The effect of variation in boldness and aggressiveness on the reproductive success of zebrafish

Boldness and aggressiveness are two behavioural traits that have received extensive attention in the field of animal behaviour. However, relatively little is known about how these traits are maintained in populations and the fitness of individuals that exhibit them. We tested the effect of boldness and aggressiveness on the reproductive success of zebrafish, Danio rerio. Using behavioural tests, we established groups of males that varied consistently in their boldness (bold, middling and shy) as well as groups of males that differed in their aggressiveness (aggressive, middling and nonaggressive) and paired them with randomly selected females. We found no difference in the total number of eggs laid by females mated to bold or aggressive males. However, the number of fertilized eggs differed, with the boldest and the most aggressive males fertilizing more eggs than the other groups. Furthermore, the proportion of fertilized eggs differed between groups. These results show that an individual’s reproductive fitness can be associated with behavioural variations in boldness and aggressiveness.