Trade-offs between fighting and breeding: a social network analysis of bison male interactions

Abstract

In most polygynous species, males compete for access to females using agonistic interactions to establish dominance hierarchies. Typically, larger and stronger males become more dominant and thus gain higher mating and reproductive success over subordinate males. However, there is an inherent trade-off between time and energy invested in dominance interactions versus courtship and mating activities. Individuals may overcome this trade-off by selectively engaging in more effective mating tactics. North American bison (Bison bison) are a species of conservation concern that exhibit female-defense polygyny with two predominant mating tactics: (1) tending individual females; or (2) challenging tending males as a satellite and then mating opportunistically. Here, we use social network analysis to examine the relationship between position in the agonistic interaction network of bison males and their mating, reproductive success, and reproductive tactics and effort. To assess the potential for social network analysis to generate new insights, we compare male (node) centrality in the interaction network with traditional David’s score and Elo-rating dominance rankings. Local and global node centrality and dominance rankings were positively associated with prime-aged, heavy males with the most mating success and offspring sired. These males invested more effort in the “tending” tactic versus the “satellite” tactic, and they tended more females for longer periods during peak rut, when most females were receptive. By engaging in the most effective mating tactic, dominant males may mitigate the trade-off between allocating time and energy to agonistic interactions that establish dominance, versus courtship and mating. While less dominant males participated more in the alternative mating tactic, network analysis demonstrated that they were still important to the interaction network on both a local and global scale.

Delayed benefits for fallow bucks: more fights decrease same day mating success, but may increase matings the next day

“…changes in how their calls sound). Some individuals may start switching to self-maintenance behaviors, reducing overall investment in intra-sexual competition towards the end of the rut (Pitcher et al, 2014; Vannoni & McElligott, 2009; Wyman et al, 2021). By contrast, lower-ranked males are involved in more than one fight less often during the rut, and some of them may increase their effort later in the mating season (Mason et al, 2012; Nieminen et al, 2016).…”

“…Mating tactics used by dominant males tend to be more effective, while subordinates are likely to use alternative mating tactics in order to secure a mating while avoiding contact with higher-ranked males (Foley et al, 2018;McElligott et al, 2001;McElligott & Hayden, 2000). As the mating season advances, body condition of males decreases and information on these changes in body condition is communicated to the rest of the population through, for example, changes in the structure of their vocalizations (McElligott et al, 2003;Mysterud et al, 2008;Vannoni & McElligott, 2009;Wyman et al, 2021). Furthermore, at the end of the mating season, dominant males are likely to decrease investment in mating-related activities and switch to self-maintenance behaviors (e.g.…”

“…In species with polygynous mating systems, dominance status depends on male competitive ability, and impacts on female mate choice and reproductive success (Fanjul & Zenuto, 2017; Farrell et al, 2011; Willisch et al, 2012; Wyman et al, 2021). High degrees of polygyny are characterized by strong reproductive skew, where a few males get most copulations during a mating season (Emlen & Oring, 1977).…”

Greater spear nosed bats commute long distances alone, rest together, but forage apart

“…All three measures were highly correlated with each other, and to simplify analysis we used the network strength of each individual in subsequent analyses. Network strength is the sum of all edge weights connected to the focal individual, and is a reasonable metric for time and effort spent in close proximity (Wyman et al, 2021). The proximity networks showed large amounts of variation within and among the groups in how much time was spent in close proximity, with individuals of some groups not showing any locations near another tagged individual.…”

“…We then calculated single values for strength centrality and degree for each individuals in asnipe (Farine, 2013), with edge weights as a simple ratio index of individuals within 30 m of each other. We used strength centrality of each individual in subsequent analyses as it reflects how much the focal bat was within 30 m of other individuals (Wyman et al, 2021), as well as degree (the number of different individuals that came within 30 m, Supplemental Table 1). We also wanted to identify if there were particular spatial locations that were used by bats across the social groups since groups were tracked sequentially and did not overlap in time.…”