Taking a risk: how far will male fiddler crabs go?

Courtship is costly for males when it increases their energy expenditure and predation risk. There are several ways in which males might be able to mitigate these costs, or compensate for them by elevating the benefits of courtship. First, they could selectively court more profitable females. Second, they could adjust the amount of risk they take against their residual reproductive value. Third, they could sometimes use cheaper signals to deceive females. In the fiddler crab Leptuca terpsichores (Crane, 1941), males risk losing their burrow to another crab and falling prey to a bird when they leave their burrow to intercept a mate-searching female and lead her back to the burrow for mating. Some males build sand hoods at their burrow entrances, which are landmarks that attract females and allow males to relocate their burrows quickly with little error. Here, we show that (1) males took greater risks when courting larger females by travelling farther away from their burrows; (2) the distance a male moved from his burrow did not depend on his size (hence, age); and (3) males with sand hoods did not travel farther away from their burrows than males without hoods, and they were not more likely to reach females. Taking greater risks when courting larger (more fecund) females appears to be a key means through which male fiddler crabs can achieve a more favourable balance between the costs and benefits of courtship. Significance statement: Courtship and mate choice can be costly for males. Males may improve the balance between courtship costs and benefits by modifying their risk-taking during courtship according to the perceived value of the female, or their expectations of future reproduction. Male fiddler crabs move away from their burrows to court females, which is risky because they may lose the burrow or be attacked by predators. We used the distance a male travels from his burrow as an index for the level of risk he is willing to take. We explored the effects of female size, male age, and the presence of a sand hood at the burrow entrance on distance travelled during courtship. Males took greater risks to court larger females, but did not adjust risk-taking according to their age (expected future reproduction) or whether their burrow had a sand hood

The Next Frontier in Understanding the Evolution of Coral Reef Fish Societies

Research on sociality in marine fishes is a vibrant field that is providing new insights into social evolution more generally. Here, we review the past two decades of research, identifying knowledge gaps and new directions. Two coral reef fishes, with social systems similar to other cooperative breeders, have emerged as models: the clown anemonefish Amphiprion percula and the emerald goby Paragobiodon xanthosoma. In these systems, non-breeders do not forgo their own reproduction to gain indirect genetic benefits. Rather, they do so because they stand to inherit the territory in the future and there are strong ecological and social constraints. The reasons why breeders tolerate non-breeders remain obscure, though it is plausibly a combination of weak kin selection, bet-hedging, and benefits mediated via mutualistic interactions with cnidarian hosts. The latter is particularly interesting, given the parallels with other social animals with mutualistic partners, such as acacia ants. Looking beyond the two model species, our attention is turning to species with more complex social organization, such as the damselfish Dascyllus aruanus. Here, variable group stability, conflict intensity, and reproductive skew provide opportunities to test theories of social evolution that have only been tested in a few taxa. New methods like social network analysis are enabling us to uncover more subtle effects of ecology on social interactions. More recently, comparative methods have yielded insights into the correlates of interspecific variation in sociality in the genera to which our model species belong. Phylogenetically controlled contrasts within the genus Gobiodon, have revealed the role of ecology, life history traits, and their interaction in sociality: smaller bodied species are more social than larger bodied species, which are only social on large corals. As climate change affects coral reefs, there is a pressing need to understand the many ways in which environmental disturbance influences these unique social systems. In sum, coral reef fishes have enabled us to test the robustness of current theories of social evolution in new taxa and environments, and they have generated new insights into social evolution that are applicable to a wider variety of taxa

Multi-level framework to assess social variation in response to ecological and social factors: modeled with coral gobies

Understanding variation in social organization that lacks a strong phylogenetic signal represents a key focus of research in behavioural ecology. Accordingly, we established a framework that identifies whether a range of ecological and social factors are affecting the social maintenance of taxa across multiple categories of social variation (ranging from large to fine-scale): 1) forms of sociality, 2) degree of sociality, 3) social plasticity and 4) hierarchy maintenance. Each category of variation can then be assessed in combination to provide an outlook for social maintenance in light of predictor factors. We modelled this framework by quantifying each category over time, space and disturbance regime using multiple species of coral-dwelling gobies, genus Gobiodon. Gobies are an interesting model system as they vary in social structure, have within-group cooperation, and form mutualisms with coral hosts, which are vulnerable to climatic disturbances. We found that gobies varied in forms of sociality – from being more solitary or pair-forming in high disturbance regimes, versus group-forming in moderate disturbance regimes at some locations. Only low or moderate degrees of sociality were observed in gobies, with location or disturbance regime affecting some species. The size of coral hosts influenced the social plasticity of gobies, which was affected by climatic disturbances. Gobies did not exhibit direct changes to hierarchy maintenance, as location and disturbance regime did not affect their size-based hierarchies. Lastly, by combining the four categories of variation, we found a high loss of sociality in coral-dwelling gobies due to environmental disturbances, which likely affects overall goby survival as group-forming can improve survival and fitness. By using our structured framework, we identified which categories of social variation were influenced by ecological factors like location and disturbance. This framework therefore provides an excellent tool for predicting future responses of animal societies to environmental stressors

Large-scale coral reef rehabilitation after blast fishing in Indonesia

The severely degraded condition of many coral reefs worldwide calls for active interventions to rehabilitate their physical and biological structure and function, in addition to effective management of fisheries and no-take reserves. Rehabilitation efforts to stabilize reef substratum sufficiently to support coral growth have been limited in size. We documented a large coral reef rehabilitation in Indonesia aiming to restore ecosystem functions by increasing live coral cover on a reef severely damaged by blast fishing and coral mining. The project deployed small, modular, open structures to stabilize rubble and to support transplanted coral fragments. Between 2013 to 2015, approximately 11,000 structures covering 7,000 m2were deployed over 2 ha of a reef at a cost of US$174,000. Live coral cover on the structures increased from less than 10% initially to greater than 60% depending on depth, deployment date and location, and disturbances. The mean live coral cover in the rehabilitation area in October 2017 was higher than reported for reefs in many other areas in the Coral Triangle, including marine protected areas, but lower than in the no-take reference reef. At least 42 coral species were observed growing on the structures. Surprisingly, during the massive coral bleaching in other regions during the 2014–2016 El Niño–Southern Oscillation event, bleaching in the rehabilitation area was less than 5% cover despite warm water (≥30°C). This project demonstrates that coral rehabilitation is achievable over large scales where coral reefs have been severely damaged and are under continuous anthropogenic disturbances in warming waters