Question: How does the evolution of a key adaptation affect niche breadth during an adaptive radiation?

Organisms: Cenozoic horse and camel species, as well as Pleistocene ungulates.

Predictions: Niche breadth theoretically could increase, decrease or remain the same as attainment of a key adaptation facilitates a niche shift. Simpson predicted a decrease in niche breadth (ecological specialization) when key adaptations lead to adaptive radiations. I test Simpson’s prediction by examining ecological response to attainment of high-crowned teeth (hypsodonty). The evolution of hyposdonty represents a key adaptation involved in many ungulate adaptive radiations.

Methods: To test whether hypsodont ungulates have potentially wider or narrower niche breadth in respect to their non-hypsodont, pre-adaptive radiation ancestors, I analysed δ¹³C values in the tooth enamel of Pleistocene ungulates as a proxy for dietary breadth. For Cenozoic horses and camels, I measured the total number of biogeographic provinces and the total number of fossil localities in which individual taxa were found to assess breadth of habitat use. I considered these two parameters (dietary breadth and habitat breadth) as two major niche axes from which I qualitatively estimated niche breadth. I also compared taxon survival between low-crowned and high-crowned taxa, reasoning that if high-crowned taxa had less broad niches, their probability of extinction should be higher and their temporal duration shorter.

Results: The δ¹³C values of herbivores from the Pleistocene of Florida revealed that high-crowned taxa fed on a diet of both C₃ and C₄ forage, while low-crowned taxa confined their feeding to C₃ plants. In the Cenozoic horse and camel clades, there was no statistically significant difference between high-crowned and low-crowned taxa in the number of biogeographic provinces or localities occupied. Nor were there significant differences between high-crowned and low-crowned taxa in the duration of time a particular species survived.

Conclusions: Simpson’s prediction that key adaptations that lead to adaptive radiation also result in decreased niche breadth is not supported in the case of the evolution of hypsodonty by the ungulates. Instead, the attainment of hypsodonty in these taxa broadened niche space along one of the studied axes (dietary variety) and had no discernible effect on the other (habitat occupancy).