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The Challenge Hypothesis postulates that male vertebrates can respond to social challenges, such as simulated territorial intrusions (STI), by rapidly increasing their concentrations of plasma androgens, such as testosterone (T). This increase may facilitate the expression of aggressive behavior and lead to persistence of this behavior even after withdrawal of the challenge, thus potentially promoting territoriality and the probability of winning future challenges. The validity of the Challenge Hypothesis was investigated in socially monogamous free-ranging male Cassin’s Sparrows, Peucaea cassinii. Exposure to STI at the beginning of the vernal nesting season stimulated aggressive behavior but did not increase plasma T. Furthermore, plasma T did not correlate with the duration of exposure to STI and the behavioral response to STI did not differ in males that were challenged a second time shortly after the first challenge. As birds were investigated at a stage of their reproductive cycle when plasma T is presumably seasonally high due to photostimulation, the lack of hormonal response to STI may have been due to the hypothalamo-pituitary-gonadal axis secreting hormones at maximum rates. This was not the case, however, because administration of gonadotropin-releasing hormone I (GnRH-I) rapidly stimulated the secretion of luteinizing hormone (LH) and T, and treatment with ovine LH rapidly stimulated T secretion.

Chronic stress generally inhibits the activity of the reproductive system. Acute stress also is often inhibitory, but the mechanism involved and its persistence of action once animals are no longer exposed to the stressor are poorly understood. We investigated the effect of capture and restraint stress on plasma testosterone (T), luteinizing hormone (LH), and corticosterone (CORT) in free-ranging male rufous-winged sparrows, Peucaea carpalis. Stress decreased plasma T between 10 and 30 min after capture and restraint but did not influence plasma LH, the main hormone that controls T secretion, suggesting that stress did not decrease plasma T by inhibiting LH secretion. The stress-induced decrease in plasma T was associated with elevated plasma CORT, but there was no evidence that these effects were functionally related. Plasma stress-induced T was positively related to plasma initial T measured within 2 min of capture. This relationship was, however, complex as plasma T decreased proportionally more in response to stress in sparrows with high than low plasma initial T. The relative sensitivity to a same stressor was, therefore, individually variable and this variation was related to initial plasma T. Birds caught and restrained for 30 min, and then released on their breeding territory before recapture up to 6 hours later, maintained depressed plasma T, indicating that the effect of acute stress on this hormone persists after the stressor removal. These studies provide new information on the effects of acute stress on plasma T in free-ranging birds. In particular, they are among the first to characterize the time course and to describe the persistence of these effects. The findings also contribute to identifying factors that are associated with individual differences in plasma hormone levels.

Chronic stress generally inhibits the activity of the reproductive system. Acute stress also is often inhibitory, but the mechanism involved and its persistence of action once animals are no longer exposed to the stressor are poorly understood. We investigated the effect of capture and restraint stress on plasma testosterone (T), luteinizing hormone (LH), and corticosterone (CORT) in free-ranging male rufous-winged sparrows, Peucaea carpalis. Stress decreased plasma T between 10 and 30 min after capture and restraint but did not influence plasma LH, the main hormone that controls T secretion, suggesting that stress did not decrease plasma T by inhibiting LH secretion. The stress-induced decrease in plasma T was associated with elevated plasma CORT, but there was no evidence that these effects were functionally related. Plasma stress-induced T was positively related to plasma initial T measured within 2 min of capture. This relationship was, however, complex as plasma T decreased proportionally more in response to stress in sparrows with high than low plasma initial T. The relative sensitivity to a same stressor was, therefore, individually variable and this variation was related to initial plasma T. Birds caught and restrained for 30 min, and then released on their breeding territory before recapture up to 6 hours later, maintained depressed plasma T, indicating that the effect of acute stress on this hormone persists after the stressor removal. These studies provide new information on the effects of acute stress on plasma T in free-ranging birds. In particular, they are among the first to characterize the time course and to describe the persistence of these effects. The findings also contribute to identifying factors that are associated with individual differences in plasma hormone levels.

The Challenge Hypothesis postulates that male vertebrates can respond to social challenges, such as simulated territorial intrusions (STI), by rapidly increasing their concentrations of plasma androgens, such as testosterone (T). This increase may facilitate the expression of aggressive behavior and lead to persistence of this behavior even after withdrawal of the challenge, thus potentially promoting territoriality and the probability of winning future challenges. The validity of the Challenge Hypothesis was investigated in socially monogamous free-ranging male Cassin’s Sparrows, Peucaea cassinii. Exposure to STI at the beginning of the vernal nesting season stimulated aggressive behavior but did not increase plasma T. Furthermore, plasma T did not correlate with the duration of exposure to STI and the behavioral response to STI did not differ in males that were challenged a second time shortly after the first challenge. As birds were investigated at a stage of their reproductive cycle when plasma T is presumably seasonally high due to photostimulation, the lack of hormonal response to STI may have been due to the hypothalamo-pituitary-gonadal axis secreting hormones at maximum rates. This was not the case, however, because administration of gonadotropin-releasing hormone I (GnRH-I) rapidly stimulated the secretion of luteinizing hormone (LH) and T, and treatment with ovine LH rapidly stimulated T secretion.