Patrick A. Green

Signaling between a sender and a receiver mediates many behavioral interactions. In reciprocal signaling systems, both partners can send and receive signals, raising questions about the circumstances under which each party signals, and whether and how an interaction's outcome is impacted by the signaling behavior of each party. In marine cleaning mutualisms, cleaner shrimp or fish consume ectoparasites and dead skin from larger “client” fish partners. Cleaners signal their intent to clean to clients, and clients often signal their intention to be cleaned by changing color. Here, we used in situ observations of cleaner–client interactions to investigate signal-mediated cleaning interactions between the cleaner shrimp Ancyclomenes longicarpus and its client fish partners. We asked if client characteristics (size, status as a predator) influence signaling behaviors in both partners, and explored how signals and client identity (ie predatory status) impact the outcome of cleaning interactions. Our findings showed that larger client species were more likely to both signal and be signaled to. Additionally, interactions involving signals from either partner were more likely to result in cleaning than those that did not involve signals, and cleaning tended to last longer when only clients signaled or when clients were predatory species. Our results suggest that signals from both parties are key factors determining whether cleaning occurs, while client signaling and predatory status influence cleaning duration. Overall, we highlight the complex ways in which assessment of mutualistic partners via reciprocal signaling is both influenced by partner identity and affects the interaction's outcome.

In contests over resource access, competitors aim to win, gaining or retaining the resource while minimizing the costs of fighting. Considerable research has tested contest “assessment strategies,” which describe how animals gather information about fighting ability, resource ownership, resource value, and other factors and how this information informs fighting effort. Historically, assessment strategies were considered fixed within a population or species, yet in recent years, there has been a push to quantify how strategies vary according to resource ownership or inherently among individuals. We tested for variation in the assessment strategy of competing Gonodactylus childi, an Indo-Pacific species of mantis shrimp (Stomatopoda) that exchanges high-force strikes during contests over access to protective burrows: holes in dead coral or rock. By staging contests over access to a mock burrow, we found that larger individuals won more often, but “residents” already in the burrow were most likely to win. Our results most closely supported a self-assessment fighting strategy in residents only: Losers based their fighting effort on their own body size, and not that of eventual winners. Analyses of contest behavioral progressions showed little ability to discriminate assessment strategies. Finally, we revealed among-individual variation that was unexplained by factors such as resource ownership. We compare our results to a prior study of an ecologically similar and closely related mantis shrimp that assesses both own and opponent fighting ability. Overall, our results highlight multiple axes of variation that, while informing future theory, form the substrate for behavioral evolution in contests.

Contests over resources are widespread in nature. To optimize outcomes, animals assess fighting abilities, deciding to escalate conflicts based on their own strength (self-assessment) or comparing their own strength with that of their rival (mutual assessment). While most research focuses on one-on-one (dyadic) contests, the assessment strategies employed by groups remain poorly understood, even though animal groups from ants to humans engage in intergroup conflict. Mutual assessment is frequently assumed, as more information is thought to improve decision-making; however, this assumption has rarely been tested. Here we used a dataset spanning 21 years and 633 intergroup contests in a banded mongoose (Mungos mungo) population in Queen Elizabeth National Park, Uganda. Our results support a model of self-assessment: groups with many males tend to escalate conflicts regardless of the rival group's strength, thus contrasting the commonly held assumption that decisions during intergroup contests are made by mutual assessment. We suggest that assessing rival group strength during conflict could be disproportionately costly, compared with assessing own group strength, which can be done over longer time periods and is easier to obtain. Greater understanding of these dynamics can shed light on the drivers and escalation patterns of intergroup conflict across social species, including humans.

Synopsis: When animals compete over essential and limited resources, how they gather information about fighting ability is a crucial factor influencing their decision-making. Most research in animal contests asks how decisions are made when facing a single competitor; however, in many cases, individuals face multiple potential opponents and may incorporate information on this social environment. In addition, recent research suggests that animals perceive contest-relevant stimuli like body size in a proportional, not absolute, manner; this proportional processing has rarely, if ever, been incorporated into studies of contest assessment. Green swordtail fish (Xiphophorus hellerii) live in social aggregations, in which males may defend females from multiple potential opponents. Here, we asked how focal male green swordtails defended live females when presented with two simulated males that differed by known sizes. We found that focal males spent less time near the larger, more salient, of the two competitors as the mean size of both simulated competitors increased. That is, focal males mainly used information on the social environment to make competitive decisions, as opposed to information about own or relative fighting ability as commonly assumed in most contest theory. We also found that males who spent less time with the largest competitor shifted their attention to the defended female, devoting more time near this resource. Our findings suggest that, when there are multiple potential competitors, common models of decision-making in contests may be less applicable than previously assumed. Further, given the common use of proportional processing across animals, we suggest that future work on contests incorporates this type of perception.

In humans and other animal societies, groups engage in intergroup conflicts over resources. The success of groups in these conflicts depends on individual contributions to collective fighting, yet individuals may have personal fitness incentives to defect rather than fight, which could undermine group performance. Here we test the hypothesis that personal fitness incentives affect intergroup conflict success in wild banded mongooses ( Mungos mungo ). In this species, intergroup fights are sometimes initiated by estrous females, who gain outgroup matings while their male group-mates contribute most of the fighting effort. We found that group fighting success was highest when a group’s females were in estrus, suggesting that, although females may initiate fights, their male group-mates seem motivated to chase away rival groups to defend their paternity. Surprisingly, we found that groups that won fights conceded more paternity to their rivals than groups that lost. In other words, behavioral “wins” did not always result in fitness “wins”. Younger males were more successful at attaining paternity between groups compared to within their own groups, suggesting that they may forego intergroup fighting to focus on intergroup mating. Overall, our results suggest that personal fitness incentives—here, in the form of paternity—vary widely among group members and can undermine rather than promote collective fighting performance. Such conflicts of interest are likely inherent in group combat and can contribute to variation in the frequency and costliness of intergroup violence.

Animals deliver and withstand physical impacts in diverse behavioral contexts, from competing rams clashing their antlers together to archerfish impacting prey with jets of water. Though the ability of animals to withstand impact has generally been studied by focusing on morphology, behaviors may also influence impact resistance. Mantis shrimp exchange high-force strikes on each other's coiled, armored telsons (tailplates) during contests over territory. Prior work has shown that telson morphology has high impact resistance. I hypothesized that the behavior of coiling the telson also contributes to impact energy dissipation. By measuring impact dynamics from high-speed videos of strikes exchanged during contests between freely moving animals, I found that approximately 20% more impact energy was dissipated by the telson as compared with findings from a prior study that focused solely on morphology. This increase is likely due to behavior: because the telson is lifted off the substrate, the entire body flexes after contact, dissipating more energy than exoskeletal morphology does on its own. While variation in the degree of telson coil did not affect energy dissipation, proportionally more energy was dissipated from higher velocity strikes and from strikes from more massive appendages. Overall, these findings show that analysis of both behavior and morphology is crucial to understanding impact resistance, and suggest future research on the evolution of structure and function under the selective pressure of biological impacts.

Animals deliver and withstand physical impacts in diverse behavioral contexts, from competing rams clashing their antlers together to archerfish impacting prey with jets of water. Though the ability of animals to withstand impact has generally been studied by focusing on morphology, behaviors may also influence impact resistance. Mantis shrimp exchange high-force strikes on each other’s coiled, armored telsons (tailplates) during contests over territory. Prior work has shown that telson morphology has high impact resistance. I hypothesized that the behavior of coiling the telson also contributes to impact energy dissipation. By measuring impact dynamics from high-speed videos of strikes exchanged during contests between freely-moving animals, I found that over 20% more impact energy was dissipated as compared to a prior study that focused solely on morphology. This increase is likely due to behavior: because the telson is lifted off the substrate, the entire body flexes after contact, dissipating more energy than exoskeletal morphology does on its own. While variation in the degree of telson coil did not affect energy dissipation, higher velocity strikes resulted in greater energy dissipation, suggesting striking individuals may vary their behavior to affect impacts. Overall, these findings show that analysis of both behavior and morphology is crucial to understanding impact resistance, and suggest future research on the evolution of structure and function under the selective pressure of biological impacts. Summary statement Freely competing mantis shrimp dissipated over 90% of the energy of high-force strikes by raising their impact-resistant tailplates off the substrate; faster strikes led to greater energy dissipation.

Conflicts between social groups or “intergroup contests” are proposed to play a major role in the evolution of cooperation and social organization in humans and some nonhuman animal societies. In humans, success in warfare and other collective conflicts depends on both fighting group size and the presence and actions of key individuals, such as leaders or talismanic warriors. Understanding the determinants of intergroup contest success in other warlike animals may help to reveal the role of these contests in social evolution. Using 19 y of data on intergroup encounters in a particularly violent social mammal, the banded mongoose (Mungos mungo), we show that two factors, the number of adult males and the age of the oldest male (the “senior” male), have the strongest impacts on the probability of group victory. The advantage conferred by senior males appears to stem from their fighting experience. However, the galvanizing effect of senior males declines as they grow old until, at very advanced ages, senior males become a liability rather than an asset and can be evicted. As in human conflict, strength in numbers and the experience of key individuals combine to determine intergroup contest success in this animal society. We discuss how selection arising from intergroup contests may explain a suite of features of individual life history and social organization, including male eviction, sex-assortative alloparental care, and adult sex ratio.