Why There Are No Modern Equids Living in Tropical Lowland Rainforests

Presently, the equid lineage occurs completely outside tropical rainforest environments, which is thought of as the cradle of Perissodactyls and early equid ancestors. The ancestral food of those early equids was based on seeds, fruits, foliage and C3-grasses. The CO2-content of the atmosphere was very high, and C4-grasses had not evolved yet. Zebras and horses are considered to be typical grazers often on C4-grasses, even though Asian equids (and the mountain zebra) have a high proportion of browse in their diet. In comparison to more open environments, present-day tropical forests represent marginal habitats for large ungulates in comparison to more open environments. The suite of traits of large ungulates is not very well adapted to this environment, but equids should not be considered the pinnacle of adaptation to open environments; they retain basal tropical forest traits and lack certain derived open environment traits. Also, equids should not be considered some “non-ruminating ruminant”: their physiology is very different from that of the artiodactyls with which they are often compared; in particular, they are much better at digesting starch and other soluble carbohydrates. We present four storylines on why extant equids may be absent from tropical rainforests: one centred on carbon dioxide, one on chemical plant defence, one on metabolism, and finally a parasitism storyline. Storylines are helpful to envisage how things could have evolved but, of course, do not provide proof.