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Even closely related and ecologically similar cichlid species of Lake Tanganyika exhibit an impressive diversity of social systems, and therefore these fishes offer an excellent opportunity to examine the evolution of social behaviour. Sophisticated social relationships are thought to have evolved via a building block design where more fundamental social behaviours and cognitive processes have been combined, incrementally modified, and elaborated over time. Here, we studied two of these putative social building blocks in two closely related species of cichlids: Neolamprologus pulcher, a group-living species, and Telmatochromis temporalis, a non-grouping species. Otherwise well matched in ecology, this pair of species provide an excellent comparison point to understand how behavioural processes may have been modified in relation to the evolution of sociality. Using social assays in both the laboratory and in the field, we explored each species' motivation to interact with conspecifics, and each species' conflict resolution tactics. We found that individuals of the group living species, N. pulcher, displayed higher social motivation and were more likely to produce submission displays than were individuals of the non-grouping species, T. temporalis. We argue that the motivation to interact with conspecifics is a necessary prerequisite for the emergence of group living, and that the use of submission reduces the costs of conflict and facilitates the maintenance of close social proximity. These results suggest that social motivation and conflict resolution tactics are associated with social complexity, and that these behavioural traits may be functionally significant in the evolution and maintenance of sociality., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Social aggression is one of the most conspicuous features of animal societies, yet little is known about the causes of individual variation in aggression within social hierarchies. Recent theory suggests that when individuals form queues for breeding, variation in social aggression by non-breeding group members is related to their probability of inheriting breeding status. However, levels of aggression could also vary as a temporary response to changes in the hierarchy, with individuals becoming more aggressive as they ascend in rank, in order to re-establish dominance relationships. Using the group-living fish, Neolamprologus pulcher, we show that subordinates became more aggressive after they ascended in rank. Female ascenders exhibited more rapid increases in aggression than males, and the increased aggression was primarily directed towards group members of adjacent rather than non-adjacent rank, suggesting that social aggression was related to conflict over rank. Elevated aggression by ascenders was not sustained over time, there was no relationship between rank and aggression in stable groups, and aggression given by ascenders was not sex-biased. Together, these results suggest that the need to re-establish dominance relationships following rank ascension is an important determinant of variation in aggression in animal societies.

Simple Summary: Aggressive invasive species can outcompete native species in contests over resources, which can lead to the exclusion of a native species by the invader. Invasive freshwater crayfish are often more aggressive than their native counterparts, however contests between invasive and native crayfish are typically investigated under laboratory conditions and are rarely examined in a natural setting. We used a baited underwater camera in a stream located in New South Wales, Australia to examine behavioural interactions between an invasive crayfish and a critically endangered native crayfish to determine which species was dominant. We found the native species dominant over the invader when larger, however when the species were size-matched the dominance of the native was lost and neither species exhibited a significant pattern of dominance. This outcome indicates the invasive crayfish represents a threat to the native since it may be able to outcompete the native over resources. Further, this outcome contrasts to previous laboratory findings, highlighting the importance of field observations in establishing the extent of impact an invader may be having on a native species. Competitive behavioural interactions between invasive and native freshwater crayfish are recognised as a key underlying mechanism behind the displacement of natives by invaders. However, in situ investigations into behavioural interactions between invasive and native crayfish are scarce. In Australian freshwater systems, the invasive Cherax destructor has spread into the ranges of many native Euastacus species, including the critically endangered Euastacus dharawalus. Staged contests between the two species in a laboratory setting found E. dharawalus to be the dominant competitor, however, this has yet to be corroborated in situ. Here, we used baited remote underwater video (BRUV) to examine in situ intra- and inter-specific behavioural interactions between E. dharawalus and C. destructor. We sought to evaluate patterns of dominance and differential contest dynamics between the species to provide indications of competition between the two species. We found E. dharawalus to be dominant over C. destructor based on pooled interspecific interaction data and size-grouped interactions where C. destructor was the smaller opponent. Alarmingly, however, when C. destructor was within a 10% size difference the dominance of E. dharawalus was lost, contrasting with the outcomes of the laboratory-staged study. In addition, we report that small C. destructor initiated significantly more contests than larger conspecifics and larger E. dharawalus, a pattern that was not observed in smaller E. dharawalus. Further, intraspecific interactions between C. destructor were significantly longer in duration than intraspecific interactions between E. dharawalus, indicating a willingness to continue fighting. Concerningly, these outcomes point towards inherent and greater aggressiveness in C. destructor relative to E. dharawalus and that only larger E. dharawalus hold a competitive advantage over C. destructor. Therefore, we conclude that C. destructor represents a substantial threat to E. dharawalus through competitive behavioural interactions. Further, due to the disparity between our findings and those produced from laboratory-staged contests, we recommend the use of in situ studies when determining the behavioural impacts of invasive crayfish on natives. [ABSTRACT FROM AUTHOR]

Population densities of invasive species fluctuate spatially and temporally, suggesting that the intensity of their aggressive interactions with native species is similarly variable. Although inter‐specific aggression is often thought to increase with population density, it is often theorized that it should be exceeded by intra‐specific aggression since conspecifics share a greater degree of resource overlap. Yet, the magnitude of intra‐specific aggression is seldom considered when examining aggressive interactions, particularly those between invasive and native species. Here, we manipulated the density of the invasive eastern mosquitofish, Gambusia holbrooki, and observed its aggressive interactions with juveniles of the native Australian bass, Macquaria novemaculeata in a laboratory setting. For both species, the magnitudes of intra‐ and inter‐specific aggression were recorded. Regardless of density, the native M. novemaculeata was more aggressive towards heterospecifics than G. holbrooki was. In addition to this, M. novemaculeata was more aggressive to G. holbrooki than towards conspecifics, at both low‐ and high‐density conditions. In contrast, G. holbrooki was similarly aggressive towards M. novemaculeata and G. holbrooki at a high density, yet at low density, displayed significantly more aggression towards conspecifics than M. novemaculeata. These findings demonstrate the importance of considering intra‐specific aggression when exploring behavioural interactions between native and invasive species. [ABSTRACT FROM AUTHOR]

Anthropogenic climate change is expected to result in dramatic shifts in the abiotic conditions within estuaries, including an elevation of temperature and salinity levels. Even so, few studies have addressed the impacts of multiple abiotic stressors on the behaviour and life history of key estuarine species, such as those of biological and commercial importance. Here we used a cross-factored experimental design to tease apart the effects of temperature and salinity on intraspecific aggression and growth rates of a native estuarine fish species, the Australian bass ( Macquaria novemaculeata). Juvenile bass were exposed to one of four treatment conditions: 1) baseline temperature and salinity, 2) elevated temperature, 3) elevated salinity, and 4) elevated temperature and salinity. Elevated salinity increased rates of aggression, and elevated temperature decreased rates of growth, although the effects of both factors were mediated by the body size of individual bass. These results therefore highlight complex and variable effects of abiotic stressors and body size, emphasising the importance of considering individual-level attributes when evaluating the impacts of climate change on estuarine fishes. [ABSTRACT FROM AUTHOR]

In social groups, hierarchies are the fundamental organizational unit and integral to the structure of social groups. For many social fishes, rank is determined by body size and conflict over rank is resolved via aggressive threats from dominants and growth restraint by subordinates. However, this balance may be offset by an alteration of abiotic factors, such as elevated temperature expected from climate change, which could thereby disrupt the usual mechanisms of conflict resolution. Here, we determined the effect of elevated temperature on hierarchy structure, stability, and conflict resolution in the Eastern mosquitofish, Gambusia holbrooki. Body size was significantly related to dominance rank, and aggression was more commonly directed toward subordinates and was heightened between individuals of adjacent rank, demonstrating that conflict over rank occurs in size-based hierarchies. Temperature did not affect overall levels or directionality/adjacency of aggression but substantially altered subordinate growth patterns. In only the high-temperature groups, growth rates of subordinates decreased as the size ratio between themselves and their immediate dominant approached 1.0, whereas growth rates of dominants were unaffected. This unique finding suggests that only under high temperatures, subordinates may adopt growth regulation to resolve conflict, when the costs of conflict with dominants are greater. This provides the first causal link between abiotic stressors and changes to hierarchical structure and functioning, providing a springboard for further research into implications of temperature-dependent subordinate growth alteration at higher levels of ecological organization. [ABSTRACT FROM AUTHOR]

A rich theoretical framework exists for understanding animal conflict. When two opponents fight over a resource, the duration, intensity and outcome of the fight ought to be determined in large part by the relative difference in resource-holding power between contestants. While our understanding of one-time conflict resolution is excellent, our knowledge is still limited of how these rules scale up when contests occur in a social context where individuals have long-term interactions. Here, we use a convenient model system, Neolamprologus pulcher, a small cooperatively breeding cichlid fish, to explore decisions in pairwise contests over resources in a species where two individual contestants are likely to remain in the same social group, and regularly and repeatedly interact. Contests began after approximately 1min, with a short display phase, and continued in an aphasic manner for an average of 10min before a clear winner emerged. Information about opponents’ body size was important when deciding on the giving-up point, but contestants’ own body size was not, suggesting that assessment of opponent size is paramount in contest decision making. No sex differences were detected in contest structure, duration or intensity, and contests between males or between females were indistinguishable. These results offer an important window on conflict in a cooperative breeder and shed insight on rules of engagement within hierarchical social groups. [Copyright &y& Elsevier]