SOCIAL MODULATION OF EXERCISE ENDURANCE IN A LIZARD (SCELOPORUS-UNDULATUS)

We conducted two experiments to investigate how exercise endurance is modulated by the social environment of male lizards (Sceloporus undula tus) and whether this response is dependent on endogenous androgens. M ale lizards were held in captivity for either 8 or 9-11 wk prior to me asurements of running endurance. In both experiments lizards were held either in complete isolation or in a single crowded group, and half o f the isolated lizards in the 8-wk experiment were castrated to remove the predominant endogenous source of androgens. Endurance in the 9-11 -wk experiment was threefold greater, and cardiac muscle mass was abou t 20% greater, in isolated than in crowded lizards. Testis, epididymis , and kidney morphology indicated that androgenic activity was higher in isolates. In the 8-wk experiment, running endurance was twofold gre ater in intact isolates than in either of the other two groups, whose values were indistinguishable. Plasma androgens were reduced in castra tes compared with those in the other two groups. The intensity of comm unicative ventrolateral and gular pigmentation, which is known to be i nfluenced by androgens, was significantly more vivid in intact isolate s than in castrated or crowded lizards, Our results show that exercise endurance can change substantially within individuals over fairly, sh ort rime periods. The expression of high endurance in isolates was ass ociated with morphological evidence of higher androgenic activity than that in crowned males and required intact testes. Thus, the differenc e in endurance between crowded and isolated male lizards is associated with a probable difference in cardiac performance and seems likely to be mediated at least in part by androgenic steroids.