The Effect of Sex Ratio and Male Age Structure on Reindeer Calving

Abstract

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 (14-1 5 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.

Effects of age, density and sex ratio on reproductive effort in male reindeer (Rangifer tarandus)

“…However, our results are not necessarily in contrast to this. In our study, condition was similar before the rutting season, as density was similar in all years except during the rutting season when herds were manipulated (Holand et al, 2003). The reason for the density-dependent effect in red deer males was suggested to be because they entered the rut in poor condition at high population density (Yoccoz et al, 2002).…”

“…These are data from the same manipulations as we have reported on reindeer calving from years 1996-97 (Holand et al, 2003). Males were weighed (to nearest kg) both before (second half of September) and after (first half of November) the rutting season.…”

“…The study was conducted at the Kutharju Field Reindeer Research Station, in Kaamanen, Finland (69 • N, 27 • E) (Holand et al, 2003). The total area of 43 km 2 is fenced and sub-divided into several smaller enclosures.…”

“…The habitat composition is similar in the 2 enclosures. The experimental herd is supplementary fed during winter only (Holand et al, 2003).…”

“…Tests were undertaken on whether reproductive effort increases with age (Kojola, 1985;Yoccoz et al, 2002), and on whether effort in yearling males depends on density and levels of competition for access to females (Solberg et al, 1994). In addition to the insight into male life history, understanding male rutting behaviour may also have implications for population dynamics (Mysterud, Coulson & Stenseth, 2002), because it affects subsequent survival of males (Ditchkoff et al, 2001) and also timing of calving (Holand et al, 2003).…”