One would not expect closer co-operation to be exhibited between complete strangers, but Nichola Raihani and colleagues found exactly that in Cleaner wrasse (Labroides dimidiatus).
Publishing the results of their study in a forthcoming issue of the Proceedings of the Royal Society B: Biological Sciences, the authors found that female Cleaner wrasse behave more co-operatively (by feeding against their preference more frequently) when paired with an unfamiliar male than with their regular partner.
Cleaner wrasses sometimes work in mixed-sex pairs to clean a client, but the fish prefer to feed on mucus of the client rather than the ectoparasites.
If a cleaner were to cheat and feed according to its preference, it causes the partner to lose out on a food source as the client terminates the session once a cleaner cheats.
Male Cleaner wrasse are larger than females, and are known to aggressively punish females for cheating. In response to this punishment, females feed more frequently against their preference and provide a better service quality.
The authors sequentially paired 24 female Cleaner wrasse with their familiar and an unfamiliar male partner. Using Plexiglas plates on which food items were glued to simulate the client, the authors observed the feeding behaviour of the fishes and their interactions to each other.
They found that female Cleaner wrasse were more cooperative when paired with an unfamiliar male. They also found that male wrasse punish unfamiliar females more aggressively for cheating, which helps explain why females are more co-operative in the presence of unfamiliar males.
For more information, see the paper: Raihani, NJ, AS Grutter and R Bshary (2012) Female cleaner fish cooperate more with unfamiliar males. Proceedings of the Royal Society B: Biological Sciences, doi:10.1098/rspb.2012.0063
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