When a male cichlid fish captures a piece of prime real estate, his gonads go wild. In the 15 August Journal of Neuroscience, scientists report another bonus: his stress hormones drop dramatically. Meanwhile, stress levels of wimpier males, who lose out in territorial competition, almost double and they appear to be unable to mate.
Male cichlid fish battle for stretches of lake-bottom territory where females lay their eggs. Only those who emerge victorious will develop sexually and be able to fertilize the eggs. They also change from camouflage gray to bright blue and yellow. In previous work, neuroscientist Russell Fernald of Stanford University connected victory in territorial battles to a reversible brain change in dominant males. He found that after a rise in social status, certain neurons enlarge eight-fold and produce more of a reproductive hormone called gonadotropin releasing hormone.
Fernald and his colleagues Stephanie White, Helen Fox, and Mimi Kao have now found that the victors also appear to be far less stressed out than their hapless foes. First, the researchers created an unstable social situation in a fish tank. They combined 20 males and females who had never lived together, and let the males vie for territory and social position. "We watched them form their fish society from scratch," says White. Every 2 weeks, the team measured blood levels of a stress hormone called cortisol; they sampled quickly, before the stress of capture changed background levels. The cortisol levels of males who established territories and matured sexually dropped to anywhere from 2% to 10% of the level when they were subordinate. But for those who lost out in this macho competition, stress hormones almost doubled, the researchers say.
"It's a fascinating model," says Richard Weiner, a neuroendocrinologist at the University of California, San Francisco, who notes that stress-related hikes in cortisol correlate with an inhibition of reproductive function in people, too. To further test the link, Fernald now plans to manipulate cortisol levels in fish and look for changes in reproductive function and rank.