The results, recently published in the journal Biology Letters, suggest a species of cichlid fish employs a complex decision-making process when joining a social group. Safety in numbers wins out over an increased chance of sex when it comes to a native African fish, a new study finds. Lead author University of New South Wales biologist Alex Jordan says the strategy employed by the fish is likely to be replicated in other group-joining species.
For the research, Jordan examined the choices made by Neolamprologus pulcher found in Lake Tanganyika in Tanzania.
He says N. pulcher is unusual because it lives in social groups that include both breeders and helpers. Jordan says the breeding pair can have between two to seven helpers who do tasks such as cleaning the nest, defending territory and looking after the eggs.
The helpers do not breed, instead waiting until a breeding position becomes available through the death of a more senior member of the group.
Previous genetic work has shown that N. pulcher within a group are unrelated, suggesting the young disperse to other groups. Jordan says he wanted to find out what affected their choice of a new group.
For the experiment, he and colleagues from McMaster University collected whole groups and relocated them to a two-metre enclosure near the edge of the lake that replicated the original home environment. A foreign fish was then placed at a rocky area in the middle and could then choose to join a group or remain alone. In each instance the fish could choose between a group where they would be a senior fish with greater access to food and shelter and be next in line for the breeding position. Or they could join a larger social group where they would be at the bottom of the queue for breeding.
Jordan says the fish would typically swim between the two groups and look at both before joining the larger group where they would be the most junior.
"They all passed up the golden opportunity," he says. "The question was, do you take an early gamble and risk a lot or do you go for the safe option?
"The [N. pulcher] always took the conservative option, they were not gamblers."
Ironically, in becoming a member of the larger group, the new fish also faced increased aggression within the group and were constantly "beaten up," says Jordan.
But, he says, what seems like a poor decision on the surface actually offers benefit in the form of increased protection to the new subordinate fish.
Jordan says Lake Tanganyika has a high predation risk for the N. pulcher, which at its largest grows to about 11 centimetres.
Among its main predators are eels and large carnivorous fish.
"It looks like these social systems offer protection from this predation to smaller fish like N. pulcher," says Jordan.
"Individuals are willing to sacrifice short-term breeding success and pay the costs of increased aggression to join groups that offer greater safety from predation."
Animal behaviour: Lyndon A. Jordan, Marian Y. L. Wong and Sigal S. Balshine - The effects of familiarity and social hierarchy on group membership decisions in a social fish. Biol. Lett. published online before print December 9, 2009, doi:10.1098/rsbl.2009.0732
In the fish species N. pulcher, new arrivals prefer to start at the bottom of the pecking order, sacrificing sex for safety. (Alex Jordan/University of New South Wales)
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