Assessing the consequences of personality traits on reproductive success is one of the most important challenges in personality studies and critical to understand the evolutionary implications of behavioural variability among animals. Personality traits are typically associated with mating acquisition in males, and, hence, linked to variation in their reproductive success. However, in most species, sexual selection continues after mating, and sperm traits (such as sperm number and quality) become very important in determining post-mating competitive success. Here, we investigate whether variation in personality traits is associated with variation in sperm traits using the guppy ( Poecilia reticulata ), a species with high levels of sperm competition. We found a positive association between boldness and sperm number but not sperm velocity, suggesting that bolder males have increased post-copulatory success than shyer individuals. No association was found between exploration and sperm traits. Our work highlights the importance of considering post-copulatory traits when investigating fitness consequences of personality traits, especially in species with high levels of female multiple matings and hence sperm competition.

The ability to recognize familiar and unfamiliar individuals is important as it plays a central role in many social interactions. Previous research has found that some animal species can discriminate among conspecifics, and recent findings indicate that some species are also able to discriminate among heterospecifics, including humans.

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Recent research has highlighted how trappability and self-selection—the processes by which individuals with particular traits may be more likely to be caught or to participate in experiments—may be sources of bias in studies of animal behaviour and cognition. It is crucial to determine whether such biases exist, and if they do, what effect they have on results. In this study, we investigated if trappability (quantified through ‘ringing status’—whether or not a bird had been trapped for ringing) and self-selection are sources of bias in a series of associative learning experiments spanning 5 years in the Western Australian magpie ( Gymnorhina tibicen dorsalis ). We found no evidence of self-selection, with no biases in task participation associated with sex, age, group size or ringing status. In addition, we found that there was no effect of trappability on cognitive performance. These findings give us confidence in the results generated in the animal cognition literature and add to a growing body of literature seeking to determine potential sources of bias in studies of animal behaviour, and how they influence the generalizability and reproducibility of findings.

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