Foraging competition in larger groups overrides harassment avoidance benefits in female reindeer (Rangifer tarandus)

Abstract

Male harassment toward females during the breeding season may have a negative effect on their reproductive success by disturbing their foraging activity, thereby inducing somatic costs. Accordingly, it is predicted that females will choose mates based on their ability to provide protection or will aggregate into large groups to dilute per capita harassment level. Conversely, increasing group size may also lead to a decrease in foraging activity by increasing foraging competition, but this effect has rarely been considered in mating tactic studies. This study examined the importance of two non-exclusive hypotheses in explaining the variations of the female activity budget during the breeding season: the male harassment hypothesis, and the female foraging competition hypothesis. We used focal observations of female activity from known mating groups collected during the breeding season from a long-term (15 years) study on reindeer Rangifer tarandus. We found that females were more disturbed (i.e., spent less time feeding) in the presence of young dominant males, and marginally disturbed in the presence of satellite males, which supports the male harassment hypothesis. We also found that female disturbance level increased with group size, being independent of the adult sex ratio. Consequently, these results rejected the dilution effect, but strongly supported the foraging competition hypothesis. This study therefore highlights a potential conflict in female behaviour. Indeed, any gains from harassment protection were negated by an increase of 6-7 females, since adult males lead larger groups than young males.

Mismatch between calf paternity and observed copulations between male and female reindeer: Multiple mating in a polygynous ungulate?

“…If multiple mating is primarily driven by male harassment, we would expect increased instance of multiple mating at higher male percent composition. An increased sex ratio should increase the opportunity for harassment by increasing the number of potential harassing males per female, and possibly by providing an environment that favors male targeting of specific females ( Uccheddu et al 2015 ). Indeed, previous studies have indicated that greater percentage of males limits the ability of high-ranking males to effectively monopolize females by increasing intraspecific competition ( Reale et al 1996 ; Clutton-Brock et al 1997 ).…”

Evolution of ungulate mating systems: Integrating social and environmental factors

“…Risk of predation is hypothesized to affect group size of ungulates via benefits of increased group size in open‐land species that accrue because of more eyes, ears, and noses with which to detect predators at distances that make successful pursuits unlikely (Roberts, 1996). Costs related to competition, however, may be associated with large groups (Uccheddu, Body, Weladji, Holand, & Nieminen, 2015). For forest‐dwelling ungulates, where avoidance of detection is thought to be the primary antipredator strategy, benefits ostensibly ensue from being solitary or living in small groups (Hirth & McCullough, 1977).…”

Increased Exploration Capacity Promotes Group Fission in Gregarious Foraging Herbivores

“…By joining a large group, foragers should benefit not only from a decrease in predation risk (e.g., dilution effect [11], collective detection [12]), but also from an increase in social information about spatio-temporal patterns in resource availability [13–15]. These benefits, however, can be shadowed by faster resource depletion, higher exploitative and interference competition [16–19], and slower collective movements that are typical of larger groups [20]. Overall, foragers in larger groups should experience a faster decrease in food intake rate (i.e., faster patch depression, sensu [21]) and, therefore, have to rely more often upon an extensive search mode, which might be less effective because of the slower movement of large groups.…”