The social and ecological costs of an ‘over-extended' phenotype

Abstract

Extended phenotypes offer a unique opportunity to experimentally manipulate and identify sources of selection acting on traits under natural conditions. The social cichlid fish Neolamprologus multifasciatus builds nests by digging up aquatic snail shells, creating an extended sexual phenotype that is highly amenable to experimental manipulation through addition of extra shells. Here, we find sources of both positive sexual selection and opposing natural selection acting on this trait; augmenting shell nests increases access to mates, but also increases social aggression and predation risk. Increasing the attractiveness of one male also changed social interactions throughout the social network and altered the entire community structure. Manipulated males produced and received more displays from neighbouring females, who also joined augmented male territories at higher rates than unmanipulated groups. However, males in more attractive territories received more aggression from neighbouring males, potentially as a form of social policing. We also detected a significant ecological cost of the ‘over-extended' phenotype; heterospecific predators usurped augmented nests at higher rates, using them as breeding sites and displacing residents. Using these natural experiments, we find that both social and ecological interactions generate clear sources of selection mediating the expression of an extended phenotype in the wild.

Environmental change mediates mate choice for an extended phenotype, but not for mate quality

“…; Jordan et al. ). These studies generally show directional female preferences for exaggerated condition‐dependent structures that reflect male quality (Borgia ; Jordan et al.…”

“…These studies generally show directional female preferences for exaggerated condition‐dependent structures that reflect male quality (Borgia ; Jordan et al. ). Previous studies of nest building in sticklebacks have found that aspects of nest structure (including nest compactness) are condition‐dependent (Stanley ; Wootton , ; Barber et al.…”

Structural manipulations of a shelter resource reveal underlying preference functions in a shell-dwelling cichlid fish

“…Neolamprologus multifasciatus is one of the smallest cichlid species in Lake Tanganyika [17] and forms stable social groups consisting of up to ~20 individuals, with one to three adult males, up to five adult females, and the rest being juveniles and immature offspring [14,18]. N. multifasciatus are mostly found living on large shell beds, made almost exclusively of empty N. tanganyicense snail shells that they use as both shelters from predators and as brood chambers for females to lay their eggs and tend to their larvae.…”

“…The choice of which shells to occupy represents the first step in the process of niche construction at the individual level, producing shelters that individual N. multifasciatus must actively excavate and maintain, and also at the community level, creating structured shell beds on which numerous other species live -this is why the digging behaviour of shell-dwelling cichlids in general has also been referred to as 'ecosystem engineering' [13]. The environmental modifications associated with shell digging have clear fitness consequences for the excavating fish and may also have secondary effects when larger heterospecifics take the shells from smaller shell-dwelling N. multifasciatus [14], suggesting that larger species are too big to clear out shells of internal sediment themselves, and need to forcefully take shells from smaller 'producer' species.…”

Male and female preferences for nest characteristics under paternal care

“…. Moreover, the nest may be more reliable as an extended phenotype if maintenance costs are high (Jordan et al, 2016), but less so if there is a risk that other individuals take over the nest (Bisazza & Marconato, 1988). Given such variation in the relevant costs and benefits, female mate choice may optimally draw from multiple independent cues (Bro-Jørgensen, 2010;Candolin, 2003).…”