Environmental factors determine whether immigrants are accepted by cooperatively breeding animals
Cichlid fish are more likely to accept immigrants into their group when they are under threat from predators and need reinforcements, new research shows. The researcher suggests that there are parallels between cooperatively breeding fish's and humans' regulation of immigrants. The research was published today, 6 February 2013, in the journal Proceedings of the Royal Society B.
The Princess of Lake Tanganyika (Neolamprologus pulcher), a cichlid fish which is popular in home aquariums, are cooperatively breeding fish with a dominant breeding pair and several 'helper' fish that do not normally breed but instead assist with raising offspring. Helpers also play a crucial role in defending the group's territory against outsiders – although helpers also compete with the breeders for resources and reproductive opportunities.
The researchers, led by Dr Markus Zӧttl of the University of Cambridge, wanted to find out how environmental pressures might influence the acceptance of new immigrants. He said: "All animal societies are affected in one or another by immigration and when we seek to understand social organisation we need to understand which environmental factors influence processes like immigration."
A subdivided tank was used to carry out a series of tests in which different scenarios were observed. For the study, a breeding pair (which would be responsible for deciding whether a newcomer would be allowed to join the group) was placed in one compartment next to a compartment containing either a fish predator, an egg predator, a herbivore fish or no fish at all. An immigrant was placed in a third, adjoining compartment. The breeding pair was then exposed to the different fish in compartment two. The researchers then observed whether the type of fish they were exposed to would affect whether they accepted the immigrant fish from the third compartment.
The researchers found that breeders are less aggressive to immigrants and more likely to accept the unknown and unrelated fish as a member of their group when they are simultaneously exposed to predators. They concluded that cichlid fish are more likely to accept immigrants into their group when they are under threat from predators and can be used to increase their defences.
Dr Zӧttl added: "Our fish resemble human societies' view of immigration in two crucial aspects: The need for help in the territory takes precedence, and it seems to be a strategy of the territory holders to accept immigrants only when they need assistance with territory defence. This resembles human societies which organise immigration according to the demand in the society by encouraging skilled immigration when certain types of labour are in short supply."
The researchers also found that the fish appear to consider future threats. When egg predators were presented to a breeding pair who had not yet spawned they accepted new incomers. Their acceptance of reinforcements in the form of immigrant fish suggests that that the egg predator was viewed as a threat.
Dr Zӧttl concluded: "This behaviour suggests that breeders of this species might be able to anticipate a potential threat – and it seems to resemble the future planning evident in birds and apes."