Embryonic selfish-herding blurs the line between brood parasitism and mutualism for communal-breeding stream fishes

Mutualisms are complex, interspecific relationships, which sometimes create "selfish-herds" as individuals of each species compete to maximize their own fitness. Nest association, where individuals of different species spawn on a nest created by a host species, is a reproductive interaction characteristic of some minnows (Leuciscidae) and is considered mutualistic despite mimicking the behavior labeled "brood parasitism." We studied the spawning behaviors of bluehead chub (Nocomis leptocephalus) and its nest associates, testing the hypothesis that bluehead chub exploits the selfish-herd dynamic in a novel manner by arranging embryos within its nest to maximize the survival of its own offspring at the expense of the nest associates' offspring. Our results show that embryos were not uniformly distributed within a nest, as one section representing one-sixth of the nest's total volume contained a disproportionate percentage of embryos (x¯ = 40.0% ± 6.1% SE). We found three-quarters of host embryos within deeper nest sections safer from embryo predators, whereas only a third of all associate embryos were found in the same sections. These results support our hypothesis that male Nocomis leptocephalus create "embryonic selfish-herds" within their nests. This is the first study to document the existence of embryonic selfish-herds, a phenomenon that warrants the reexamination of some vertebrate reproductive interactions labeled as brood parasitism.