Reduction in Predator Defense in the Presence of Neighbors in a Colonial Fish

Abstract

Predation pressure has long been considered a leading explanation of colonies, where close neighbors may reduce predation via dilution, alarming or group predator attacks. Attacking predators may be costly in terms of energy and survival, leading to the question of how neighbors contribute to predator deterrence in relationship to each other. Two hypotheses explaining the relative efforts made by neighbors are byproduct-mutualism, which occurs when breeders inadvertently attack predators by defending their nests, and reciprocity, which occurs when breeders deliberately exchange predator defense efforts with neighbors. Most studies investigating group nest defense have been performed with birds. However, colonial fish may constitute a more practical model system for an experimental approach because of the greater ability of researchers to manipulate their environment. We investigated in the colonial fish, Neolamprologus caudopunctatus, whether prospecting pairs preferred to breed near conspecifics or solitarily, and how breeders invested in anti-predator defense in relation to neighbors. In a simple choice test, prospecting pairs selected breeding sites close to neighbors versus a solitary site. Predators were then sequentially presented to the newly established test pairs, the previously established stimulus pairs or in between the two pairs. Test pairs attacked the predator eight times more frequently when they were presented on their non-neighbor side compared to between the two breeding sites, where stimulus pairs maintained high attack rates. Thus, by joining an established pair, test pairs were able to reduce their anti-predator efforts near neighbors, at no apparent cost to the stimulus pairs. These findings are unlikely to be explained by reciprocity or byproduct-mutualism. Our results instead suggest a commensal relationship in which new pairs exploit the high anti-predator efforts of established pairs, which invest similarly with or without neighbors. Further studies are needed to determine the scope of commensalism as an anti-predator strategy in colonial animals.

Benefits of coloniality: communal defence saves anti‐predator effort in cooperative breeders

“…Our results differ somewhat from those found in a laboratory study of the colonial, biparental congener Neolamprologus caudapunctatus (Schädelin, Fischer & Wagner ), where newly introduced pairs reduced their anti‐predator behaviour and free rode on the effort of already established neighbours. This also led to stable levels of anti‐predator effort at the focal pair's territory, but the investment of the two parties was asymmetric, with one party clearly exploiting the effort of the other.…”

“…The key findings of these two studies are not easily comparable with our work. The experimental asymmetry between pairs in the laboratory study of Schädelin, Fischer & Wagner () differs from our groups in the field, where both focal and neighbouring groups had defended their territories for at least 2 months prior to the experiments (and probably much longer). Similarly, the conditions in Hellman & Hamilton's laboratory study () differed significantly from our field experiment: their groups had no physical contact with each other and lived in an otherwise predator‐free environment.…”

“…Alternatively, higher densities of prey can cause individuals to reduce their defence effort, as the combined effort of other individuals may compensate for this reduction; in this case, similar per capita predation risk may ensue across a wide range of prey densities (Daly et al . ; Schädelin, Fischer & Wagner ). In case such compensatory effects do not apply, higher prey densities might even result in increased predation risk, for instance because of an unproportionally large reduction of total anti‐predator behaviour of individuals or due to an increased attractiveness of higher prey densities to predators.…”

Opposing selective pressures may act on the colony size in a waterbird species

“…However, taking all these into account, large colonies are likely to provide better safety from predation in comparison to smaller nesting aggregations, which has been supported by empirical evidence in birds (Raveling 1989;Wiklund and Andersson 1994). There is also experimental evidence from other taxa, such as fish, that nesting in aggregations reduces the costs of anti-predator efforts, thus providing further support for anti-predatory functions of animal colonies (Schädelin et al 2012).…”

Phage selection for bacterial cheats leads to population decline

“…[16,19,20], but see [21]), or constitute a cost that potentially impacts other cooperative behaviours (e.g. resource access and sharing, quorum sensing [19,22,23]).…”