By Susan Milius
A male fish produces a burst of hormones as he fights off an intruder. Now, researchers say that this surge isn’t triggered simply by fighting.
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“Apparently, the fish need to know whether they are winning,” says Rui Oliveira of the Institute of Applied Psychology in Lisbon, Portugal.
Oliveira and his colleagues took this view after exposing each fish to a perfectly matched opponent: his image in a mirror. When a male cichlid sees his reflection, he displays increasingly menacing postures, as if he’s facing off with a live intruder. However, he doesn’t show a surge of male hormones, the researchers report in the Sept. 8 Nature.