Two disparate survivors of the Pleistocene megafauna in the Americas

Everyone knows that, at the end of the Pleistocene about 11,000 years ago, most of the large species of mammals in the Americas went abruptly extinct.

However, who understands why the few exceptions managed to survive?

Let us adopt a criterion of 200 kg, which is about the body mass of adult females of the sable antelope (Hippotragus niger,
https://upload.wikimedia.org/wikipedia/commons/3/3a/Sable_%28Hippotragus_niger%29_female_crossing_the_road_%2816635641913%29%2C_crop.jpg and https://animaldiversity.org/accounts/Hippotragus_niger/pictures/collections/contributors/susan_hoffman/sable/), adult females of the gemsbok (Oryx gazella, https://dewetswild.com/2016/09/02/gemsbok/#jp-carousel-22394), adult females of the eastern white-bearded wildebeest (Connochaetes albojubatus, https://www.flickr.com/photos/gridarendal/31251992874 and https://es.123rf.com/photo_62252521_mother-and-baby-wildebeest-in-amboseli-park-kenya.html), and adult females of two subspecies of the Asian wild ass (Equus hemionus onager, https://upload.wikimedia.org/wikipedia/commons/a/a3/Onager_Asiatischer_Wildesel_Equus_hemionus_onager_Zoo_Augsburg-11.jpg and https://printmeposter.com/poster_photo__176560507.html and Equus hemionus khur https://www.researchgate.net/figure/Figure-41-Indian-wild-ass-on-the-agro-pastoral-landscape-Source-Parmar-2016_fig25_330576869).

According to this criterion of body size:

Which species of the Pleistocene megafauna, with adult female body mass exceeding 200 kg, survived in the Americas?

More precisely, which of these species were present on this supercontinent 13,000 years ago, and remain today?

The answer is only two species: the moose (Alces alces https://en.wikipedia.org/wiki/Moose and https://bmcgenomics.biomedcentral.com/articles/10.1186/s12864-020-07208-3#:~:text=Modern%20moose%20(Alces%20alces)%20first,of%20the%20Pleistocene%20%5B17%5D) in North America and Baird's tapir (Tapirus bairdii https://en.wikipedia.org/wiki/Baird%27s_tapir and https://www.dreamstime.com/stock-images-baird-s-tapir-adult-strolling-water-image31211024#_ and https://www.dreamstime.com/stock-images-baird-s-tapir-adult-strolling-water-image31211024#_) in central and South America.

What is remarkable about these two survivors is that, although they both have proboscis-like facial specialisations (https://www.researchgate.net/publication/236183185_Structural_and_functional_comparison_of_the_proboscis_between_tapirs_and_other_extant_and_extinct_vertebrates), they are otherwise disparate in body form, habitat, life history strategy and anti-predator adaptations.

The moose is not just a 'Pleistocene giant' but combines the following three attributes. It is unusually versatile in its foraging behaviour, unusually fecund for its body size (https://pub.epsilon.slu.se/11098/1/malmsten_j_140401.pdf and https://link.springer.com/article/10.1007/BF00328601 and http://www.adfg.alaska.gov/static/education/educators/curricula/alaskawildlifecurriculum/pdfs/how_fast_can_a_moose_population_grow.pdf), and unusually adept at self-defence by means of kicking.

The fecundity of the moose seems to be a fortuitous - and fortunate - facet of its adaptation to a habitat, the boreal forest, which even during the Ice Ages was subject to episodic wildfires. A capacity for rapid reproduction allowed the moose to fluctuate in population with the cycle of post-fire regeneration - and then allowed it to survive the mysterious onslaught that defeated all other species of ungulates of its body mass or more in North America.

Baird's tapir has a different profile. It belongs to a family of 'living fossils', archaic forms originating before the Pleistocene and surviving mainly by virtue of the refugial nature of its habitat: dense forest on steep slopes and with sparse populations of other large prey. Baird's tapir was not particularly adapted to either the Ice Ages or pressure from predators, but happened to be relatively inaccessible.

The difference in the reproductive rates of moose and Baird's tapir can be seen in gestation period (243 days vs 400 days) and litter size (opportunistic twinning vs only one newborn per birth). And the difference in braininess between these two survivors is typical of the contrast between versatile and archaic forms: relative to body mass, the brain mass of the moose is about double that of Baird's tapir (file:///C:/Users/Antoni%20Milewski/Downloads/rspb20053283s04.pdf and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1560022/).

Footnote:
Although some individual adult females of the muskox (Ovibos moschatus, see https://www.researchgate.net/publication/238035671_Body_composition_of_muskoxen_Ovibos_moschatus_and_its_estimation_from_condition_index_and_mass_measurements) and the South American tapir (Tapirus terrestris) exceed 200 kg, these species are excluded on the basis of averages. The North American bison (Bison bison) does not qualify because as a species it evolved within the Holocene, despite belonging to a genus already present in the Pleistocene.

Anotado por milewski milewski, 24 de septiembre de 2021 a las 08:46 AM

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I really appreciate all your posts. There is always a lot of information new to me in all of them.

Anotado por marshall20 hace 20 días (Advertencia)

@marshall20 Hi Marshall, Many thanks for your encouraging words, from Antoni.

Anotado por milewski hace 20 días (Advertencia)

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