ARCUS | Arctic Research Consortium of the United States

6th Annual ARCUS Award for Arctic Research Excellence


Submitted by   Robert Bertrand Weladji
Authors   Robert B. Weladji and Ø. Holand
Category   Life Science
Title   Sex ratio variation in reindeer: a test of the extrinsis modification hypothesis
Affiliation   Animal Science, Agricultural University of Norway, ÅS, Norway

Abstract

Offspring sex ratio has been reported to differ from parity in many mammal populations. Evolutionary theories based on adaptive modification by maternal investment, are generally forwarded to explain the variation in offspring sex ratios, but are repeatedly inconsistent. Recently, factors beyond the control of the mother, such as density and climate, were reported to affect offspring sex ratios, termed the extrinsic modification hypothesis (EMH). We investigated sex ratio variation in 5509 reindeer (Rangifer tarandus) calves during an eight-year period. Using separated models, we tested how offspring sex ratio covaried with density, and respectively local winter weather, local summer weather, and global weather (measured by the North Atlantic Oscillation (NAO) during winter). Calf birth sex ratios differ from parity only in one year. Consistently, the proportion of males in the herd declined with density. We also show that offspring sex ratio declined with temperature and precipitation the summer preceding conception, as well as the state of the NAO when calves were in utero and one year before (NAO-1). Local winter precipitation when calves were in utero also had a negative effect on the proportion of male calves. As significant factors are consistently related to environmental condition, we suggest that their effects are associated with nutritional stress, affecting gender determination at conception and/or differential fetal mortality. The effect of both summer weather and NAO-1 suggest a potential effect of weather pre-conception, which has not been shown previously. Evidence of extrinsic sex ratio adjustment was conclusive and our findings therefore support the “extrinsic modification hypothesis” of sex ratio variation.