Kin discrimination in polyphenic salamander larvae: trade-offs between inclusive fitness and pathogen transmission

Abstract

Some larval amphibians can develop into two distinct morphological types: a small-headed “normal” morph and a rarer broad-headed morph with specialized adaptations to feed on the normal morph. Cannibalism confers nutritional benefits that accelerate development, essential for survival in transient environments, but incurs potential inclusive fitness costs. Selective cannibalism of non-kin thus should be favored. However, subjects may be more vulnerable to contracting disease from non-siblings to which they lack immunological defenses. We investigated kin discrimination and pathogen transmission among Korean salamander (Hynobius leechii) larvae. We placed broad-headed morph larvae into a circular arena together with two normal morph larvae, one their sibling and the other a non-sibling. To test for kin discrimination, we recorded all behavioral interactions among them. To study pathogen transmission, we fed broad-headed larvae either a sibling or non-sibling normal larva that we previously had infected with the bacterium Aeromonas hydrophila. Two days after ingestion, we determined whether the bacterium had systemically infected cannibals by quantitative PCR (qPCR). Broad-headed larvae discriminated between sibling and non-sibling normal larvae, directing aggressive behaviors mostly toward siblings. Infection loads varied more widely among broad-headed larvae that cannibalized non-siblings than those that cannibalized siblings, but the highest infection loads were recorded after ingestion of non-siblings. Cannibalizing non-siblings thus may increase the risk of contracting disease. Broad-headed larvae discriminate most strongly between siblings and non-siblings late in development, when inclusive fitness costs of cannibalizing relatives diminish and vulnerability to novel pathogens transmitted by non-relatives rises.