This experiment aimed to assay the levels of aggression in male and female bluefin killifish (both within and between the sexes) and to determine the extent to which aggression led to detrimental effects on survival. We manipulated the sex ratio and density of bluefin killifish in 110-liter tanks and observed behavior and survival. We manipulated sex ratios and density to create treatments where competition and aggression were likely to differ between males and females. We created a treatment with a male-biased sex ratio (3 males: 1 female) and a female-biased sex ratio (1 male: 3 females). We also had two controls with an even sex ratio to control for the density of each sex (1 male : 1 female and 3 males : 3 females). Six replicates were performed across the four treatments (24 tanks in total). The experiment was carried out between February 2020 – July 2020. The experimental tanks were initially set up in late February 2020. Due to the COVID-19 pandemic, they remained in those conditions for six months. During this time, tanks were censused, fish mortality was noted, and dead fish were replaced. We measured survival as the proportion of individuals alive in each treatment, which was the total number of individuals of each sex alive divided by the total number placed in the tank. We conducted behavioral observations from June - to July 2020. Fish were marked using a fluorescent elastomer dye injected underneath their skin, enabling individual identification within a tank (Northwest Marine Technology, Inc.). Each fish in each tank was observed once for a duration of five minutes. During these observations, we recorded the number of chases and attacks exhibited by both males and females. The number of fin flares and courtship bouts was also noted for males. For chases and attacks, we recorded the sex of the recipient of aggression by the focal individual.

The process of obtaining mates, mating, and (potentially) caring for offspring is costly. While there are inherent costs to reproduction, behavioral interactions among individuals are often the primary drivers of reproductive costs. Males frequently compete for territories, and females may compete for food or males; males often harass females. Here, we sought to determine whether reproductive costs were primarily due to male/male competition, female/female competition, or male/female interactions in the bluefin killifish. In this species, males guard small spawning territories where females visit them daily to spawn. To manipulate the potential for male and female competition and male/female interactions, we altered the sex ratio and density of each sex across four treatments (1 male: 1 female, 1 male : 3 females, 3 males: 1 female, 3 males: 3 females). Female mortality was higher than male mortality. Surprisingly, female mortality and male aggressive behaviors towards females (i.e., chases) were highest in treatments with a single male. Male-male aggression was present, but males often resolved these disputes via signaling by flaring their fins. There was little evidence for overt aggression among females. When males lack rivals, they turn their territorial defense towards females. These costs help explain why, in nature, females promptly leave male territories following spawning and join loose shoals with conspecific females and minnows.

Funding provided by: N/ACrossref Funder Registry ID: 0