It pays not to put all your eggs (or fry) in one basket…

A cichlid from Lake Tanganyika exchanges its young with other parents to reduce the chances that its entire brood will be eaten by predators.

Neolamprologus caudopunctatus is monogamous and pairs construct nesting caves to protect their eggs and fry from predators. By diving 12 metres to the lake floor, scientists were able to collect DNA samples from over 350 parents and fry from over 30 nests. Sophisticated genetic techniques were then applied to investigate the parentage of fry in individual nests.

Most nests were found to contain fry that were unrelated to both "parents", with some nests containing fry produced by several pairs of parents.  

Because the locations of the nests were known, the scientists were able to show that fry had been born in nests that were separated by less than one metre to over 40 metres from their adoptive nests. Although very small fry may be able to swim several metres to a new cave without being eaten, it is highly unlikely that they could travel much longer distances.  Instead it is probable that they were carried to new nests in the mouths of their parents.

Transporting the fry to fairly distant nests would ensure that some young are protected even if all the nests in the immediate neighbourhood are predated or destroyed, so it is easy to rationalise why parents should do this. But why should other fish be willing to adopt fry that are unrelated to them?

Franziska Schaedelin of the Konrad Lorenz Institute of the University of Veterinary Medicine, Vienna, suggests that foster parents may accept unrelated fry as a way of diluting predation of their own offspring. If this is so, parents should adopt fry that are not larger than their own young, as smaller fry are known to be predated first.

The researchers were indeed able to show that adopted fry were the same size as native fry within broods, although they were generally larger than fry that were not offered out for adoption. It seems that parents selectively allow unrelated fry to assimilate into their own broods while also delivering their fry for adoption by others.

Sharing the care of broods among different families thus represents a kind of insurance policy against the predation of a nest. Schaedelin summarises the findings neatly: "in a species that is so highly predated, it must have been important to develop a strategy to ensure that at least some of the young survive. It seems that fish do this by not putting all their eggs (or young) in one basket."

The paper "Nonrandom brood mixing suggests adoption in a colonial cichlid" by Franziska C. Schaedelin, Wouter F.D. van Dongen and Richard H. Wagner is published in the current issue of the journal "Behavioral Ecology".

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