Dominant females can play an important part in controlling breeding in social fish a study by Canadian scientists has found.
The study focussed on the social cichlid from Lake Tanganyika Neolamprologus pulcher. This species of cichlid is unusual in that it lives in relatively large permanent groups of up to 20 individuals.
The groups comprise of a dominant breeding pair and subordinate male and female helpers that assist in territory defence, maintenance and brood care.
The researchers from McMaster and Queen s university located groups in Lake Tanganyika where there were large helpers and removed the dominant males; leaving a vacancy for the large subordinate male helpers to fill.
Larger testesDominant male breeders have larger testes and higher hormone levels than subordinate males of a similar size. This suggests that helpers are much less likely to sire many offspring in natural populations.
Whether or not a subordinate male ascended to fill the breeding gap left was found to be dependent on four factors including the initial male size, the size of the dominant breeding female and the relative sizes and ratios of the males and their female breeders.
This means that the ascending male was nearly always larger than the female breeder.
Female N.pulcher lay eggs only once a month following the lunar cycle so they limit breeding significantly.
In addition there is a high turnover of males in the dominant male breeding position so males must be large and immediately ready to breed.
This study found that the ascending males adapted extremely quickly to their new breeding position. Within a day the newly ascended males performed significantly more dominance behaviours and received more submissive behaviours from the dominant females.
Maintaining social positionThese behaviours would serve to secure the new dominant social position and research suggests that in this species the cichlids would need to continue behaving aggressively to maintain their position in the society.
Within one week the ascended males testes had increased to a size similar to that of the removed dominant male and were significantly larger than that of subordinate males.
This growth was found to be at the expense of overall growth and was responsible for a corresponding increase in reproductive output. Once the male had established social dominance, it would then stop growing its testes and start to grow in overall size again in order to secure other breeding positions by taking over neighbouring territories.
This study highlights how complex the interactions between social status, reproductive physiology and group dynamics can be in a highly social species.
A recent study on Neolamprologus brichardi and N. pulcher found evidence to suggest that the fish were actually a single species. (See Neolamprologus brichardi and pulcher are same species, News, 16th October 2007.)
For more information see: J.L Fitzpatrick, et al. Proc. R. Soc. B. (2008) 275, pp 926-936. doi:10.1098/rspb.2007.1449