Plasticity in physiological plasticity in an avian range expansion

Background/Question/Methods

Phenotypic plasticity can be important for the spread of species outside their native ranges.  However, diverse plastic responses, such as developmental plasticity (e.g., metamorphosis) and shorter-term plastic (flexible) responses (e.g., homeostasis) are often equivocated. Moreover, some plastic responses themselves can be modified by plasticity.  Subsequently, when one is interested in revealing how extensive phenotypic differentiation can arise in light of the short spatiotemporal scales and genetic challenges accompanying invasions, equivocation of types of plasticity could be misleading. For these reasons, we attempted to discriminate the roles of long-term and short-term physiological plasticity of the house sparrow (Passer domesticus) in Kenya.  Specifically, we asked whether plasticity in plastic stress hormone (corticosterone) responses could explain a previous observation in this system: that birds at the range-edge release more corticosterone in response to a stressor.   To test whether new populations exhibit different plasticity in plasticity than old populations, we compared stress hormone regulation among an old, intermediate, and young site over a period of one week in captivity.  House sparrows perceive captivity as a psychological stressor, inducing them to alter stress hormone regulation including subsequent stress responses.

Results/Conclusions

As we found before, baseline corticosterone from wild caught birds did not differ among populations, but younger populations (Nakuru and Nairobi) released more corticosterone in response to a restraint stressor than the oldest site (Mombasa). A week of captivity increased baseline corticosterone for most birds, and most continued to mount robust stress responses over captivity, a trait not shared with captive North American house sparrows introduced in the 1850’s.  In regards to plasticity of plasticity though, the oldest population (Mombasa) showed the greatest captivity-induced change in corticosterone regulation.  Captivity caused these sparrows to mimic the stress responses of birds at the range-edge whereas stress responses of younger populations remained similar to their wild-caught form.  Ongoing analysis is resolving whether individual heterogeneity in sensitivity to captivity underlies this outcome.  At present though, our data suggest substantial plasticity in plasticity in a key homeostatic system of an invading bird.