The acute phase response (APR) is the first line of defense that many vertebrates employ during a pathogenic challenge. This response is composed of a suite of physiological, behavioral, hormonal, and metabolic changes that include fever, iron sequestration, anorexia, adipsia, somnolence, and activation of the hypothalamo-pituitaryadrenal (HPA) axis and suppression of the hypothalamopituitary-gonadal (HPG) axis. Although well-studied in mammals and domesticated birds, the APR of passerines is virtually unexplored. Here, we characterize the APR in several species of Emberizidae and examine seasonal variation. Captive and free-living sparrows were treated with lipopolysaccharide (LPS), an immunogenic agent that triggers the APR without actually causing infection. LPS treatment activates the HPA axis, suppresses the HPG axis, decreases activity and food and water intake, and induces short-term hypothermia in captives, as well as inhibiting territorial aggressive behavior and song in free-living males. The magnitude of the APR also varies seasonally in males, implicating a tradeoff between physiological processes within particular life-history stages, such as reproduction. The proximate mechanisms underlying this seasonal modulation may include hormonal suppression by the steroid testosterone and seasonal differences in energy stores, which are rapidly depleted to a minimum body mass threshold as a result of APR-induced sickness behavior. We conclude by comparing this variation in APR to seasonal variation of avian stress responses.

No abstract

No abstract

We examined plasticity of the stress response among three populations of the white-crowned sparrow (Zonotrichia leucophrys). These populations breed at different elevations and latitudes and thus have breeding seasons that differ markedly in length. We hypothesize that in populations where birds raise only one or rarely two broods in a season, the fitness costs of abandoning a nest are substantially larger than in closely related populations that raise up to three broods per season. Thus individuals with short breeding seasons should be less responsive to stressors and therefore less likely to abandon their young. In our study, baseline and handling-induced corticosterone levels were similar among populations, but corticosteroid-binding globulins differed, leading to a direct relationship between stress-induced free corticosteroid levels and length of breeding season. There were also population-specific differences in intracellular low-affinity (glucocorticoid-like) receptors in both liver and brain tissue. Although investigations of population-based differences in glucocorticoid secretion are common, this is the first study to demonstrate population-level differences in binding globulins. These differences could lead to dramatically different physiological and behavioral responses to stress.