Foetal sex ratios in wild reindeer Rangifer tarandus in relation to maternal condition and age

Abstract

Sex and total weight were recorded in 215 wild reindeer Rangifer tarandus tarandus foetuses from six ranges evaluated as good, medium, and poor based on maternal weights. The proportions of males per 100 females in the good, medium and poor ranges were 76, 112 and 100, respectively, and 94 overall. None of these ratios were statistically different from a 50:50 sex distribution. Examination of individual year and weight classes within areas revealed no difference in foetus sex ratio from a 50:50 ratio. Male and female foetus mean weight differences varied between 28 and 216 g within areas when measured in February and March. None of these differences were significant. Male foetus weights were significantly higher than female weights in April in a pooled sample from the good ranges. Although the sample size is too small for a conclusive statement, the data do not support the Trivers and Willard hypothesis that females should produce more male offspring when resources are abundant.