Male–male behavioral interactions drive social-dominance-mediated differences in ejaculate traits

Abstract

Higher social status is expected to result in fitness benefits as it secures access to potential mates. In promiscuous species, male reproductive success is also determined by an individual’s ability to compete for fertilization after mating by producing high-quality ejaculates. However, the complex relationship between a male’s investment in social status and ejaculates remains unclear. Here, we examine how male social status influences ejaculate quality under a range of social contexts in the pygmy halfbeak Dermogenys collettei, a small, group-living, internally fertilizing freshwater fish. We show that male social status influences ejaculate traits, both in the presence and absence of females. Dominant males produced faster swimming and more viable sperm, two key determinants of ejaculate quality, but only under conditions with frequent male–male behavioral interactions. When male–male interactions were experimentally reduced through the addition of a refuge, differences in ejaculate traits of dominant and subordinate males disappeared. Furthermore, dominant males were in a better condition, growing faster, and possessing larger livers, highlighting a possible condition dependence of competitive traits. Contrary to expectations, female presence or absence did not affect sperm swimming speed or testes mass. Together, these results suggest a positive relationship between social status and ejaculate quality in halfbeaks and highlight that the strength of behavioral interactions between males is a key driver of social-status-dependent differences in ejaculate traits.

Behavioral Variation in the Pygmy Halfbeak Dermogenys collettei: Comparing Shoals With Contrasting Ecologies

“…However, additional surveys and experiments are necessary to validate this hypothesis. Contrary to our hypothesis, as well as a recent laboratory experiment demonstrating that adding a visual obstruction to the environment reduces aggression between male halfbeaks (Reuland et al, 2020b), in this study aquatic vegetation was not associated with differences in agonistic interactions in natural halfbeak populations. It is plausible that the specialized surface-dwelling lifestyle of halfbeaks may reduce the importance of both water depth and aquatic vegetation in shaping sociosexual behaviors, particularly when compared with other fish species that swim throughout the water column.…”

“…Halfbeak males (and to a lesser degree females, see the Supplementary video) perform frequent agonistic interactions both within and between sexes (Berten and Greven, 1991;Greven, 2006;2010;Reuland et al, 2020b). We recorded agonistic interactions between individuals, which were clearly visible when observing fish from above (see Supplementary video).…”

“…We generated an index of the total number agonistic behaviors for each shoal by summing the shoal-mean values of the total number of chase/being chased, displace/being displaced, frontal threatening and biting behaviors observed, separately for males and females. Under laboratory conditions, halfbeak males form dominance hierarchies, where dominant individuals are more likely to perform agonistic behaviors and subordinate individuals are more likely to receive agonistic behaviors (Reuland et al, 2020b). Therefore, by including agonistic behaviors that were both performed and received by focal fish during the observations, our metric of the total number agonistic behaviors captures overall levels of shoal agonistic behaviors, regardless of a focal individual's position in a potential dominance hierarchy.…”

“…Male halfbeaks routinely perform a series of well-characterized courtship behaviors, whereas agonistic interactions are observed in both sexes (see the methods of this study and Greven, 2010;Baker and Lim, 2012;Ho et al, 2015). However, male halfbeaks are usually more aggressive than females (Greven, 2010) and form dominance hierarchies in laboratory conditions (Reuland et al, 2020b). These attributes make halfbeaks an ideal system for conducting non-invasive behavioral observations of natural populations across a range of environmental conditions.…”

The Effect of Brief or Prolonged Bouts of Winning or Losing Male-Male Contests on Plasticity in Sexually Selected Traits

“…Indeed, many studies report measurable differences between dominant and subordinate males in both pre-and postcopulatory sexual traits (e.g. Montrose et al 2008;Simmons and Buzatto 2014;Reuland et al 2021). There is also evidence that these differences reflect plastic responses to changes in social ranking.…”