How baboons and like-size, coexisting gazelles differ in their most basic anti-predator strategies, part 1

Gazelles and baboons both survive the intense predatory regime of the Serengeti ecosystem (https://en.wikipedia.org/wiki/Serengeti), but by means of different biological strategies.

This exemplifies the fundamental adaptive differences between ruminants and primates.

Eudorcas nasalis (https://en.wikipedia.org/wiki/Thomson%27s_gazelle and https://www.jungledragon.com/specie/1837/thomsons_gazelle.html) and Papio anubis (https://en.wikipedia.org/wiki/Olive_baboon) share habitat (https://www.youtube.com/watch?v=wFhiq_vFX60).

However, they show the great variation in pace of life in mammals, hence the different potentials of prey species to support predators.

The comparison is nice because both of these species:

• weigh about 20 kilograms in adulthood,
• are gregarious,
• metabolise more rapidly than humans do,
• gestate for about half a year, and
• bear about a dozen offspring in a full maternal lifetime.

However, the gazelle breeds three times faster than the primate, as shown by time from:

• conception to time of first giving birth (16 months vs 4 years),
• birth to weaning (6 months vs 18 months),
• birth to social maturity in males (3 years vs 9 years), and
• one birth to the next (9 months vs 25 months).

In the case of females, there is a particularly great difference in the time taken to reach sexual maturity: one year in the gazelle vs 4.5 years in the baboon.

Accordingly, Thomson's gazelle lives only a third as long as the olive baboon (15 years vs 45 years) - even if it survives to become elderly.

Because the ruminant spends energy threefold faster on reproduction and growth than the primate does, it is the one that makes energy available to predators.

Thomson's gazelle can afford this productivity because it eats grass and other herbaceous plants (https://videohive.net/item/thomsons-gazelle-grazing/16966247) that are:

• abundant compared to the fruits, tubers, flowers and small animals eaten (besides green grass) by the baboon, and
• digested more thoroughly by the ruminant than are the foods (which include green brass) eaten by the monogastric primate.

(Note also that the gazelle can keep chewing its food at night, while the baboon sleeps.)

Given the same hypothetical density of population of ruminant and primate, the ruminant can support an offtake rate by predators threefold greater than that affordable to the primate. Nearly a third of the adult population of Thomson's gazelle can die each year without the population necessarily declining, because that many individuals are routinely replaced by breeding. By contrast, if more than one tenth of the population of the olive baboon dies each year, year after year, it risks extermination.

The result is that Thomson's gazelle supports the guild of predators with which the olive baboon has to contend. This is true despite the fact that the carnivore most dependent on gazelles, namely the cheetah (Acinonyx jubatus, https://en.wikipedia.org/wiki/Cheetah), is different from the one most likely to predate baboons, namely the leopard (Panthera pardus, https://en.wikipedia.org/wiki/Leopard).

Both Thomson's gazelle and the olive baboon have sharp eyes, collective vigilance, and tactics of flight: extremely rapid running in Thomson's gazelle vs refuge in trees in the olive baboon.

They both also, at least nominally, have sharp weapons, at least in adult males (https://upload.wikimedia.org/wikipedia/commons/3/3d/Thomsons-gazelle_horns.jpg and https://en.wikipedia.org/wiki/Olive_baboon#/media/File:Papio_anubis_skulls.jpg and http://skullbase.info/skulls/mammals/olive_baboon.php). However, fighting back against attacking predators is hardly effective in either species.

Thomson's gazelle is, like most ungulates, too inept with its horns and hooves to harm its predators with significant frequency. For its part, the olive baboon lacks sharp teeth in females; and although the masculine canines are formidable there is not necessarily much defence of females or juveniles by males.

Whereas the gazelle relies mainly on fecundity, the baboon relies mainly on intelligence.

Given similar body masses, the brain weighs about twice as much in the olive baboon as in Thomson's gazelle. This means an encephalisation quotient (https://en.wikipedia.org/wiki/Encephalization_quotient) of 2 instead of 1. And the brain, like the pregnant uterus, is among the most energetically expensive organs in the body.

All of this resolves to the following:

The olive baboon has a life history strategy in which what is emphasised is not fecundity but instead life-experience. Cognition takes time to build, and the extended longevity of primates both provides this time and allows successful reproduction to be less urgent than in ruminants.

In a sense, this means that the metabolic 'hotspot', crucial to survival in the Serengeti, is the uterus in Thomson's gazelle vs the brain in the olive baboon. The facts and figures of growth and reproduction fall into place around this central divergence in adaptive emphasis.

to be continued in https://www.inaturalist.org/journal/milewski/81934-how-baboons-and-like-size-coexisting-gazelles-differ-in-their-most-basic-anti-predator-strategies-part-2#...

Posted on Μάρτιος 15, 2022 0210 ΠΜ by milewski