Selection on female body colouration through male mate choice in Alpine newts, Ichthyosaura alpestris

Abstract

Colourful animal ornaments have been a core research focus of the field of sexual selection in evolutionary biology. Traditionally, females are considered to be the choosing sex, while males compete for access to females and advertise themselves through extravagant colourations and decorations. However, in many species not only males but also females showbright body colourations, giving rise to the potential of quality differentiation much in the same way as in males. Male mate choice and the signalling content of bright female colourations remains a contentious issue, and empirical evidence for male mate preferences and honesty in female ornamentation in conventional sex-role species is scarce for vertebrate species other than birds and fish. In this thesis, I investigated a link between female belly colouration, male preference and female fecundity in the Alpine newt, Ichthyosaura alpestris, a medium-sized, aquatic salamander. Both sexes of this species have colourful bellies that vary naturally from pale yellow to bright red or dark orange among individuals. Because male Alpine newts are likely to be limited in their mating capacity, males would benefit when they mate with females of particular high quality. I hypothesised that the belly colouration of females may act as an indicator of individual quality and that males therefore respond to variation in this colouration. The first paper describes the mate preferences of male Alpine newts, in an experimental setup. Males had a choice between females that varied in belly redness (red versus yellow). Additionally, I tested the preference of males in different light conditions (control and blue filter) to determine whether male choosiness solely depends on the female’s colour signal. I found that males generally spent more time courting red females but increased the time when a yellow female was particularly interested. This verifies the existence of male mate choice in Alpine newts; a species that had previously been thought of as being a traditional model of female choice only. The second paper examines the temporal pattern of mating activity in Alpine newts. I observed mating behaviour in undisturbed groups of males and females during light and dark conditions over several days. I recorded most courtship behaviour during light conditions, which demonstrates that males use mating opportunities when visual cues can be utilised optimally and when they can properly assess females by their colour. The last paper provides first evidence for the signal content of the female belly colouration. By counting the number of eggs of females that varied in belly colouration, I showed that more orange-bellied females were more fecund and could lay their eggs faster than yellower ones. This enabled more orange-bellied females to cease egg laying at a similar time as yellow females in spite of a higher number of eggs. This result presents an explanation for the preference patterns described before because it suggests a direct benefit for males that mate with ornamented females. Taken together, the results of this thesis indicate that female belly colour is subject to sexual selection through male mate choice in Alpine newts. My results are further consistent with the general idea of mate choice based on colourful ornaments and give additional insight into the function of female colouration in diverse taxa.