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Abstract(s)
Dominance hierarchies are formed within groups of fish in response to social competition for limited resources such as food, mates and space. In a dominance hierarchy, a fish displays dominant behaviours toward individuals lower in rank and submissive behaviours toward those higher in rank. Dominant fish exhibit agonistic behaviours, such as chasing and biting while defending a nest, and sexual behaviours, such as more frequent courtship. During and after formation of dominance hierarchies, a variety of physiological (e.g., endocrine), morphological (e.g., coloration), and behavioural (e.g., courtship) changes can occur. In Mozambique tilapia (Oreochromis mossambicus), dominant males release urine containing pheromones when interacting with other males or pre-ovulatory females. Male urine has been shown to contain two molecules, 20α- and 20β-pregnanetriol-3-glucuronates (P3Gs), which act as sex pheromones and stimulate ovulation in females. Males also appear to be able to discriminate the sexual state of females using olfactory cues present in urine and faeces. Furthermore, behavioural experiments, such as those using the mirror assay, have shown that urine from dominant males can reduce aggression in focal males, but P3Gs alone do not affect aggression. The identity of dominance signals present in urine is largely unknown. However, there is evidence that the signal in the urine of dominant males that controls aggression may be a multicomponent pheromone. In the laboratory, during the mouthbrooding period, females have occasionally been observed to display dominant behaviours and create territories for themselves, suggesting that females may compete with each other. We hypothesized that these behaviours might be mediated by chemical signals. This thesis aimed to identify the urinary dominance pheromones released by dominant males and pre-ovulatory females of Mozambique tilapia, using analytical chemistry, electrophysiology and behavioural analysis. The basic methodology consisted of collecting urine and faeces samples from dominant and subordinate males and pre-ovulatory and post-spawning females and using solid phase extraction (SPE) and high-performance liquid chromatography (HPLC) to fractionate the odorants involved. Electrophysiological methods tested the olfactory potency of the fractions, and those with the highest bioactivity were isolated to identify their chemical structure using liquid chromatography associated with mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR). The effect of putative pheromones on modulating aggression was tested using the mirror assay. Furthermore, it was tested whether mature males use chemical signals to discern the ovulation status of females and for mate choice through a preference assay. The results showed that dominant males release amino acids through urine, but high-performance liquid chromatography (HPLC) fractions containing amino acids did not affect male aggression. Similarly, a mixture of amino acids did not induce a significant preference response in focal males. Tilapias had olfactory sensitivity to extracts obtained from different solid phase media, including C18 eluate, C18 filtrate, mixed-mode cation exchange (MCX) bases, and mixed-mode anion exchange (MAX) acids—but none of the extracts separately affected male aggression, supporting previous suggestions of a multicomponent dominance pheromone. Mass spectrometry identified higher concentrations of cholic acid and taurocholic acids in the faeces of pre-ovulatory females than in post-spawning females. To assess the possible involvement of faeces in modulating aggressive behaviours in Mozambique tilapia, mirror assays were conducted; however, HPLC fractions containing bile acids (used as stimuli) did not affect aggression in focal males. However, male tilapia showed a preference for pre-ovulatory conditioned water, as well as their faeces (and bile acids within), compared to post-spawning females. However, males preferred equally pre-ovulatory female-conditioned water and water containing 17β-estradiol 3-glucuronate (released by pre-ovulatory females), suggesting this compound could be part of a pheromone bouquet to synchronise reproductive activity between males and females or as a token of female quality. Taken together, the findings of this thesis significantly enhance our understanding of the nature of chemical compounds involved in chemical communication in Mozambique tilapia. Moreover, it offers valuable information for future strategies in identifying dominant pheromones, from designing the behavioural experiments and monitoring the experimental fish to isolation and fractionation of samples for structural analysis.
Description
Keywords
Mozambique tilapia Chemical communication Behaviour Olfaction Dominance index Tilápia moçambicana Comunicação química Comportamento Olfato Índice de dominância