To harass or to respect: the economy of male persistence despite female refusal in a damselfly with scramble mate competition

In sexual conflict, males are often thought to gain fitness benefits from harassing females over mating. Yet when harassment itself incurs costs to males and if alternative, receptive females are available in a local population, theory predicts that when confronted with a female refusal, a male’s choice of persisting or retreating is determined in part by the likelihood of achieving a mating. We tested that prediction in the damselfly Enallagma hageni, whose males compete by intense scramble competition, resulting in widespread mating harassment toward females, which have a high level of control over mating. Using captive individuals of E. hageni in outdoor insectaries, we quantified male persistence in mating after refusals by pre- and post-oviposition focal females whose egg content we quantified after observations. We documented a novel, context-dependent head-turning refusal signal of sexual non-receptivity, most often displayed in tandem pairs by post-oviposition females that typically carried few mature eggs for males to fertilize. Male persistence was less likely to result in mating with post-oviposition females compared with pre-oviposition females carrying a clutch of mature eggs. Accordingly, males were less likely to persist following refusal signals given by post-oviposition females, supporting the theoretical prediction. Compared with a refusal signal known as wing spread, head-turning was significantly more effective in deterring harassing males. Our results suggest that despite on-going sexual conflict over mating, cooperation benefits both sexes when females use the honest signal of non-receptivity because they carry few mature eggs that males could fertilize.

Larger is not better: No mate preference by European Common Frog (Rana temporaria) males

According to classical sexual selection theory, females are the choosy sex in most species. Choosiness is defined as the individual effort to invest energy and time to assess potential mates. In explosive breeding anurans, high intrasexual competition between males leads to a sexual coercion ruled mating system, where males could have evolved preferences for specific female traits. In the current study, we tested male mating preference in the explosive breeding European Common Frog without intrasexual competition. We hypothesized that males show preferences towards larger female body size in the absence of male competition. We conducted mate choice experiments, placing a male and two differently sized females in a box and recorded their mating behavior. Males did not show any preference considering female body size, neither in the attempt to grab a female nor during the formation of pairs. We witnessed a high failure rate of male mating attempts, which might make the evolution of mate choice too costly. However, small males are faster in attempting females, which could be an alternative strategy to get access to females, because their larger competitors have an advantage during scramble competition. Nonetheless, in successfully formed pairs, the females were on average larger than the males, an observation which deviated from our null-model where pairs should be of similar size if mating would be random. This indicates that selection takes place, independent from male mating preference or scramble competition.

Dissecting the two mechanisms of scramble competition among the Virunga mountain gorillas

Two mechanisms have been proposed to explain why scramble competition can increase the travel requirements of individuals within larger groups. Firstly, individuals in larger groups may be more likely to encounter food sites where other group members have already eaten, leading to greater asynchronous “individual” travel to find fresh sites. Secondly, when food sites are aggregated into patches, larger groups may need to visit more patches to obtain the same amount of food per capita, leading to greater synchronous “group” travel between patches. If the first mechanism can be mitigated by increasing group spread, then we expect the second mechanism to be more sensitive to group size. Here, we examine the individual travel and group travel of the Virunga mountain gorillas, along with potential implications for the two mechanisms of scramble competition. Asynchronous individual travel accounted for 67% of the total travel time, and the remainder arose from group travel. Group spread increased significantly for larger groups, but not enough to prevent an increase in individual travel. Contrary to expectations, group travel decreased with size among most groups, and we found only limited evidence of patch depletion that would cause the second mechanism of scramble competition. Collectively, our results illustrate how the influence of group size can differ for individual travel versus group travel, just as it differs among species for overall travel. Studies that distinguish between the two mechanisms of scramble competition may enhance our understanding of ecological constraints upon group size, including potential differences between frugivores and folivores. Significance statement Feeding competition provides insight into how group size can influence the foraging patterns of social animals, but two key mechanisms are not typically compared. Firstly, larger groups may visit more patches to access the same amount of food per capita (group travel). Secondly, their individuals may also need to move past more spots where another member has already eaten (individual travel). Contrary to expectations, we found that group travel decreased with size for most groups of mountain gorillas, which may reflect extra travel by smaller groups to avoid larger groups. Individual travel increased with size in most groups, even though gorillas in larger groups compensated by spreading out over a broader area. The two mechanisms revealed patterns that were not apparent in our previous study of overall travel. Our approach may help to explain potential differences between folivores and frugivores.

Investigating the Effect of Flock Size on Vigilance in the American Coot (Fulica americana) in Relationship to Habitat

Among many anti-predator behaviors, vigilance is observed in many species and plays an important role in survival. In this study, we investigated the effect of flock size on vigilance in American Coots (Fulica americana)foraging on land and water, by observing individual birds in these habitats and recording the time spent scanning (i.e., vigilance). Mean flock size was larger on land compared to water and vigilance negatively correlated with flock size. Birds in water were more vigilant compared to on land, regardless of whether they were foraging alone or in flocks. However, the effect of flock size on vigilance showed a weak linear correlation as it was possible that other factors (e.g., human habituation, food kleptoparasitism, or scramble competition) could have also played a role in shaping vigilance. These results suggest that there is a relationship between flock size and vigilance, which are related to previous researches that show a negative correlation between vigilance and flock size. KEYWORDS: Birds; American Coot; Vigilance; Scanning; Foraging; Flock Size; Habituation; Competition; Behavior

Feeding competition inferred from patch depletion in a supergroup of Rwenzori black-and-white colobus monkeys (Colobus angolensis ruwenzorii) in Rwanda

Competition for food is often a cost associated with living in a group, and can occur in an indirect (scramble) or direct (contest) form. We investigated feeding competition in a supergroup of Rwenzori black-and-white colobus monkeys (Colobus angolensis ruwenzorii) in Rwanda, with the aim of establishing whether freedom from scramble competition allows these monkeys to form supergroups. We used the patch depletion method, measuring intake rate coupled with movement rate, to assess if food patches become depleted over the occupancy period. Resource depletion was evident when the colobus fed on young leaves, but not when feeding on mature leaves. Scramble competition was inferred from a negative correlation between group size and change in intake rate over patch occupancy. Between-group contest competition was inferred from displacement from patches. Although feeding competition exists for select resources, limited competition for mature leaves may enable Rwenzori colobus to live in a supergroup of hundreds of individuals in this montane forest.

Male mating success is related to body condition and stress-induced leukocyte response in an anuran with scramble competition

Competition among males can be a decisive factor for successful mating in anuran explosive breeders with a male-biased population sex ratio. The Yungas Red-belly Toad (Melanophryniscus rubriventris (Vellard, 1947)) is an explosive breeder where males exhibit a classic scramble competition. We studied the body size, body condition, and stress-induced leukocyte response of M. rubriventris males under natural scenarios of scramble competition. We hand-captured adult males (categorised as single, in pairs, or in mating balls) and determined their body mass and length, body condition, and their leukocyte profile. We found that males in pairs had significantly higher body mass than single males and those in mating balls. We found the highest percentage of individuals with better body condition within the paired males who showed significantly higher body condition index value than singles or those in mating balls. Amplectant males (those collected in pairs or in mating balls) had a significantly lower proportion of lymphocytes and higher neutrophil–lymphocyte ratios than singles. Amplectant males showed significantly higher proportions of neutrophils or eosinophils than singles. Our results suggest that M. rubriventris males with better body condition can be more successful in acquiring mates, although scramble competition among males can trigger a stress response.

Differential distribution of resources for females on a dioecious plant affects the small–scale distribution of male of an oligolectic bee

Females of the solitary digger bee Andrena florea Fabricius, 1793 (Hymenoptera, Andrenidae) nest in aggregations and collect pollen almost exclusively on dioecious plants of the genus Bryonia, making this species a good model to study the relationship between nest density, male density, male behaviour and female lecty. At a study site in the valley of the river Serio, in Italy, an aggregation of this bee showed low density of randomly distributed nests and was closely surrounded by B. dioica plants. Female nectar foraging and male feeding and mate–searching activity, confined to the host plants, peaked at similar hours across the day, while female pollen foraging peaked earlier. Males fed on plants of both sexes but seemed to perch waiting for females more frequently on male B. dioica leaves. Individual males more often visited only one of the male plants, for up to four days; here they did not interact aggressively with conspecifics, suggesting scramble competition in resource–based home ranges and not territoriality. These findings are preliminarily in accordance with the predicted resource–based rendez–vous sites at low nest density for oligolectic bees and the predicted occurrence of scramble competition in case of high male density. Additionally, males would maximize their mating opportunity by mainly perching on male plants, the only source of the most limited resource for females (pollen).