Renn Lab Thesis Project
2008 Reed Graduate
The Dominant Female: A Look at the Possible Sex-Role Reversal in Julidochromis marlieri, from a Behavioral and Hormonal Perspective.
The fish family Cichlidae is characterized as monogamous and biparental. Typically, the males are larger and more aggressive and hence are able to dominate their mates. The genus Julidochromis or “Julies” as they are affectionately called, is part of this family of fish localized to Lake Tanganyika in Eastern Africa. Julidochromis marlieri, is one of five species belonging to the genus. They are unique, in that they differ from other species by having the females larger than their male counterparts and hence, females develop the potential to dominate the males. Size differences between males and females may lead to the development of a polyandrous mating system, though they are typically monogamous. Such differences have led researchers and cichlid enthusiasts to suggest that J. marlieri are a sex-role reversed species. Some of the work in this area has centered on the size difference between the males and the females of the species.
The aim of this investigation was to examine possible sex-role reversal from the perspective of social interactions between males and females. I set up different treatments with different size combinations of males and females and recorded the levels of aggression of each individual.
1) Conventional Size Condition with females larger than males.
2) Size Matched Condtion with females and males of the same size
3) Size Reversed Condition with males larger than females.
The results corroborated the idea that in situations when the female is larger, the female tends to be more aggressive and dominant; and in situations where the male is the larger of the two sexes, the male becomes more aggressive and hence dominates its mate.
Hormone samples were collected from test subjects using a new, non-invasive method to detect correlations between aggression and the concentrations of key sex hormones. Hormone data suggested a correlation between the levels of aggression and the concentration of testosterone released by the individuals sampled.
For 17b-estradiol, progesterone and 11-ketotestosterone there was no correlation between hormone level in the excreted water sample and and aggression levels.
Interestingly, each treatment had one individual that was more aggressive, compared to others, suggesting that in a social setting there may be a social hierarchy of some kind, which is typically headed by the largest individual. This study supports the idea that J. marlieri has dominance characteristics of a sex-role reversed species, given that in typical social settings, the females are bigger, more aggressive and hence dominate their male counterparts.
This research project was supported in part by a grant from the James F. &Marion L. Miller Foundation made to Reed College.
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