Larger female cichlids bully smaller ones into not reproducing, according to research by a Swiss scientist published in the latest issue of the journal Biology Letter.
Dik Heg studied the cooperatively breeding Neolamprologus pulcher and carried out experiments in which groups of three unrelated females (one each of a large, medium and small individual) were maintained in aquaria with a male (pairs consisting of a medium-sized female with a male were maintained as controls).
The groups and pairs were checked daily for courtship, spawning and brood care activity.
In the first set of experiments, the author replaced the largest female in each group with another large female, while in the second set of experiments, the largest female was replaced by a very small female.
Suppressed spawningsThe author found that within groups of three unrelated females, subordinate (smaller) females were reproductively suppressed and that this suppression was ...due to medium- and small-sized females laying less frequently compared with large females, and compared with medium females in control pairs.
In the replacement experiments where the largest female was replaced by a very small female, the author found that the medium females ...immediately seized the dominant breeding position in the group and started to reproduce as frequently as control pairs, whereas clutch size and egg mass did not change.
The suppressing mechanism within the group remains unknown, the author writes t remains to be established whether suppression comes about by active interference of the dominant female in subordinate reproduction, by indirect interference or by commitment of the subordinates. Evidence for active interference has been found in this species.
Egg eatingDominant females eat subordinate eggs on the day of spawning and subordinates can only effectively circumvent this by securing an exclusive breeding patch. Indirect interference may involve the dominants harassing the subordinates, which increases the subordinates cortisol levels and through a cascade of other physiological effects may reduce the subordinates reproductive potential.
Analyses of stress-induced subordinate reproductive suppression via hormonal changes have given mixed results. Alternatively, some subordinates may simply not be interested in current reproduction to avoid eviction.
However, the author notes that the dominant females in each group also do not completely suppress reproduction in subordinate females.
Dominant females may be selected to concede some reproduction to their female subordinates to acquire their continued help in brood care. Indeed, the dominant females benefited from alloparental care, as they reduced their care behaviour concurrently (~load-lightening ).
For more information, see the paper: Heg, D (2008) Reproductive suppression in female cooperatively breeding cichlids. Biology Letters 4, pp. 606"609.