Some aspects of comparison between Sarcophilus and Speothos

A comparison between Sarcophilus harrisii (https://www.inaturalist.org/taxa/40232-Sarcophilus-harrisii) of Australia and Speothos venaticus (https://www.inaturalist.org/taxa/42089-Speothos-venaticus and http://68.media.tumblr.com/89a69e2b8fc47ffb9f86d84fc0598028/tumblr_nszd94TsOF1uubmz3o1_1280.jpg and https://c1.staticflickr.com/6/5475/11768525476_af99f34332_b.jpg and http://elelur.com/data_images/mammals/bush-dog/bush-dog-03.jpg and https://www.animal.photos/mamm1/dog-bush.htm and https://imgaf.s3.eu-central-1.amazonaws.com/public/artwork/products/320437/zoom/bush-dog-img-7345-sq.jpg?1307629750 and https://media1.britannica.com/eb-media/27/143527-004-F1FD0BB8.jpg and https://thumbs.dreamstime.com/t/bush-dog-speothos-venaticus-wild-37541129.jpg and http://www.naturephoto-cz.com/photos/maly/bush-dog-36x_2243.jpg and http://d3nehc6yl9qzo4.cloudfront.net/img/original/862_06_cachorro_vinagre.jpg and http://www.hlasek.com/foto/speothos_venaticus_11264.jpg and http://www.stmaartenzoo.com/wp-content/gallery/mammals/DSC_0746.jpg and https://s-media-cache-ak0.pinimg.com/564x/ae/c3/da/aec3da4bed83417168e91e2cb3243ffc.jpg and http://l7.alamy.com/zooms/291f656a2f1f409d9f3089c5a1650d60/bush-dog-speothos-venaticus-bk4ybr.jpg and https://w-dog.net/wallpaper/waldhund-bush-dog-a-predatory-mammal/id/262752/ and http://img01.deviantart.net/0153/i/2014/122/e/9/the_bush_dog_by_picturebypali-d7gtm5a.jpg) of South America may be informative.

Speothos is similar to Sarcophilus in body size, and looks superficially similar in various ways.

The following show that juveniles of Speothos somewhat resemble Sarcophilus in colouration, including subtle emphasis of the eyes (http://img09.deviantart.net/505d/i/2015/013/1/6/pz_bush_dog_puppies_portrait_1_by_orangeroom-d8dq3c3.jpg and http://www.zooborns.com/.a/6a010535647bf3970b019aff1c93a0970d-500wi).
 
I have previously suggested that a basic difference between Sarcophilus and like-size Carnivora is that the marsupial has a limited pace of life, consistent with the limited availability of resources in Australia. One aspect is that Sarcophilus has relatively slow somatic growth.

The time from conception to weaning in Sarcophilus is about 215 days. This is because gestation is 21 days, attachment to the teat lasts 100 days, and suckling continues after departure from the pouch for another 90 days at least.
 
The time from conception to weaning in Speothos is about 103 days. This because gestation is 75 days, and suckling continues for 28 days after birth.
 
Approaching the same topic a different way:
 
Sarcophilus: from conception to sexual maturity takes about 21 days plus about 2 years, = about 730 days. This is approximate because the figure of 2 years is approximate.
 
Speothos: from conceptual to sexual maturity takes about 75 days plus about one year, = about 440 days.
 
So, either way the rate of growth – assuming that both Sarcophilus and Speothos reach sexual maturity at near-maximum body mass – is far greater in Speothos than in Sarcophilus. The figures are in the ratio 215/103 (at least two-fold) if we consider weaning, and 730/440 (more than 1.5-fold) if we consider sexual maturity.
 
So, Sarcophilus seems to grow at about half the rate of Speothos.

As I understand it, this difference in rates of growth is in line with the difference in metabolic rates, which in turn seems in line with the general (average) difference in metabolic rates between marsupials and eutherians.
 
It is only when this relationship is appreciated that the paradox of the ‘semelparous tendency’ of Sarcophilus really emerges (see my Post on antechinuses).

Based on the fact that the marsupial has a relatively slow pace of life, one would logically expect Sarcophilus to be the form with the greater longevity. In fact it is the other way around: Sarcophilus both grows relatively slowly and lives a relatively short life.

As I see it, such a combination (even more typical of genera Antechinus, Phascogale, and Dasyurus) can best be understood in the context of a total environmental regime in Australia that consists of limited amounts and reliability of food, limited predation pressure, and limited specialisation in habitat and diet.
 
Given that Sarcophilus lives a surprisingly short life, it seems, at face value, to compensate for this by intense reproduction (as exemplified by the fact that up to 50 neonates can emerge in a single birth). However, when put into proper perspective, the rate of reproduction of Sarcophilus is, in a sense, as limited as its longevity. In a way, the current threat of extinction of Sarcophilus reflects a limited fecundity relative to comparable eutherians.

Another aspect of this comparison between Sarcophilus and Speothos is the anatomy of their skulls.

Overall, I would not describe these skulls as showing evolutionary convergence. In appearance the two species are only superficially similar, and about as similar as Sarcophilus is with any eutherian carnivore (including Gulo, as I have described in a recent Post).

I see nothing in the skull or dentition of Speothos that indicates any encroachment into a scavenging niche. Instead what we have in Speothos is a miniature hunting dog, in line with Lycaon and Cuon but remarkably small and short-legged with amphibious tendencies at the expense of enduring cursoriality. I infer that in the Neotropics the niche of bone-eating has been cornered by large rodents. The role of Speothos is not to compete with these rodents for bones, but instead to semi-specialise on eating the soft flesh of the same rodents - which have no counterparts in Australia past or present.

The following show the far thicker zygomatic arch (https://en.wikipedia.org/wiki/Zygomatic_arch) in Sarcophilus than in Speothos, indicating a far stronger bite in the marsupial than in the eutherian carnivore.

Speothos:
http://www.digimorph.org/specimens/Speothos_venaticus/specimen.jpg

Sarcophilus:
https://assets.catawiki.nl/assets/2017/2/5/b/c/0/bc0ac474-eb9a-11e6-8c34-7585d9ed6476.jpg

Although the lower canine seems similar in size in the two forms, the rest of the dentition if more massive in Sarcophilus than in Speothos. The skulls are indeed consistent with Sarcophilus being able to crush bones, while Speothos is the more specialised meat-shearer (using its carnassial premolars).

A surprise is to find how much more massive the muzzle (particularly the nasal area) is in Sarcophilus than in Speothos.

The sizes of the crania are consistent with the brain of Speothos being larger than that of Sarcophilus. The following photo-pair shows also that the occipital condyles (https://en.wikipedia.org/wiki/Occipital_condyles) of Speothos project to the posterior far more noticeably than do those of Sarcophilus.

Speothos:
http://www.biolib.cz/IMG/GAL/BIG/7216.jpg

Sarcophilus:
https://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Tasmanian_Devil_skull.jpg/220px-Tasmanian_Devil_skull.jpg

The sagittal crest (https://en.wikipedia.org/wiki/Sagittal_crest) is more noticeable in the marsupial than in the eutherian carnivore. This seems to be partly because the cranium is narrower dorsally in the former than in the latter, consistent with a difference in the size of the brain.

Although Speothos has smaller upper canines than those of Sarcophilus, it has a more clamp-like interdigitation of upper and lower canines together with the outermost upper incisor – something we saw also in Gulo in a previous Post.

The following again show the difference in the sizes of the crania, as well as the relatively long muzzle of Sarcophilus. The difference in size of the upper canine is particularly clearly shown.

Speothos:
https://www.mindenpictures.com/cache/pcache2/80166935.jpg

Sarcophilus:
https://pixels.com/featured/tasmanian-devil-skull-ucl-grant-museum-of-zoology.html

Posted on June 8, 2022 12:26 AM by milewski