ORCID

Abstract

Animals experience temporally and spatially heterogeneous environments throughout their lives. Therefore, individuals that are able to plastically adjust their behaviour to the current environment, are able to maximise their fitness. These behavioural adjustments are particularly important where there are trade-offs between investment in different life-history traits. Trade-offs between reproduction and survival have been widely documented, in addition to a body of evidence for trade-offs between mating related traits. How an individual adjusts their mating behaviour in response to their social and biotic environments are important aspects of selection. Changes in the social environment, including sex ratio or population density changes, will have direct impacts on the risk of mate competition and the opportunity for mate choice. At the same time, biotic environmental changes in the environment could have impacts on nutritional availability, and therefore the condition state of individuals.In Drosophila melanogaster, males have been repeatedly observed to increase their copulation duration, when they perceive there to be a risk of sperm competition. Increasing copulation duration confers a benefit during sperm competition, yet males are adjusting their behaviour only in response to certain contexts. This suggests the trait is costly, and that males are forced into a trade-off to enable its expression. To further explore these effects, in chapter two, I assessed the interplay between two social factors, sex ratio and density, and the nutritional composition of the male rearing environment, to determine how the variation in these factors may impact reproductive behaviours and fitness outcomes. Nutritional restriction lowers individual quality, and so should also increase the expression of trade-offs between traits, due to their impaired energy reserves. I found no clear directional response across the sex ratio, density and nutritional treatments. However, by assessing the correlations between dependent variables, I found that longer mating durations were associated with fewer viable offspring and lower hatching success. Low-quality males took longer to initiate mating, providing evidence that nutritionally restricted males may be investing less in courtship, and so are less attractive to females. The effects of sperm competition risk on copulation duration were not found in this experiment, but it is possible the physical separation of the focal male inhibited the sensory cues of their rivals. Alternatively, the previously-observed changes in copulation duration may not be consistent between populations and may be an artefact of selection under laboratory rearing.Chapter three further investigated the mating behavioural response of males to sperm competition risk, removing the physical separation of the focal male, inaddition to varying the quality of both males and females, in a fully factorial design. Males who had not been pre-exposed to sperm competition risk were faster to initiate courtship, had a longer average courtship bout duration, and were quicker to begin mating. Whereas males who had been pre-exposed to sperm competition risk initiated more total courtship bouts, and, when males were also high-quality, performed more courtship bouts per minute. However, these findings appear to be highly influenced by increased output in pairings with low-quality females. Low-quality females produced significantly fewer viable offspring, regardless of their mate’s quality. Males pre-exposed to sperm competition risk were quicker to mate with high-quality females. Additionally, high-quality females were significantly more likely to mate overall. However, this is the only evidence of males positively increasing investment when courting high-quality females. Males pre-exposed to sperm competition significantly increased their mating duration, adding to a significant body of evidence demonstrating males plastically adjust this trait in response to sperm competition risk. Reduced mating latency was significantly associated with more courtship bouts per minute, suggesting females prefer higher courtship intensity, as opposed to longer courtship bout length. There appeared to be evidence that low-quality males were less able to maintain their level of courtship when compared to high-quality males, in some instances.Chapter four explored inter-individual differences in activity and found differences in movement frequency, which were consistent over time and measures. As consistent inter-individual differences in behavioural responses to the environment can be defined as personality, this chapter provides evidence of personality in D.melanogaster. In this chapter I further investigated the correlation between personality type and reproductive traits, in addition to the effect of male social environment (isolated, or pre-exposed to sperm competition risk). There was no effect of behaviour type on any of the measured reproductive traits. Males not pre-exposed to sperm competition risk were both quicker to initiate courtship, and had a higher hatching success rate, out-performing males pre-exposed to sperm competition risk, overall. As the male pre-exposed to sperm will likely have been energy depleted, due to the associated costs on fighting with rival males, this finding may represent trade-offs between investing in male-male competition and mating related traits.Chapter five investigated the different temporal measures of courtship in D. melanogaster, to provide further understanding of the bout and pause structure, as well as their effects on fitness outcomes. Beneficial fitness outcomes are not associated with a higher number of total courtship bouts, or longer courtship duration. Instead, males that performed more bouts per minute, and had shorter pauses between bouts, had a shorter mating latency, suggesting that females prefer males that court with high intensity. Males that initiated more courtship bouts per minute had shorter pauses between their final four courtship bouts, had shorter total gaps between courtship bouts, overall, and had a higher number of viable eggs. An individual’s ability to court intensely, with shorter gaps, and more bouts per minute, sustained through the entirety of courtship, appears to be an honest signal of male quality, resulting in direct fitness benefits. Males that were not pre-exposed to sperm competition were faster to initiate courtship and appear to invest more heavily in courtship, resulting in these males being more likely to secure a mating. This may be due to these males being reared in social isolation, therefore perceiving the likelihood of encountering further mates to be minimal, and consequently investing heavily in this interaction as a strategy to increase their fitness.This thesis has found evidence of personality in D. melanogaster, where future research should further investigate the relationship between behaviour type and reproductive traits. Additionally, this study highlights the suitability of this model system to further explore the pace-of-life syndrome hypothesis due to the finding of personality in this species, in addition to a wealth of evidence of life-history trade-offs. I found inconsistent support for the increase in mating duration with risk of sperm competition. Future work should explore the variation in this behaviour, across different strains and species to ascertain its function and fitness implications. Finally, courtship intensity appears to be an honest signal of male quality, and there appears to be a trade-off with investment in male-male competition. This thesis provides evidence that despite D. melanogaster being a model organism, many of their behaviours are varied and complex, and understanding this variation is vital to when extrapolating findings to other taxa or the evolutionary mechanisms more broadly.

Awarding Institution(s)

University of Plymouth

Supervisor

Mark Briffa, Calvin Dytham

Keywords

Drosophila melanogaster, Sexual selection, Personality, Evolutionary Biology, Ecological, Trade-offs

Document Type

Thesis

Publication Date

2025

Embargo Period

2025-08-11

Deposit Date

August 2025

Creative Commons License

Creative Commons Attribution-NonCommercial 4.0 International License
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License

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