Meeting Abstract
Maternal effects are a powerful way to influence offspring phenotype and fitness, thus strong selection is expected. Although data from other vertebrates suggests a prominent role for maternal thyroid hormones (TH) on offspring development and survival, the causes and consequences of variation in avian maternally-derived THs (via eggs) have been largely neglected. We studied environmental variation in egg TH levels (thyroxine, T4 and triiodothyronine, T3) in wild great tits (Parus major) and its heritability in captive-bred siblings of wild origin. We experimentally elevated egg TH levels in a wild population to study the effects on fitness-related traits. Yolk T4 (but not T3) concentration was correlated negatively with temperature, positively with timing of breeding and increased with laying order within clutches. Interestingly, T3 (but not T4), was heritable (h2 = 0.25). Experimental egg TH elevation showed a sex-specific effect on offspring growth: it increased the growth of male nestlings, but decreased growth in female nestlings, relative to controls. Egg TH elevation had no effect on behavior, metabolic rate, hatching and fledging success. These studies demonstrate, for the first time in a wild bird species, the biological relevance of variation in maternal thyroid hormones in egg yolk on offspring development, and open a new, interesting avenue for further research in the field of hormone-mediated maternal effects.
