Watching the weight of your female mate is a good way of keeping tabs on your sexual rivals if you're a Cleaner wrasse!
Cleaner wrasse (Labroides dimidiatus) often feed together as male-female pairs. Although they typically feed by picking ectoparasites off of larger client fish, some females are tempted to bite a chunk out of client fish instead and prematurely end the feeding session as the disgruntled client swims away.
The females that cheat, however, receive a severe punishment from the male for their indiscretion.
Nichola Raihani and colleagues show in a study to be published in a forthcoming issue of the Proceedings of the Royal Society B that males not only lose a meal when females cheat, they may also gain a sexual rival, thus doubling their incentive for punishing wayward females.
All cleaner wrasse are born female and live in groups consisting of a (physically larger) male with a harem of up to 16 females. When one of the females become the largest fish in the group, they become male. One would thus expect the male fish to have a vested interest in maintaining a size disparity over the females.
In the first experiment, the authors maintained 12 pairs of cleaner wrasses separately, and offered them Plexiglas plates to which both prawn and fish flakes were glued. The plates would be immediately removed whenever one of the fishes ate the prawn.
Presenting the fish with high- (eight flakes and two prawns) and low-value (four flakes and two prawns) plates in their experiments, the authors recorded the intensity of male punishment (number of aggressive chases) in the 30 seconds following plate removal if the female fish ate the prawn. The size of the female relative to the male was also recorded for each pair. Data from trials where females cheated (by eating the prawn item) were used to investigate how relative female size and client value affected male punishment intensity.
The second experiment was designed to explicitly test whether male punishment of cheating females increased with relative female size, and whether females behaved more co-operatively in response to more severe punishment.
The authors created ‘similar-sized’ pairs and ‘dissimilar-sized’ pairs using a total of 11 females and 15 males. Each experiment then consisted of presenting each pair with two plates separated by a 60-second delay. As in the first experiment, each plate was removed once one of the pair ate the prawn.
Again, the authors recorded the intensity of male punishment (number of aggressive chases) in the 60 seconds following plate removal if the female fish ate the prawn. The authors also recorded whether or not the female fish ‘cheated’ and ate the prawn again when the second plate was offered.
The authors found that male punishment of females was most severe, when females cheated high value clients and when the female was relatively close in size to the male. However, males did not appear to significantly adjust punishment intensity according to client value when the female was relatively small in size.
The results of the second experiment supported the idea that relative female size influenced male punishment intensity.
Females that caused plate removal (by eating a prawn item) received more severe punishment from similar-sized males than from dissimilar-sized males. The authors also found that when the males punished the females more severely, they were less likely to repeat their transgression.
This supported the notion that relatively large females represent a more immediate threat to male fitness owing to the increased likelihood of sex change compared with females that are relatively smaller in size.
For more information, see the paper: Raihani, NJ, AI Pinto, AS Grutter, S Wismer and S Bshary (2011) Male cleaner wrasses adjust punishment of female partners according to the stakes. Proceedings of the Royal Society B: Biological Sciences, doi:10.1098/rspb.2011.0690
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