Mammals with transverse stripes in different places: numbat vs ringtail

 
 (writing in progress)

Something I have not seen pointed out in the literature is that the pattern of colouration ninth numbat (Myrmecobius fasciatus) is unusual on a global basis, among otherwise comparable mammals. Here I elaborate on this observation w.r.t. the Americas.
 
As far as I know there is only one lineage of mammals in the Americas comparable with the numbat in having transverse striping. The striping in the case of said lineage is restricted to the tail. Furthermore, there is no ‘crossing out’ of the eye in the most clearly striped members of the raccoon family (Procyonidae) to which I refer.
 
This raises the question of why the numbat does not have a plain body and a striped tail instead, and why the numbat’s tail is not more substantial than it is.
 
I also doubt that the following has been pointed out before: the best example of a transverse-striped smallish mammal in the Americas happens also to be a mammal with an extraordinarily well-developed tail relative to its body. This seems to suggest a function in distraction rather than simple camouflage. Please bear in mind that Bassariscus astutus lives in semi-desert, not dense vegetation.
 
The bottom line seems to be: no mammal in the America resembles the numbat in colouration, and the closest resemblance (Bassaricus astutus) differs in suggesting distraction rather than the camouflage invoked for the numbat.
 
The only American mammals with considerable transverse striping are in genus Bassariscus, which consists of two species of small relatives of raccoons. The tail in one of these species is particularly strongly marked with transverse stripes, in such a way that both camouflage and distraction are plausible functions. However, the eyes differ from those of the numbat, being conspicuous rather than ‘crossed out’. The differences in colouration between Bassariscus and the numbat may partly be explained by the fact that Bassariscus is nocturnal, whereas the numbat is diurnal. However one interprets the colouration of procyonids and Bassaricus, comparison with the Americas shows how odd the colouration of the numbat really is.
 
Bassariscus astutus:
 
Bassariscus astutus:
 
The panel on far left shows the whole raccoon family (Procyonidae), with Bassariscus astutus in the middle of the top row. As you can see

• all members of this family are unlike the numbat in having essentially plain bodies,
• although transverse striping is a common theme in this family it is Bassariscus, particularly B. astutus, which has this pattern best-developed, and

• the particularly clearly striped tail of Bassariscus is correlated with a particularly large tail, relative to the body, in this genus.
   
  The following view shows that the transverse striping on the tail of Bassariscus is interrupted on the ventral side, suggesting a function more consistent with camouflage in this context provided the species has a behavioural repertoire of freezing when alarmed, which I assume that it does have.
   
  Bassariscus astutus:

The following is an attempt to be as clear as possible about the value of comparing zebras with the numbat.
 
Firstly, I don’t think it’s ever been pointed out previously that the numbat is analogous to zebras in being one of the few mammals of approximately its body size on Earth that possesses transverse striping on its body. (The most similar are the banded mongoose and one species of palm civet, but in neither of these are the stripes so clear.)
 
Secondly, both zebras and the numbat differ from most typically camouflaged mammals in not being nocturnal. (Zebras are cathemeral, while the numbat is diurnal.)
 
Thirdly, in both zebras and the numbat, the function of the striping remains a puzzle despite the animals being favourite subjects for study and illustration in zoology. (In the case of the numbat this is because nobody has yet studied the normal behaviour of the species relative to its natural regime of predation, and the natural pattern of light and shade in its typical environment.)
 
Fourthly, in neither zebras nor the numbat does a function of distraction/confusion of predators make sense. (In the case of zebras this is mainly because all ungulates have been evolutionarily selected by predation but no other ungulate is thought to use bold colouration to foil the aim of pouncing predators. In the case of the numbat it is mainly because comparable mammals use the tail, not the body, to distract predators at the moment of attack.)
 
Fifthly, in both zebras and the numbat there is evidence of a ‘duplex’ anti-predator strategy comprising at least two unrelated defences: striping on the body and a skin-shield on the rump. (In both cases the predators foiled by the skin-shield are different from those hypothetically foiled by the striping.)
 
Sixthly, in both zebras and the numbat the striping and the duplex nature of the anti-predator defences, and the extreme degree of striping, seem to be associated with ‘demographic liability’. (In the case of zebras this liability results from the lesser rate of reproduction of growth of perissodactyls than the coexisting ruminants which support the prevailing density of population of diverse carnivores; in the case of the numbat this liability results from a lesser rate of reproduction and growth than in coexisting marsupials) plus a body size ‘falling between two stools’ relative to anti-predator strategies.)
 
Finally, in both zebras and the numbat the unusual degree and quality of the striping ultimately corresponds to a trophic poverty of the animals relative to the guilds of which they form part. (In the case of zebras this is the monogastic strategy instead of the prevailing ruminant strategy of digestion; in the case of the numbat it is a specialisation on termites, which are nutrient-poor.)
 
The overall conclusion, then, is that in both zebras and the numbat the unusual striping has evolved because of a particular pressure to boost anti-predator defences above those needed by most coexisting and comparable mammals. (In the case of zebras the function is likely to be foiling of the targeting of vulnerable individuals by a cursorial predator, the spotted hyena; in the case of the numbat the function is likely to be shadow-camouflage which foils the spotting of the numbat by diurnal raptorial birds.)

(writing in progress)

Publicado el julio 5, 2022 07:21 MAÑANA por milewski