Female sperm use and paternity distribution in the promiscuous tortoise Testudo hermanni hermanni

Chelonian species are one of the most outstanding examples of promiscuity and long-term female sperm storage. Such characteristics lay the basis for post-copulatory processes such as sperm competition and cryptic female choice. In the last years most of the researches concerning sexual selection in Chelonia tended to focus on turtles (i.e. aquatic species) and till now little is known about the factors influencing mounting and reproductive success of males in tortoises (i.e. terrestrial species). Moreover, no studies investigated the presence and eventually the extent of a cryptic female choice in favour of determinate males: some authors, in fact, have already suggested that courtship intensity could be used by females as an honest signal of male’s health and good genes. Within this framework, my PhD research focused on Testudo hermanni hermanni, being an example of promiscuous species able to store sperm for years. This taxon is one of the three species of Testudo endemic to Europe, and it is becoming endangered in the wild. The main aim of my research was to determine, combining a behavioural and a genetic approach, which factors are more likely to influence male’s success, both in the courtship phase and in the fertilization of eggs. For performing my experiments I used both captive and wild individuals. Thanks to a series of planned controlled matings and to parental genetic assignment of the hatchlings produced by experimental females, first of all I demonstrated that mating order does not influence nor mounting neither reproductive success of males, on the contrary of what observed in other chelonian species. The pattern of paternity distribution found in successive clutches seems instead to support the idea of an optimized use of stored sperm by females, following a sperm viability criterion. Secondly, the time a male spent in copulation with a female does not correlate with his siring success: this result seems to indicate that the male probability of egg fertilization is not linked to the amount of sperm released in the female genital tract, at least during the first reproductive season after mating. Finally, under inter-male competition conditions, my experiments clearly demonstrated that a hierarchy is soon established between the two competitors and that alfa males are more motivated in courting females and achieve a higher mounting success than the beta ones. Notwithstanding, males of higher hierarchy do not obtain a reproductive advantage over males of lower rank. This finding supports the idea that in spite the existence of qualitative differences between partners, females do not seem to operate any post-copulatory active choice in favour of determinate males. I then suggest that best quality males may produce more viable sperm, and thus can acquire a long-term reproductive advantage over others, both in terms of fertilization success and offspring survival. Surely more long-term studies are needed, but this thesis provides an original and fundamental contribution that may help at clarifying, for the first time, the mechanism at the basis of pre- and post- copulatory sexual selection in tortoises.