Tamanduas have converged with African pangolins except in anti-predator defences

Pangolins (https://en.wikipedia.org/wiki/Pangolin) are the most widespread of the mammals specialised for a diet of ants and termites. They occur extensively in both Africa and tropical Asia (https://en.wikipedia.org/wiki/Pangolin#/media/File:Manis_ranges.png).

Therefore, one might expect some mammal in the tropics of the Americas to be convergent with pangolins.

Among the obvious candidates, based on body size, diet and arboreal tendency, are the two species of Tamandua (https://en.wikipedia.org/wiki/Tamandua and https://www.youtube.com/watch?v=CtQiSGHJAEI).

Pangolins and tamanduas belong to different orders of mammals, namely Pholidota vs Pilosa. Therefore, any similarities - such as toothlessness, extreme reduction of the mandibles, extreme protrusion of the tongue (http://www.animalsanimals.com/results.asp?image=MAM%20020NEL004%2001 and https://www.youtube.com/watch?v=T6bMlm-oUM8), gizzard-like stomach, and extreme development of the fore claws - are evidence of evolutionary convergence in adaptation to similar habitats and niches.

In this Post, I focus on two of the African species of pangolins, namely the white-bellied pangolin (Phataginus tricuspis, https://en.wikipedia.org/wiki/Tree_pangolin) and the black-bellied pangolin (Uromanis tetradactyla, https://en.wikipedia.org/wiki/Long-tailed_pangolin).

Tamanduas (two species complementing each other in distribution, https://en.wikipedia.org/wiki/Northern_tamandua and https://en.wikipedia.org/wiki/Southern_tamandua) weigh about 4.5 kg, comparable with our two species of pangolins (about 2.5 kg).

Tamanduas and our pangolins are similar in that they are incapable of running; their metabolism is slower than is typical of eutherian mammals; and they have slow reproduction - gestating for about 145 days and bearing one infant at a time.

Both tamanduas and pangolins qualify as myrmecophagous, and some of the genera of ants (e.g. Crematogaster) and termites (e.g. Nasutitermes) they eat are the same on the two continents. However, our pangolins are the more specialised in habitat and diet.

Tamanduas range beyond forests and, although their diet is mainly termites and ants, they also eat other invertebrates (e.g. see https://www.youtube.com/watch?v=_p0aL5nQDps) and fleshy fruits (e.g. see https://bioone.org/journals/edentata/volume-12/issue-1/020.012.0110/Fruit-Eating-by-an-Obligate-Insectivore--Palm-Fruit-Consumption/10.5537/020.012.0110.full).

By contrast, the black-bellied pangolin occurs only in swamp forest and seems to have a strict diet of arboreal ants (mainly Crematogaster https://en.wikipedia.org/wiki/Crematogaster and Cataulacus https://en.wikipedia.org/wiki/Cataulacus). The partly terrestrial white-bellied pangolin, although ranging into clearings in the forest where there has been recent cultivation, seems to eat only termites and ants, in that order of importance.

Both tamanduas and our pangolins have prehensile tails. However, in accordance with its particular arboreal specialisation, the black-bellied pangolin has an extremely long tail with the most caudal vertebrae (46-47) known in any mammal.

Tamanduas and pangolins seem equally adept at swimming, despite their arboreal tendencies and usually slow movements (https://www.researchgate.net/publication/216376186_Swimming_in_the_Northern_Tamandua_Tamandua_mexicana_in_Panama). Both of our species of pangolins seem sometimes to escape from predators by dropping into rivers.

Both tamanduas and pangolins release noxious secretions from anal glands to defend themselves from predators.

Despite all the above convergences, the remaining aspects of the anti-predator adaptations are divergent.

Tamanduas - unlike pangolins - fight formidably with their fore claws (https://en.wikipedia.org/wiki/Southern_tamandua#/media/File:T_tetradactyla_1.jpg and https://twitter.com/brkbru/status/1198070601749073920). Their colouration seems to combine partial aposematism (https://en.wikipedia.org/wiki/Aposematism) with enough individual variation to thwart the formation of search-images by predators (https://www.shutterstock.com/nb/image-photo/tamandua-mexicana-arborical-ant-eater-central-1639865707 and https://www.dreamstime.com/southern-tamandua-tetradactyla-brazil-rain-forest-animal-central-america-image174784097 and https://www.shutterstock.com/nb/image-photo/southern-tamandua-on-branch-tetradactyla-1816839440 and https://alchetron.com/Tamandua#tamandua-963bdc1c-883e-4307-94b0-1268b1b5dd6-resize-750.jpeg).

By contrast, our pangolins rely on the armour unique to their order of mammals. All pangolins have a mainly passive strategy against predators: they curl up, protected by tough scales which can additionally scissor to slice any attacker which tries to prize open the ball.

What is particularly remarkable about the black-bellied pangolin is that it combines armour with arboreal specialisation. Tropical America does have armoured mammals in the form of armadillos (order Cingulata), but these are terrestrial and, at the body sizes concerned here, not mymecophagous.

The shapes of the skulls of tamanduas and pangolins differ considerably, in a way possibly explained by the anti-predator strategies.

Compare Tamandua tetradactyla (https://www.researchgate.net/figure/Dorsal-ventral-and-lateral-views-of-skull-and-lateral-view-of-mandible-of-an-adult_fig1_274102852 and https://www.valleyanatomical.com/product/tamandua-skull/1016) with Phataginus tricuspis
(http://pierce.wesleyancollege.edu/faculty/brhoades/woc/mammals/pangolin.html and http://digimorph.org/specimens/Manis_tricuspis/skull/).

It is immediately noticeable that in our pangolins the snout is relatively short. This seems consistent with being able to roll up into a tight ball when threatened.

How can the non-convergences in anti-predator adaptations be explained?

Tropical forests in Africa and America differ considerably in the intensities of their predatory regimes. At body masses of less than 5 kg and with the constraints on metabolic capacity imposed by a relatively energy-poor diet of social insects, fighting may be less effective than armour under the intense predation typical of Africa.

In summary:

Tamanduas and pangolins are certainly convergent enough to qualify as ecological counterparts on different continents. However, tamanduas are not as specialised on ants and termites as are the two species of pangolins most similar to them in Africa.

More intriguingly, tamanduas have diverged from both pangolins and armadillos in lacking armour, instead opting for a combination of clawing, chemical defence and warning colouration. The same divergence has, in the case of an otherwise tamandua-like species of pangolin in equatorial Africa, produced the only extremely arboreal mammal on Earth which is armoured. And, more remarkably still, an extremely long and prehensile tail which is armoured.

Publicado el octubre 25, 2021 07:43 MAÑANA por milewski