Why was the wild horse of the Eurasian steppes not a zebra?

In previous Posts, I have explained the striping of zebras as an adaptation to demographic liability.

The essence of the argument is that

• zebras live as a minor part of communities of like-size ruminants, that support an intense predatory regime,
• the relatively slow reproduction of equids means that, without additional anti-predator adaptations, the zebras are vulnerable to natural extermination,
• the most important carnivores rely on visual detection of lame, pregnant, old, or ill individuals, and
• accordingly, a compensatory anti-predator defence has arisen, in the form of colouration designed to interfere with the flicker-sensitive eyesight of the carnivores.

The ancestor of the domestic horse (Equus caballus) is an extinct species (Equus ferus, scroll in https://prehistoric-fauna.com/Tarpan nd scroll in https://prehistoric-fauna.com/Equus-ferus-latipes).

The wild horse weighed about 300 kg, if similar to a surviving form, the takhi (https://en.wikipedia.org/wiki/Przewalski%27s_horse). Six spp. of Equus from the Pleistocene in North America seemed to have weighed 300-575 kg (please see screenshot below).

The wild horse formerly lived somewhere in the treeless grasslands stretching West to East across Eurasia, from Hungary to Manchuria (https://upload.wikimedia.org/wikipedia/commons/7/79/Eurasian_steppe_belt.jpg and http://3.bp.blogspot.com/-6e7JOcOlovI/UJuP8ApWvJI/AAAAAAAAAAg/3CFL8i_Vrss/s1600/an390469.f1.gif and http://s155239215.onlinehome.us/turkic/btn_Archeology/Chernykh/Chernykh2008Fig1.jpg).

So, why was E. ferus not striped, like zebras?

Is it possible that, in the ecosystem of the Eurasian treeless grasslands, there was no demographic liability for equids?

I argue here that the answer is 'yes'.

Eurasia is a vast and complicated continent. Yet, in some ways, the ‘steppe belt’ is remarkably uniform, latitudinally.

And the latitudes involved are far greater than those of treeless grasslands in Africa, South America, and the USA.

For example, the latitudes of the Eurasian treeless grasslands, if projected to the southern hemisphere, would be located far south of the southern tip of Africa, at the level of Patagonia and the South Island of New Zealand.

Even within the Northern hemisphere, there is a difference from North America.

Only the northernmost 'prairies', in e.g. North Dakota and Alberta, conform to the latitudes of the Eurasian 'steppes'.

This means that treeless grasslands in 14 states of the USA (https://upload.wikimedia.org/wikipedia/commons/4/49/United_States_Prairies.svg) lie farther south than any similar vegetation in Eurasia (https://www.britannica.com/place/the-Steppe), other than an outlying patch in Turkey.
 
The far-northerly latitudes of the treeless grasslands of Eurasia suggest an extremely seasonal climate, prohibitive to most wild ruminants, even during the Holocene.

And indeed, across this vast ecosystem, there seems to have been only one genus of indigenous ‘antelope’, and none similar in body mass to wild equids.

In this way the treeless grassland biome of Eurasia was extremely different from the Highveld (https://en.wikipedia.org/wiki/Highveld) of South Africa, which had an extremely rich fauna of 'antelopes' (see Table 1 in https://www.sciencedirect.com/science/article/pii/S0254629915003051#:~:text=2.2.-,Frost,those%20of%20the%20African%20continent. and https://www.researchgate.net/publication/282619832_Why_was_the_Highveld_treeless_Looking_laterally_to_the_Pampas_for_global_edaphic_principles_beyond_biogeographical_accidents and https://www.researchgate.net/publication/282619832_Why_was_the_Highveld_treeless_Looking_laterally_to_the_Pampas_for_global_edaphic_principles_beyond_biogeographical_accidents/fulltext/5e601cc792851cefa1decb48/Why-was-the-Highveld-treeless-Looking-laterally-to-the-Pampas-for-global-edaphic-principles-beyond-biogeographical-accidents.pdf).

The only ruminants coexisting with E. ferus seem to have been

• Bison priscus (scroll in https://prehistoric-fauna.com/Bison-priscus and https://en.wikipedia.org/wiki/Steppe_bison), and
• Procapra gutturosa (adult female 25 kg) or a congener.

The steppe bison (http://prehistoric-fauna.com/image/cache/data/Bison_priscus-738x591.jpg ) was more massive than its extant relatives. It probably weighed about 550 kg (adult female). This is about 22% heavier than the African bovine, Syncerus caffer, https://www.inaturalist.org/taxa/42405-Syncerus-caffer), which widely coexists with zebras.

I cannot rule out some species of Gazella, perhaps G. sinensis. However, I have found scant information on this.

The saiga (Saiga tatarica, https://www.inaturalist.org/taxa/42378-Saiga-tatarica) does not qualify as a denizen of treeless grassland. This is because it is associated instead with semi-desert just to the south, where it presumably coexisted with Equus hemionus (https://www.inaturalist.org/taxa/43333-Equus-hemionus).

But even if the saiga was formerly present in the mesic treeless grassland, I consider it too small and too extremely migratory to support an argument of ‘demographic liability’, w.r.t. the wild horse.
 
The indigenous community of ungulates was exterminated several thousands of years ago, except for Procapra, which has survived in Mongolia.

Once domesticated species replaced the wild horse and the steppe bison, the ungulate community seems to have consisted mainly of domestic horse and domestic sheep (Ovis aries, https://en.wikipedia.org/wiki/Sheep). The natural predators (including the spotted hyena, Crocuta crocuta) were controlled, which is feasible by means of poisoning.

What is crucial here is that, in the Eurasian treeless grasslands,

• the wild horse was a major, not minor, component of the community of ungulates,
• this community (excluding proboscideans and rhinos) consisted of just two species: wild horse and steppe bison, the latter (body mass about 550 kg) being more massive than the former (body mass about 300 kg), and
• the reproductive rates of the equid and the bison would have been similar, because of the limitations on fecundity with increasing body size among ruminants.

I suspect that the population densities of wild horse and steppe bison were also similar.

Therefore, pressure from predators would have been set by both equid and ruminant together. This differs from the regime in Africa, where this pressure is set mainly by the ruminants - with their superior numbers, and superior rates of replacement of the dead.

On this basis, there would be no case for ‘demographic liability’ for the equid, despite the fact that its reproductive rate was indeed, relative to its body mass, certainly inferior to that of the bovine.
 
A similar rationale extends to the extant spp. of wild asses. All of these tend to dominate their ungulate communities. They therefore do not qualify for the idea of ‘demographic liability.’

In summary:
In interpreting why the wild horse (and its domestic descendant) has such different colouration from zebras, it may be important to realise that the former was quite different from the latter in terms of coexistence with like-size ruminants.

Posted on 08 de setembro de 2022, 10:16 AM by milewski