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Phylogenetic analysis of behavioural evolution : a case study using gasterosteid fishes

University of British Columbia

The hypothesis that intersexual selection has been the most important force shaping the male nuptial signal in Gasterosteus aculeatus is based upon experiments in which females were offered a choice between "red" and "nonred" males. However, only territorial males assume nuptial colouration; therefore, results from these experiments provide support for mate recognition rather than differentiation among potential mates. In this thesis I examined the intersexual selection hypothesis further by first performing a phylogenetic systematic analysis of the Gasterosteidae using only behavioural characters (chapter 2). The resultant cladogram was congruent with trees utilizing morphological and electrophoretic data. Based upon the macroevolutionary relationships between breeding colours and breeding behaviours in the gasterosteids, three predictions relevant to a discussion of nuptial colouration in G. aculeatus were proposed: nuptial colouration is (1) weakly associated with male/male interactions (intra-sexual selection), (2) moderately associated with parental care (natural selection) and (3) strongly associated with male/female interactions (intersexual selection). These predictions were investigated by scoring seven colour variables (intensity, hue and distribution of red body colour; intensity and hue of blue eye colour; intensity and distribution of black body colour) for 19 males across an entire breeding cycle (chapter 3). The variables interacted to produce four distinct male colour mosaic signals corresponding with the stage a male had reached in the breeding cycle. No single variable was sufficient for the delineation of all breeding stages; however, the distribution of red body colour reliably distinguished the courting male from nesting and parental males. The phylogenetic predictions were confirmed for the component of the nuptial mosaic showing the greatest degree of intermale variability: the intensity of red body colour peaked maximally during courtship, and secondarily during fry guarding. The relationship between male colour and male behaviour was examined by recording a solitary male's reactions to a captive, receptive female over a 5 minute test period (chapter 4). The intensities of red body and blue eye colour in dull (and possibly bright) males reliably signalled their behavioural vigour. Medium intensity males signalled that they were more vigorous than their dull conspecifics; however, there was no association between colour and behavioural intensity within this group. Medium males may be more strongly influenced by stochastic factors such as previous experiences or colour relative to neighbours. The increased importance of personal history introduces a source of disorganization into the mating system that may oppose the directional force of truth in advertising and thereby increase the ambiguity of the male colour signal. The relationship between male colour and female behaviour was examined in a sequence of experiments culminating in a female's choice between two competing territorial males (chapter 5). Cladistic analysis of 16 behavioural characters recorded during the choice tests revealed three groups of males in this population: inactive losers (losing male did not participate in choice test), active losers (losing male active during choice test) and fighting losers (subgroup of active losers defined by female behaviour during choice test). The "inactive losers" group represents a "no choice" situation. In the remaining two groups, females responded preferentially to the most intensely coloured member of the competing male pair. This preferential response was strongest during the pre-choice, captive presentation in which the majority of females oriented head-up to, and tracked, the brighter red male. Once released, the female's initial response to the brighter courtship signal was overridden by the behavioural actions of the duller intensity male in 2 out of 12 trials: 10 of 12 females spawned with the brighter red male. The three structural components of red body colour thus provide information at all levels of mate choice: hue identifies sex, species and territorial status (mate recognition), distribution across the entire ventral/lateral surface signals a courting male (breeding status recognition) and intensity provides the basis for female discrimination among courting males (differentiation among potential mates). This series of micro-evolutionary studies supports the macroevolutionary prediction of a strong association between colour and courtship in this species and confirms the original intuition of previous researchers: the intensity of red is an important component of female mate choice in Gasterosteus aculeatus.

Text

2010-08-21

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http://hdl.handle.net/2429/27590

Zoology

University of British Columbia

Graduate