Researchers at UC Davis have recently published a novel analysis of how the behavior of individuals within a group affects group-level mating behavior in water striders (Aquarius remegis). Water striders are aquatic insects that skate on the surface of ponds, lakes and streams. Water striders have long been used to study sexual conflict—as the typical mating system they display involves males actively, and aggressively, chasing females on the open water to try and mate. This often leaves females with a choice between hiding from male harassment on the water’s edge or actively skating on the main water surface despite male harassment, which they must do occasionally so they can search for food .
Recent work in the Sih lab, however, has identified that small groups of water striders can exhibit group-level mating systems that fundamentally differ from what has usually been observed . Mating systems are essentially characterized by how many individuals get to mate, ranging from situations where only one dominant individual mates, to the formation of monogamous pairs within a group, to systems where everyone just mates with everyone. These researchers found that, as opposed to the typical system where groups of males all actively search for females on the open water and mate whenever they can, small groups of water striders can sometimes transition to a system where one male dominates the main water surface and, thus, mating opportunities.
In this most recent paper, the authors analyzed the timing of these “tipping point”  transitions from the typical mating system to one in which a single male emerged dominant. The basic design of the experiment was to put water striders in small groups for 6 days and record data on each group periodically (who was active on the water, who was hiding in refuge on the water’s edge, which pairs mated with each other, etc.). Groups were then combined into large pools which are too big for any single male to monopolize. Individuals were then randomly split into new small groups and observed for another 6 days. Thus, the researchers were able to collect data on many randomly assembled small groups, a majority of which experienced this transition to being dominated by a single male at some point during the 6-day trial. Importantly, each individual was marked for individual identification and measured for size characteristics before the experiment began. Individual behavioral traits were calculated for each individual during their time in the large pool (where potentially aggressive dominant males were unable to substantially exert their influence on others).
Using a statistical method known as “survival analysis”, the authors analyzed what factors contributed to how quickly small groups transitioned to a male dominated system, if they transitioned at all. They showed that the main predictor for this transition is average male social plasticity within each group, which is essentially a measure of how much males change their activity in response to the increased activity of other males in the group. More specifically, the results indicate that groups where males are more likely to increase their activity in response to increased activity by male competitors take much longer to transition to a system dominated by a single male. The results also showed that female size characteristics are important for the timing of this transition. The authors suggest this is due to the effects of larger female size generating greater incentive for males to monopolize mating opportunities, since larger females can produce more offspring.
Overall, the study identifies the importance of individual behavioral traits within a group in determining how different groups of animals behave under identical environmental conditions (i.e. each group had the same number of individuals of each sex, were housed in the same sized pool, and given the same amount of resources). A few previous studies have identified male responsiveness to female behavior as an important determinant of mating success. However, this is one of the first studies to show the importance of male responsiveness to competitor activity in shaping collective mating outcomes in animal groups.
For more information:
*Perez A, Montiglio PO, Wey TW, & *Sih A. (2019). Male social plasticity influences transient dynamics in the emergence of alternative mating systems in water striders. Behavioral Ecology
****Link to main article: https://academic.oup.com/beheco/advance-article/doi/10.1093/beheco/arz108/5526709
*Denotes UC Davis ABGG-affiliated authors
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