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Insulin-like growth factor 1 and the hormonal mediation of sibling rivalry in eastern bluebirds (Sialia sialis)

Ridenour, Matthew
In altricial animals, young are completely dependent on parents for provisioning. Because this provisioning is limited, the ability to outcompete siblings to receive food items and parental attention has clear fitness benefits. This ability may be mediated by hormones, particularly those that influence growth. In this study, I analyzed the effects of the hormone insulin-like growth factor 1 (IGF-1) on growth and sibling rivalry in eastern bluebirds (Sialia sialis). In order to determine whether this hormone is adaptively upregulated in response to high competition environments, I performed brood size manipulations and examined the effect of these manipulations on serum IGF-1 levels, morphometrics, and behavior. Additionally, I injected young bluebirds with exogenous IGF-1 to study its impacts on growth and sibling competition. I found that serum IGF-1 levels were not related to brood size, but fledging was later in larger broods. I also found a trend toward longer wings in larger broods. IGF-1 was positively related to mass late in the nestling period. Behaviorally, I found that larger nestlings were able to spend more time at the front of the nest box near the opening than their smaller siblings, however time spent at the front of the nest was not directly related to feeding rate, nor was there a direct relationship between IGF-1 and time spent at the front of the box. In the injection experiment, male nestlings had lower IGF-1 levels late in the nestling period than females. Consequently, IGF-1 was significantly increased by injection in males but not females. However, injection did not cause an increase in growth in either sex. The evidence collected here suggests that while IGF-1 may offer certain benefits to nestlings in terms of sibling competition, it is not upregulated to adapt to this competition.