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Several lines of evidence support the hypothesis that free-living male mountain spiny lizards (Sceloporus jarrovi) given testosterone (T) implants experience increased energetic costs as a result of increased territorial aggression. More aggressive T-implanted males expended more energy by having a longer daily activity period andperforming more territorial displays and movements. T-implanted males made fewer foraging attempts, caught fewer insects, and had lower gut content mass. The change infrequency of foraging attempts suggests a time conflict between territorial aggression and foraging. T-implanted males also had less stored energy in the form of neutral lipids and lower dry lean body mass for their snoutvent lengths. Decreased lean body mass suggests that T-implanted males may be trading increased aggression for decreased growth. These data suggest that energetic considerations could explain previously observed decreases in survivorship of T-implanted males. Survivorship diferences, however, may al...

Two models of the mechanism that measures daylength in the photoperiodic testicular response in Passer domesticus have been proposed. Both explain adequately the "ultra-short-day" response of this species, which appears to be unique among avian photoperiodic species. However, the models differ in their predictions of the response to very long days and to continuous light. Because of this we have examined experimentally the responses of this species thereto and have compared them with those of Zonotrichia leucophrys gambelii, which does not respond to "ultra-short days." We measured testicular growth rates and plasma levels of luteinizing hormone (LH), both of which increased with increasing daylength up to about 18L 6D and were maximal for all photoregimes from 20L 4D through LL. The results are consistent with the predictions of the simplest form of external-coincidence model, but not with those of the internal-coincidence model of Dolnik or the class of simple models that involve coincidence in phase of...

Previous studies of the parthenogenetic whiptail lizard Cnemidophorus uniparens have provided the best evidence for an endogenous circannual rhythm of reproduction in a reptile. However, these studies were restricted to captive animals on constant photothermal conditions. We report the results of field and laboratory studies indicating that timing of reproduction is dramatically affected by seasonal temperature change. Our field data show that between-year variations in timing of breeding in natural populations are correlated with between-year variations in spring temperatures. Our laboratory data demonstrate that reproduction is delayed and synchronized by exposure to winter-like photothermal conditions. Thus, temperature appears to be the most important environmental cue regulating timing of reproduction in this species. However, the conclusion of previous studies that the natural reproductive cycle is controlled by a circannual rhythm is equivocal and requires further study.

The testes of male white-crowned sparrows, Zonotrichia leucophrys gambelii, subjected to a photoregimen of 12-h days (12L 12D) undergo quasiannual cycles of repeated growth and regression. These cycles decrease successively in period and amplitude and are not accompanied by postnuptial molts. This suggests that they are not physiologically equivalent to those of birds subjected to natural photocycles. Our results demonstrate that transfer of white-crowned sparrows undergoing testicular regression on 12L 12D to long days (20L 4D) induces both testicular growth and increases in plasma levels of luteinizing hormone at normal rates for 20L 4D. These data indicate that testicular regression on 12L 12D must be induced by mechanisms other than the development of photorefractoriness, which is the normal cause of regression on natural photocycles. This suggests very strongly that daylength functions as a driver of the independent components of the annual cycle rather than, as in some other species, a Zeitgeber for...