Laboratory studies suggest that this is because, where a female intruder is concerned, the alpha male does not just turn a blind eye: he extends a helping fin, defending her against his irate female nestmates, explain Carsten Schradin and Jürg Lamprecht of the Max-Planck Institute for Behavioural Physiology, Germany.
"Females will only tolerate one another if a male is present; once he is removed, the dominant female promptly evicts the other female newcomer," explains Schradin. These findings will be published in
"Breeding space is often fought over by males," comments Axel Meyer of the University of Konstanz, Germany, "but such 'peacekeeping' provides an interesting twist." He adds, "In several other cichlid species, males will steal shells from other males to make their territories more attractive to females."
The number of shells within each territory determines how many fish can live in each group, so females guard them fiercely for the future needs of their offspring. Schradin and Lamprecht report that females each have their own subterritories, and size matters.
"Bigger females had more shells and also laid claim to shells vacated by juveniles," says Schradin. He and Lamprecht experimented with an artificial territory, inhabited by a male and female pair, furnished with an excess of shells to entice newcomers.
Male peacekeeping is probably not completely noble in intent. Males welcome new females to their territory to broaden their sexual opportunities. They swim freely between subterritories, keeping the peace between females, but playing little part in bringing up the 'fry' (young). They spend their days defending their harems from intrusive males, while looking out for new females with which to mate.
Something similar goes on in songbird communities. The strongest male lures secondary and even tertiary females to his lair. Meyer adds, "females would rather share their nests with other females than mate with a lesser male."