Parental effects of social density on mating behaviour, reproductive success, and longevity

Abstract

Organisms can adapt to changing environments, changing their investment strategies to increase their lifetime reproductive success. Contemporary evolutionary theory would suggest that organisms have evolved mechanisms which allow them to assess competition and acquire phenotypic responses that accurately responds to the environment, allowing for rapid changes in response to the social context. Selection would favour organisms which can make nongenetic parental effects, with parents evolving an ability pass on plasticity responses that optimize offspring fitness related traits to suit ecological challenges. In this thesis , I investigate how the social density affects the mating behaviour and offspring production of the parent and whether they use their current social environment to assess the reproductive opportunities and competition their offspring are likely to encounter. To determine the impact social density has on mating behaviour and whether there is a parental effect being conferred to the offspring I will raise two generations of male and female D.melanogaster in different social density treatments and measure their mating behaviours and how many offspring they produce. Female D. melanogaster raised at high density produce fewer offspring, but of a higher quality, which are larger and have higher disease resistance. Females which have been kept in low density mixed-sex environments are less likely to reject the males mating attempt and will produce more eggs than high density female. Given these responses, I predicted that the females raised at high density would confer mating behaviour traits to their offspring to give them an advantage in the social environments they were expected face. Here the results of my study suggest that there is a parental effect on mating behaviour being conferred on to the offspring in response to the mother’s density, with high density mothers producing daughters which are more attractive to the male and which mate for longer (Chapter 2).

I then used tested whether male D. melanogaster could pass on parental effects to their offspring in response to their social environment (Chapter 3). D. melanogaster males that anticipate a higher risk of sperm competition take longer to initiate courtship, have a longer mating latency and lengthen their mating time. The results of this study suggest that the male flies which anticipate higher levels of sperm competition, delay mating with the female. This suggests that the male, anticipating fewer mating opportunities due to the competitive environment has a higher threshold that the female must pass, than the males that do not anticipate high levels of sperm competition. The low-density males, which courted faster than the males raised at high density, may try, and capitalise on the mating opportunity, as more mating opportunities are expected. The results of this study suggest that we can use male, or sperm, competition to encourage males to initiate mating. Being able to reduce male choosiness may be a useful tool in conservation where access to females is limited, such as in ex-situ conservation programmes, although further studies would need to be conducted as to whether the results found D. melanogaster are conferred in other species. This study showed that mating latency, which was used in this study as a proxy of male attractiveness, was not significantly affected by the social density the male was reared in. This suggests that keeping males at high density social groups does not impact that attractiveness to the female, which suggests that keeping males in a high social group can increase their willingness to mate with a presented female without affecting the female’s attraction to the with the male. In fact, this study shows that males who experience no rival male competition prior to mating were more likely to be rejected by the female. That the number of offspring produced were also unaffected by the male’s social environment suggests that if a breeding programme were to select a male that was raised at high density with other males then they would be more likely to choose to court the female, less likely to be rejected by the female and they would suffer no loss in quantity of offspring produced. This study also tested the impact the paternal social density would have on their son’s mating behaviour. Studies on D. melanogaster suggest that parental density treatment has significant intergeneration effects on both juvenile and adult fitness. The results of this study show that whilst the sires adapted their mating behaviour in response to the social density they experienced, these responses were not passed down to their sons. These results suggest that sons mating behaviours are unaffected by the social environment of their fathers. This would be advantageous to in-situ and ex-situ breeding programmes, as it allows males to be placed in high social environments, when necessary, without there being a negative impact on their son’s mating behaviour. This model may only be suitable for species which cultivate in ephemeral resource matches, such as D. melanogaster, where crowding in transiently available rich patches is a key component of their natural ecology. In Chapter 1, I discuss how crowding affects mating behaviours and success in mammals, where it can have a negative affect on mating success of quality of offspring produced. This study builds on the evidence that the parental social density has an impact on mating behaviour and success and suggests that future studies investigate how well D. melanogaster works as a model across species.