Stress hormones and the fitness consequences associated with the transition to a novel diet in larval amphibians.

Closely related species often specialize on different types of prey, but little is known about the fitness consequences of making an evolutionary transition to a novel diet. Spadefoot toad larvae provide a unique opportunity to reconstruct these evolutionary events. Although most anuran larvae feed on detritus or plankton, Spea larvae have also evolved the ability to consume large anostracan fairy shrimp. To investigate the changes that may have accompanied the shift to shrimp prey, we compared shrimp-induced physiological responses of Spea larvae with those of its sister genus, Scaphiopus, that has not made this transition. Although Spea larvae performed equally well on either diet, shrimp-fed Scaphiopus larvae experienced reduced growth and developmental rates, as well as elevated levels of the stress hormone corticosterone when compared with those that ate the ancestral detritus diet. These results suggest that ancestral Spea likely experienced reduced fitness when they first adopted a carnivorous feeding strategy.