The effect of crowding and density on male mating behaviour in the rose bitterling (Rhodeus ocellatus)

Abstract

Female density and resource availability are two key variables that shape mating systems. Theory predicts that reproductive skew will amplify with increased male density and decreasing availability of resources, though limited empirical evidence suggests that this may not always be the case. Here we tested mean crowding, defined as the number of males per unit of resource, and density per se, defined as the number of individuals present per unit area, to investigate their effect on the mating system of Rhodeus ocellatus, a fish with a promiscuous, resource-based mating system. Males were exposed to combinations of high and low levels of crowding and density, while the operational sex ratio was held constant. High levels of crowding significantly affected the proportion of mussel spawning sites defended by males and the proportion of mussels into which sperm was released. In contrast to theoretical predictions, neither density nor crowding influenced overall male aggressive behaviours. Density, but not crowding, had a significant effect on male courtship rate, which arose as a possible trade-off between intra-sexual competition and inter-sexual behaviour. We discuss the results in the context of mating system evolution.

Cognitive ability is heritable and predicts the success of an alternative mating tactic

“…A prediction from our results is that those males suited to a guarder role will have greater reproductive success at low densities, where the reproductive success of guarder is known to be greatest [16,26]. By contrast, males with superior cognitive ability would be predicted to perform better at high male densities, where sperm competition and male ability to optimally distribute their ejaculates plays a more critical role in male reproductive success [16,26,36,46,47]. A predicted outcome is that selection on male cognitive traits will vary among populations, and within populations rspb.royalsocietypublishing.org Proc.…”

“…The mechanism by which spatial cognition might contribute to male reproductive success was not directly measured in this study. However, male reproductive success in bitterlings is closely linked to the way the male distributes ejaculates in space and time, particularly for sneakers [16,17,26,36,37]. In nature and in the laboratory, male bitterlings systematically patrol mussels in rspb.royalsocietypublishing.org Proc.…”

“…By contrast, males with superior cognitive ability would be predicted to perform better at high male densities, where sperm competition and male ability to optimally distribute their ejaculates plays a more critical role in male reproductive success [16,26,36,46,47]. A predicted outcome is that selection on male cognitive traits will vary among populations, and within populations rspb.royalsocietypublishing.org Proc.…”

“…B 282: 20151046 among breeding seasons, thereby maintaining variance for cognitive traits. Bitterling populations occur at highly variable densities [48], and males exhibit wide variation in behavioural and morphological traits [36], offering exceptional material for examining selection on cognitive traits in nature. Further investigation of the role of cognitive traits in species that express alternative mating tactics will demonstrate the generality of our conclusions for other mating systems.…”

Influence of sex on growth of juvenile Greenland halibut (Reinhardtius hippoglossoides) in the Estuary and Gulf of St. Lawrence

“…Factors that control these differences are numerous. In fish, sexual selection (Casalini et al, 2010), parental care (Dupont Cyr et al, 2018), niche separation (Gerritsen et al, 2010), and feeding characteristics (type of food, food intake, food conversion efficiency, search for food, and vulnerability to predators; Blair Holtby and Healy, 1990; Rennie et al, 2008;Horppila et al, 2011) have all been shown to influence SSD.…”