EVOLUTION OF LIFE HISTORIES ALONG ELEVATIONAL GRADIENTS: TRADE-OFF BETWEEN PARENTAL CARE AND FECUNDITY

Abstract

Life history responses to environmental conditions include a combination of fecundity-survival schedules and behavioral strategies that yield the highest fitness in a given environment. In this study, we examined the pattern of covariation in avian life history strategies along an elevational gradient by comparing variation in life history traits, including most components of parental care, between phylogenetically paired taxa from low- and high-elevation sites. We found that high-elevation species had significantly lower annual fecundity but provided greater parental care to their offspring. However, a strong negative relationship between offspring number and duration of parental care along the elevational gradient suggested that high-elevation species were shifting investment from offspring number toward offspring quality. Although adult survival did not differ between high- and low-elevation species, higher juvenile survival may have compensated for lower annual fecundity in high-elevation species. The elevation at which breeding occurred strong- ly influenced the partitioning of parental behavior between sexes. Male participation in nestling provisioning was significantly greater in high-elevation species. In turn, altitudinal variation in the frequency of biparental care closely covaries with the intensity of sexual selection, ultimately resulting in the strong elevational pattern of sexual dimorphism. More- over, elevational variation in costs of development and maintenance of secondary sexual traits constitutes an additional effect on fecundity-survival schedules along elevational gradients. Thus, a trade-off between fecundity and parental care, and associated interactions among morphological, life history, and behavioral traits play important roles in the evolution of life history strategies in birds.