The sun and the moon play a considerable role in the reproductive cycle of some cichlids, according to the results of a study to be published in a forthcoming issue of the Journal of Zoology.
Julie Desjardins and coauthors investigated reproductive and behavioural patterns in relation to both the lunar cycle and the daily cycle from dawn to dusk in Neolamprologus pulcher.
They did this by relating the behaviour of the fish to reproductive physiology (gonadal investment patterns) in general and to plasma hormone (androgen) levels specifically. The authors additionally explored the relationship between dominants and subordinates and mated breeding pairs to explore the extent of synchrony in reproductive behaviour and physiology.
The authors studied 88 social groups living in the Kasakalawe Bay, Lake Tanganyika (Zambia), observing the behaviour of the fish once in the morning and once in the afternoon (the fish were individually marked to aid identification) for diurnal variations in behaviour.
The fish were followed at four time periods in the lunar cycle corresponding to degree of moonlight to explore lunar variation in behaviour. The authors then extracted blood samples to test the levels of testosterone and 11-ketotestosterone, and examined the ovaries and testes of the fishes, as well as tested for the swimming speed of the sperm in the case of the male fish.
The results of the study indicated that dominant female breeders dramatically increased their investment in gonads (ie. their ovaries were the largest) during the first quarter moon, and male breeders exhibited a marked increase in androgens and sperm swimming speed during the same period.
The authors also found both male and female dominant breeders to feed more in the morning than in the afternoon.
The first quarter moon is thought to represent an ideal time for spawning, as the moon would be full and the night would be brightest when the young emerge.
The authors hypothesise that full moonlight would provide allow adults the best opportunity to visually detect nocturnal predators entering the brood chamber, and to more effectively protect newly laid eggs or developing embryos.
The brightest nights of the lunar cycle would also allow the young emerging at this time to spot nocturnal predators best and afford them the best chances of survival.
The authors also found the ovaries in female subordinate helpers to be the largest during the first quarter moon, but no significant increase in sperm quality and androgen levels in the male subordinate helpers. This suggests that the multiple females (but not males) may be breeding within a single social group.
For more information, see the paper: Desjardins, JK, JL Fitzpatrick, KA Stiver, GJ Van Der Kraak and S Balshine (2011) Lunar and diurnal cycles in reproductive physiology and behavior in a natural population of cooperatively breeding fish. Journal of Zoology 284, doi: 10.1111/j.1469-7998.2011.00814.x