Higher sociability leads to lower reproductive success in female kangaroos

Abstract

In social mammals, social integration is generally assumed to improve females' reproductive success. Most species demonstrating this relationship exhibit complex forms of social bonds and interactions. However, female eastern grey kangaroos ( Macropus giganteus ) exhibit differentiated social relationships, yet do not appear to cooperate directly. It is unclear what the fitness consequences of such sociability could be in species that do not exhibit obvious forms of cooperation. Using 4 years of life history, spatial and social data from a wild population of approximately 200 individually recognizable female eastern grey kangaroos, we tested whether higher levels of sociability are associated with greater reproductive success. Contrary to expectations, we found that the size of a female's social network, her numbers of preferential associations with other females and her group sizes all negatively influenced her reproductive success. These factors influenced the survival of dependent young that had left the pouch rather than those that were still in the pouch. We also show that primiparous females (first-time breeders) were less likely to have surviving young. Our findings suggest that social bonds are not always beneficial for reproductive success in group-living species, and that female kangaroos may experience trade-offs between successfully rearing young and maintaining affiliative relationships.

Volatile social environments can favour investments in quality over quantity of social relationships

“…Carter et al (2017) showed that cooperative vampire bats may invest in quantity of social relationships at the expense of relationship quality, yet it remains unknown what strategy performs best for this species under a volatile social environment. The same applies in the other empirical studies showing some differential investment in quantity vs. quality of social relationships (Sutcliffe & Crabbe 1963, Amato 1993, Entwisle et al 2007, Sato & Zenou 2015, Vacca 2019, Silk et al 1999, Ellis et al 2019, Morrison et al 2020, Menz et al 2020, Bond et al 2021.…”

“…Losses of individuals whose networks did not help them avoid death create a process of selective disappearance within each cohort. This makes it exceedingly hard to collect unbiased data: unless one traces social bonds longitudinally and records every death, any analysis among living individuals will pay disproportionate attention to the successful subset of individuals who are presently observable by virtue of being alive — a problem which applies to a wide range of taxa (beyond bats), whenever aiming to document effects of the quantity vs. quality of social relationships (Sutcliffe & Crabbe 1963, Amato 1993, Entwisle et al 2007, Sato & Zenou 2015, Vacca 2019, Silk et al 1999, Carter et al 2017, Ellis et al 2019, Morrison et al 2020, Menz et al 2020, Bond et al 2021). Whenever disappearance is selective (not random), the problem is exacerbated by the fact that situations where selection is at its strongest also produce the most severe data collection biases.…”

“…Carter et al (2017) showed that cooperative vampire bats may invest in quantity of social relationships at the expense of relationship quality; the authors named this strategy a 'social bet-hedging strategy', as the diversified portfolio of acquaintances may protect against the worst-case outcome of none of one's few friends remaining alive. Differential investment in quantity vs. quality of social relationships has also been observed in humans (Sutcliffe & Crabbe 1963, Amato 1993, Entwisle et al 2007, Sato & Zenou 2015, Vacca 2019, in nonhuman primates (Silk et al 1999, Ellis et al 2019, Morrison et al 2020, and in other mammals such as kangaroos (Menz et al 2020) and giraffes (Bond et al 2021). There is surprisingly little theory on the evolutionary forces shaping the cooperative strategies that ultimately determine the individuals' social environment, which has potentially important consequences on survival (Snyder-Mackler 2020), and which in turn may feed back into the stability of the social environment itself.…”

“…Cognitive limitations (Dunbar 1992, Dunbar 1993, Tamarit et al 2018), and time or other resource budgets (Ozaktas 1996), create constraints such that one cannot possibly be ‘everyone’s best friend’. Differential investment in quantity vs. quality of social relationships has been observed in humans (Sutcliffe & Crabbe 1963, Amato 1993, Entwisle et al 2007, Sato & Zenou 2015, Vacca 2019), in nonhuman primates (Silk et al 1999, Ellis et al 2019, Morrison et al 2020), and in other mammals such as kangaroos (Menz et al 2020) and giraffes (Bond et al 2021). Carter et al (2017) showed that cooperative vampire bats investing in quantity of social relationships (with unrelated individuals) at the expense of relationship quality cope better with a volatile social environment; the authors named this strategy a ‘social bet-hedging strategy’, as the diversified portfolio of acquaintances protects against the worst-case outcome of none of one’s few friends remaining alive.…”

The Perils of Being Populous: Control and Conservation of Abundant Kangaroo Species

“…There are of course additional causes of mortality such as starvation induced by drought, competition or both [95], and disease [96]. There may also be densitydependent suppression of breeding success [97,98]. There is also non-human predation (e.g., [99]).…”