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

Abstract

In sexually dimorphic ungulates, male reproductive success depends on fighting with other males for access to females during a brief rutting season. Large body size is necessary for success in intrasexual competition, and a few large-sized males are often able to monopolize access to female groups. Earlier studies have reported that reproductive effort increases with age until prime-age is reached, and one study that population density lowered effort in (older) males. No study has directly assessed whether there is within-age-class variation in effort resulting from varying levels of intra-male competition. It is reported here the weight loss during the rutting season of 54 individual male reindeer Rangifer tarandus coming from eight herds with varying density (3.3-6.0 deer/km 2 ) and sex ratio (4-28% males). In agreement with earlier studies, reproductive effort was lower for young (1-to 2-year-old) than for prime-aged (3-to 5-year-old) males both on an absolute and relative scale. Among 1-year-old males (n = 33), effort was lower as sex ratio became closer to even, but density during the rutting season had no effect. This suggests that yearling males take a more active role when prime-aged males are absent. In addition to the insight into male ungulate life history, understanding male rutting behaviour may also have implications for population dynamics.

Patterns of reproductive effort in male ungulates

“…Variation in reproductive effort relative to population sex ratio/male age structure was available from four studies (Table 1). Consistent with the hypothesis, yearling bison bulls (Komers, Meisser & Gates, 1994b) and yearling male reindeer (Mysterud et al, 2003) had a higher reproductive effort as population sex ratio becomes more female-biased and when prime-aged males were absent. In moose, there was no relationship between effort and sex ratio for either prime-aged or yearling males (Mysterud et al, 2004).…”

“…In moose, effort also decreased with increasing density, but in yearlings only (Mysterud et al, 2004). There was no effect of density on effort of yearling reindeer (Mysterud et al, 2003), but in this case, only rutting densities differed giving males a similar condition at high and low density.…”

Variation in Growth Pattern of Male MooseAlces alcesAfter Two Contrasted Periods of Hunting

“…In hunted ungulate populations, sub-adult males are more likely to reproduce at a younger age because of the reduced number of competitive older males during the rutting season , Laurian et al 2000. Reproduction involves costs, especially in young males, because until the somatic growth is completed, there is a negative trade-off between survival and reproduction (Mysterud et al 2003, Garel et al 2006. Therefore, in polygynous ungulates, males often use an income breeding strategy when young and shift to capital breeding tactic at prime age (Mysterud et al 2005, Mysterud et al 2008.…”

“…1) might lead to a population where sexual maturation is attained early in life because animals which reproduce before others will be positively selected. Second, as compared with the adultbiased harvest before the 1970s, the femalebiased sex ratio and the younger age structure of males of the second period would encourage increased growth at a younger age due to reduced competition from older males (but see Mysterud et al 2003, Garel et al 2006. Further, assuming that a rapid growth rate involves costs, we hypothesised a decrease in age and size when prime age is reached.…”

Timing of Reproductive Effort Differs between Young and Old Dominant Male Reindeer

“…As a result of an increased activity and reduced feeding, condition of males typically declines as the mating season progresses (Forsyth et al 2005, Vannoni & McElligott 2009). Substantial body mass loss over the mating season is also common among these species (Bobek et al 1990, Mysterud et al 2003. Decreased condition at the end of the mating season could negatively impact future ability to reproduce and survival (Neuhaus & Pelletier 2001, Barboza et al 2004.…”