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In this issue |
Sex in Cichlid Society
kathryn{at}biologists.com
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Teenagers aren't the only species that suffer from social pressure. In fact some creatures even change sex when the social equilibrium shifts. Some male cichlid fish don't go that far, but their behaviour, appearance and fertility alter as the social hierarchy changes. Russell Fernald wondered whether a shift in social status was the trigger for the physiological changes that accompany well-established behavioural changes. Working with Stephanie White and Tuan Nguyen, he monitored male cichlid reproductive status after the fish were transferred to a new social environment. Tracking the fish as their dominance rose and fell, the scientists discovered that changing the fish's social status drove changes in the level of a key sex hormone that controls the fish's sexual maturity (p. 2567).
In the shore pools of Lake Tanganyika, only territorial, brightly coloured male cichlid fish are fertile. The remaining males, who haven't succeeded in winning a patch of pool, are dowdy and infertile until circumstances change and territory becomes available. An unaggressive male can then join the dominant group, changing both his behaviour and fertility while transferring his social status. Fernald and his team wondered whether changes in the fish's physiology and behaviour were influenced by the make up of the community around it.
White decided to test for evidence of changes in the fish's sexual maturity by following the sex hormone, gonadotropin-releasing hormone (GnRH). The peptide is expressed in neurons in the fish's hypothalamus, so White tracked levels of the hormone's mRNA template in the fish's brain. She also monitored gonad size as it changed in response to the fish's social context. Having figured out how to track the fish's physiology, she needed to persuade them to change social status.
The simplest way to make a fish `top dog' is to place him, unchallenged, in the midst of a school of females. White transferred a non-territorial male into a tank of females. With no other males around, the newly transferred fish instantly switched to behaving aggressively, but it took several days before he matured sexually.
Although promoting a male cichlid into a harem might be every cichlid's dream, it's hardly a realistic social situation. White repeated the test with non-territorial males, transferring them to a tank where they dwarfed the resident dominant males. This time, the males were more cautious, waiting until they had matured completely before beginning to act boldly. However, when White reversed the situation and demoted a dominant male by transferring him to a tank where he was dwarfed, he switched instantly into a submissive role. This fish's GnRH mRNA levels didn't immediately reflect the male's new life style, falling slowly over a period of three weeks while the gonads shrank.
White explains that these different responses to situations where the males were either promoted or demoted show that `the community make-up influences both the behavioural and the physiological changes'. She adds that although the time lag between the responses seems counterintuitive at first, White believes that it is probably geared to take advantage of the rare opportunities when territory becomes available.
White and her colleagues don't know what aspect of the fish's altered social status triggers the transcription of the hormone template, but they have evidence that social stress affects production of the all important gonadotropin-releasing hormone.
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