Many avian species of the North American Sonoran desert, e.g., the black-throated sparrow, Amphispiza bilineata, cactus wren, Campylorhynchus brunneicapillus, and curve-billed thrasher, Toxostoma curvirostre, can potentially breed from March/April to August. It is possible that, at least in summer, intense heat and aridity may have inhibitory effects on breeding by precipitating a stress response. Stress typically results in a rise in secretion of adrenocorticosteroid hormones that then inhibit reproduction by suppressing release of gonadal hormones. However, we found that plasma levels of corticosterone were not higher during summer, compared with winter, even in 1989 when summer temperatures were higher than normal. In June 1990, temperatures were also above normal and soared to the highest level recorded in Arizona (50 degrees C). Plasma levels of corticosterone during June were high in black-throated sparrows, but less so in two other species (Abert's towhee, Pipilo aberti, and Inca dove, Scardafella inca) found in more shady riparian and suburban habitat with constant access to water. The adrenocortical response to stress (as measured by the rate of corticosterone increase following capture) was reduced in the hottest summer months in black-throated sparrows, cactus wrens, and curve-billed thrashers, but less so in Abert's towhee an Inca dove. These data suggest that at least some birds breeding in the open desert with restricted access to water are able to suppress the classical adrenocortical response to stress. The response is then reactivated in winter after breeding has ceased. It is possible that this stress modulation may allow breeding to continue despite severe heat. Analysis of plasma from these species indicated that the apparent modulation of the adrenocortical response to stress was not an artifact of reduced affinity or capacity of corticosterone binding proteins.

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SYNOPSIS. An organism's phenotype results from an interaction of environment and genotype. Sex steroids play a role in translating sexual genotype into phenotype. The focus of this research has been to extend the model of sex steroid hormone action in sexual differentiation to individual variation in reproductive phenotype. The hypothesis generated, called the relative plasticity hypothesis, has been tested in a species with alternative phenotypes, tree lizards (Urosaurus ornatus). Such species are useful models for tests of these ideas because variation is extreme and easily studied. These tests have shown that permanent differentiation of the territorial and nonterritorial phenotypes is accomplished by hormonal mechanisms operating during early development. These are similar to organizational actions of hormones classically described for sexual differentiation. A unique finding of this work is that the adrenal hormone progesterone regulates differentiation of the two male types. Furthermore, the nonterritorial male tree lizard appears to conditionally switch tactics between sedentary satellite and nomadic behavior. Nomadic behavior is triggered under stressful environmental conditions. Nonterritorial tree lizards show greater inhibition of reproductive hormones following stress than do territorial tree lizards, suggesting that a differential sensitivity of reproductive hormones to stress is the endocrine basis of conditional tactic switching in this morph. This mechanism is similar to the classical activational effects of hormones. Together, these findings indicate that individual variation in sexual phenotype is produced by mechanisms similar to sexual differentiation. Further refinement of the models and integration with some evolutionary ideas is explored. 1 From the Symposium Animal Behavior: Integra-ing sexual phenotypes (Breedlove, 1992; tion of Proximate and Ultimate Causation presented at GuStafson a n d D o n a h o e , 1 9 9 4). H o w e v e r , the Annual Meeting of the Society for Integrative and h j j. frequently not emphasized Comparative Biology, 26

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