of-life-between-felis-catus-and-dasyurus-maculatus#

The following conceptual framework is, as far as I know, new in the literature on the numbat (https://www.inaturalist.org/taxa/40262-Myrmecobius-fasciatus).

However, it may be basic to our understanding of this species, including its adaptive colouration.
 
The bottom line is that the numbat is odd not only

• in colouration, but also
• in combining slow reproduction and growth with a short lifespan – a pattern shared with dasyurids (https://en.wikipedia.org/wiki/Dasyuridae), but differing from comparable placental mammals.

The numbat is endangered, and much scientific effort has gone into breeding the species in captivity. Therefore, one might assume that the demographics/life history strategy of the numbat has been understood long ago, relative to other mammals.

However, I doubt that this is the case, and I think that my assessment below is new despite being of such basic importance.
 
First some basic principles:
 
In assessing the life history strategy of the numbat, body mass is important, because in general we expect lifespan to increase and reproductive rate to decrease, the larger the adult body of the species.
 
In general, we would expect species with short lives to be the most fecund. (In fact, the numbat combines short lifespan with less than the expected fecundity for its body size.)
 
We would also expect species with rapid metabolism to be the most fecund. (In terms of metabolism, the numbat conforms to expectations because it combines slow metabolism with limited fecundity.)
 
A typical example of a short-lived, fecund taxon is Rattus (https://www.inaturalist.org/observations?place_id=any&taxon_id=44540&view=species).

Rats usually live only a few years, naturally senescing by about 3 years old (the maximum lifespan recorded in captivity for Rattus rattus is only 4 years). This limited lifespan would make sense even to the lay person, because of the great rate at which Rattus breeds.
 
At any given value for adult body size, we might expect those taxa emphasising fecundity to have lesser maximum lifespans than those taxa emphasising longevity over fecundity. (This is indeed the case in any comparison of zebras with coexisting ruminants: the equid lives longer and breeds more slowly than the like-size ruminant.)
 
Now, for the anomaly presented by the numbat:
 
Like most marsupials, the numbat has slow metabolism, and breeds slowly (by the way, the notion that kangaroos breed rapidly is a case of misinterpretation).

However, contrary to expectations, the numbat also lives a remarkably short life: even if it evades predation and other forms of mishap, it naturally senesces early.
 
This counterintuitive pattern, in which limited lifespan is combined with limited reproductive rate, is typical of Dasyuridae (https://www.inaturalist.org/observations?place_id=any&taxon_id=40153&view=species).

It is particularly well-known in Antechinus (https://www.inaturalist.org/observations?place_id=any&taxon_id=40181&view=species) and Dasyurus (https://www.inaturalist.org/observations?place_id=any&taxon_id=40165&view=species), some of the species of which can be described as ‘semelparous’ (https://en.wikipedia.org/wiki/Semelparity_and_iteroparity), because they approach a life history strategy in which each adult breeds for only a year or two before naturally senescing.
 
The semelparous life history strategy occurs in various other familiar animals, being typically associated with e.g. octopuses (https://www.researchgate.net/figure/Semelparity-versus-iteroparity-Top-Organisms-such-as-the-octopus-the-agave-or-the_fig4_249342068 and https://www.sciencedaily.com/releases/2018/09/180925140402.htm#:~:text=Octopuses%20are%20semelparous%20animals%2C%20which,once%20and%20then%20they%20die.).
 
One of the reasons why this anomaly has previously been overlooked in the numbat is that it is not an extreme example of the trend towards semelparity.

However, as the figures I present here clearly show, the numbat is indeed in line with the general trend in dasyurids, for a combination of limited lifespan and limited reproduction. I.e. like dasyurids, it tends to defy classic ‘r- vs K-selection’ theory (https://study.com/academy/lesson/graphing-population-growth-of-r-selected-k-selected-species.html#:~:text=K%2Dselected%20species%20are%20those,young%2C%20and%20exhibit%20exponential%20growth.).
 
In order to make a useful comparison, let me choose that familiar species of squirrel, Sciurus carolinensis (https://www.inaturalist.org/taxa/46017-Sciurus-carolinensis).

This comparison helps because the eastern grey squirrel is

• similar in body mass to the numbat,
• well-known biologically, and
• the opposite of endangered (it is if anything ‘weedy’ and invasive’, as seen in Britain where it has largely usurped the niche of the indigenous squirrel).

Firstly, the great difference in metabolic rates:

The squirrel metabolises at a rate more than double that of the numbat: 0.004686 watts per gram of body mass, compared with only 0.002070 watts per gram of body mass.

This is a factor of 2.25 different, despite the similarity in adult body masses, and is typical of the general difference between eutherians and marsupials. (My source is ‘AnAge’, accessed via ADW, the Animal Diversity Web, online.)

The slow metabolism of the numbat makes sense because it is not only a marsupial but also a termite-eating specialist.
 
The great difference in metabolic rates is reflected in the expected way by body temperatures: the squirrel normally has a body temperature of 38.7 degrees Celsius, whereas the corresponding value for the numbat is only 32.5 degrees Celsius. That is about as great a difference as one can find between mammals, because the squirrel runs hot even for a eutherian, whereas the numbat runs cool even for a marsupial.
 
Now for reproductive rates:
 
A sound way to compare reproductive rates is by the time taken from conception to weaning. This avoids the obvious complications of comparing a eutherian with a marsupial.
 
Time from conception to weaning is far less in the squirrel than in the numbat: 110 days vs about 292 days.  (The exact figures are squirrel: gestation 44 days plus birth to weaning 66 days; numbat gestation 14 days plus birth to weaning 9 months.)

Based on time from conception to weaning, the reproductive rate of the squirrel would be nearly 2.7-fold greater than of the numbat.
 
Litter size is about 4 in both squirrel and numbat. However, here the difference is that the squirrel usually produces two litters per year, whereas the numbat produces only one litter per year. So, based on frequency of reproduction per year, the squirrel again outbreeds the numbat by a factor of two.
 
Finally, we come to maximum lifespan:

Here the figures should surprise even the lay person with a sketchy general knowledge of the life histories of mammals.

The maximum lifespan (recorded in captivity) for the squirrel is far, far greater than that for Rattus: 23.6 years.

Compare this with the corresponding value for the numbat, which is only 11 years.

So, the squirrel not only breeds at more than double the rate found in the numbat, but it also – counterintuitively – lives more than twice as long, before naturally senescing.
 
What all of this adds up to:

The squirrel, although having a rapid pace of life in terms of metabolism, reproduction, and growth, also has an extended lifespan – the opposite of the expectation generated by the classic ‘r-selected’ animal such as Rattus.

Meanwhile the numbat, although having a slow pace of life in terms of metabolism, reproduction, and growth, has a surprisingly limited lifespan, the opposite of the expectation generated by the classic ‘K-selected’ animal.

The whole concept of ‘r-selected vs K-selected’ seems to fail when it comes to the numbat. Although the numbat is not extreme in the way some species of Antechinus and Dasyurus are, it shows the same general anomaly. (By the way, another important Australian animal showing this kind of pattern seems to be the emu, Dromaius nkvaehollandiae, https://www.inaturalist.org/taxa/20504-Dromaius-novaehollandiae.)
 
I suggest that one cannot interpret the oddities of e.g. the colouration of the numbat without taking into consideration its surprising life history strategy. And one particular difference between the numbat (transverse striping) and squirrels (longitudinal striping) seems particularly relevant.
  
http://animaldiversity.org/accounts/Myrmecobius_fasciatus/#lifespan_longevity

The numbat is not only Australia’s most distinctly transverse-striped mammal, but one of the few mammals of its body size on Earth that have transverse striping on the back and flanks. In the mind of semi-popular science, transverse striping would seem to be suitable for camouflage – as seen in the tiger.

So, why does the colouration of the numbat turn out to be exceptional?
 
Well, the numbat is also unusual at several other ways, as follows.

• Firstly, it is the only indigenous mammal in Australia specialised on diurnal activity, something necessary because it depends for food on termites active near the surface only when it is relatively warm.
• Secondly, the numbat breeds and grows slowly even relative to other marsupials owing to its nutrient-poor diet of termites, which means that without special anti-predator adaptations (i.e. mainly against raptors and snakes) it would not have survived even before fox and cat were introduced to Australia.
• Thirdly, the mallee (https://en.wikipedia.org/wiki/Mallee_(habit)) and other typically open-crowned vegetation, in which it forages, provides sparse cover but casts a suitable tracery of sparse shade on the ground and on fallen logs - into which the striping of the numbat blends, as long as it remains intermittently stationary.

All other like-size mammals, worldwide, which possess some degree of transverse striping have predatory regimes and anti-predator strategies different from those of the numbat.
 
For example:

• the one species of palm civet (Hemigalus derbyanus, https://www.inaturalist.org/taxa/41635-Hemigalus-derbyanus) marked with distinct transverse striping is nocturnal in dense forest, exemplifying typical camouflage.
• The banded mongoose (Mungos mungo, https://www.inaturalist.org/taxa/41921-Mungos-mungo) of Africa is diurnal and extremely social, and its transverse striping seems to function mainly to produce a confusing blur as the individuals move in serpentine single file in the open.
• The ringtail (Bassariscus astutus, https://www.inaturalist.org/taxa/41676-Bassariscus-astutus) of southern North America is extremely transverse-striped, but only on the bushy tail, which is wielded as a distraction when the animal is attacked.

Of these, only H. derbyanus conforms to the typical profile of a camouflaged mammal (https://www.facebook.com/watch/?v=506171570801506 and https://www.youtube.com/watch?v=5zX72v-VLYg and https://www.facebook.com/watch/?v=695754191811002 and https://www.youtube.com/watch?v=FshDn_9Qw2U). And what is surprising is that even this species is virtually restricted to several large islands of south-east Asia.
 
Zebras are so much larger than the numbat, and inhabit such open vegetation, that typical camouflage hardly applies to them.

However, there are several parallels with the numbat.

• Firstly, both the numbat and zebras have a shield of thick skin under the fur on the rump – which protects the numbat from pythons when it is holed up in tree logs and protects zebras from the lion at waterholes. So, both the numbat and zebras combine transverse striping (acting against major predators) with armour (acting against different predators), in keeping with the ‘demographic liability’ incurred by their unusually slow reproduction and growth relative to most of the prey in their communities.
• Secondly, neither zebras nor the numbat are nocturnal. Although zebras certainly are active under various conditions of illumination, they are as active by day as by night. This means that in both cases the anti-predator function of the striping plays on the light rather than simply hiding the animals.
• Thirdly, neither zebras nor the numbat are likely to depend on gross bewilderment at the moment of attack, a mechanism often assumed to occur in zebras but far better exemplified by the ringtail.

In the case of zebras, the real adaptive value of the striping is likely to be realised pre-attack, and against cursorial predators such as the spotted hyena. This carnivore does not try to pounce on top of the prey, and relies instead on exhausting the prey in a prolonged chase, after carefully choosing the most vulnerable-looking individual.

In the case of the numbat, the real adaptive value of the striping seems to be camouflage-against-shadows. And, although this marsupial tends to hold its fairly large tail erect, there is nothing about the bushiness or colouration of the tail that suggests its particular deployment for distraction of swooping raptors. 

Posted on June 13, 2022 05:57 PM by milewski