Adaptive sex ratios: Another example?

Abstract

We investigated sex bias at birth among 165 young elk (Cervas elaphus) born to freeranging, supplementally fed females in northwestern Wyoming during 1990-1992. Sex ratios of an additional 86 young born females confined and supplementally fed in pastures during 1978-1982 also were recorded. Sex ratios for cohorts of neonates were not sex-biased. Like-wise, sex ratios of neonates were not sex-biased in Grand Teton National Park, Wyoming, but they were male-biased on calving areas of lower densities of elk outside Grand Teton National Park. The proportion of males born in seven cohorts increased with earlier initiation of supplemental feeding and with digestibility of feed. Our findings appear to support the Trivers and Willard hypothesis of adaptive sex ratios.