Food supplementation experiments: A tool to reveal mechanisms that mediate timing of reproduction.

Food supplementation of free-living animals has been used to address the role of availability of resources in the timing of reproduction. A meta-analysis by Schoech and Hahn suggested that responsiveness of the reproductive axis to the supplementary cue of food is lessened at higher latitudes, presumably because the brief time during which conditions are appropriate to rear offspring has led to an evolved resistance to supplementary cues with a primary reliance on photoperiod. Unfortunately, few investigators have examined the potential underlying mechanisms that mediate this differential responsiveness to supplemental food across latitudes. Considerable research, however, links nutritional state and plasma glucocorticoid levels, both of which impinge upon the reproductive axis. Long-term research on Florida scrub-jays (Aphelocoma coerulescens) in my laboratory shows that suburban birds with access to ad libitum supplemental food express early breeding and lower plasma corticosterone (CORT) levels in comparison to jays in nearby natural habitat. Furthermore, supplementation in natural habitat advances laying, with the largest effects occurring in bad years (i.e., years defined by late breeding and poor reproductive output by non-supplemented controls). Similarly, reproductive output of supplemented jays is greater and exhibits considerably less variance than do controls, suggesting fitness benefits of supplementation that are tied to advanced breeding. Generally, CORT levels in early-breeding supplemented jays are lower than are those of controls. Also, regression analysis suggests that clutch-initiation dates of non-supplemented female breeders are predicted by baseline CORT levels. Although these data are not conclusive and trends can be obscured by year-effects, they suggest a role for CORT in timing of breeding. Whether this link might help to explain the above-referenced latitudinal trends remains to be characterized.