Mating group composition influences somatic costs and activity in rutting dominant male reindeer (Rangifer tarandus)
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In polygynous species, males devote considerable effort to reproduction during the rut. Both the number of females in the mating group and the ratio of sexually mature males to sexually mature females [adult sex ratio (ASR)] are expected to affect the amount of effort a male devotes to reproductive activities. We predicted the reproductive effort of dominant male reindeer, measured as relative mass loss, proportions of active reproductive behaviors, and frequencies of agonistic behaviors would (1) increase with an increasing number of females in the mating group and eventually level off, and (2) exhibit a dome shape with respect to ASR in the mating group. We tested these predictions using 12 years of data collected from semi-domesticated reindeer in northern Finland. We found a positive relationship between relative mass loss and the mean number of females in the mating group for mature, but not young males. The relationship between the proportion of active reproductive behaviors performed by mature males and the mean number of females in the group was quadratic while agonistic behaviors of mature males increased with the increasing female group size. We also found that active reproductive behaviors decreased with a rising mating group ASR for mature males; whereas, young males performed more agonistic behaviors as group ASR increased. Our results point to age-specific patterns of mass loss and activity during the mating season. They also indicate that both the number of females and ASR in the mating group are important factors in determining the level of reproductive effort of dominant male reindeer.
KeywordsAdult sex ratio Agonistic behaviors Male reproductive effort Mass loss Operational sex ratio Rangifer tarandus Rut
We gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (scholarship to E.M.T and research grant to R.B.W) and the Northern Scientific Training Program. Thanks also to J. Grant, M. Festa-Bianchet, D. Forsyth, and an anonymous referee for comments that improved this manuscript. We thank all students, assistants, and volunteers who have contributed to our research over the past 14 years. We are grateful to the Kutuharju Field Reindeer Station (Kaamanen, Finland), to Mikka Tervonen, and Heilki Tormanen for the assistance and logistic support.
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