The effect of sex ratio and male age structure on reindeer calving

In polygynous ungulates, biologists commonly assume that the role of males in population dynamics is negligible since the male's physiological capacity to inseminate females normally will not be a limiting factor for calving rates. Recently, however, research indicates that the role of males may be more important than previously acknowledged because availability of males may affect conception dates and hence calving dates and synchrony. In many harvested or otherwise managed populations, a sex ratio highly skewed toward females and a young male age structure often exist. Both a skewed sex ratio and male age structure may affect conception times and may delay calving dates. We manipulated the sex ratio and male age structure in herds of reindeer (Rangifer tarandus) fenced in large-scale enclosures (14-15 km(2)). We compared calving rates, dates, and synchrony as well as (1) birth and autumn weights of calves among a skewed sex ratio and yearling males only, (2) a skewed sex ratio and an even male age structure, and (3) an even sex ratio and even male age structure. As predicted, calving dates were earlier in the treatment with an even sex ratio and even male age structure compared to the 2 other treatments. Neither sex ratio nor male age structure impacted calving rates or birth synchrony. Timing of births is important for the survival of newborns, and this should be considered when harvesting or otherwise managing populations of polygynous ungulates.