Female mosquitofish prefer males that have longer genitals, according to Australian research.
“This is a relatively novel result, as selection on genitals is generally thought to occur during or after copulation,” say the authors of the study “Females prefer to associate with males with longer intromittent organs in mosquitofish,” published this month in the science journal “Biology Letters.” The research was done by Andrew Kahn, a graduate student at the Australian National University, and others.
According to the abstract of the paper, sexual selection is a major force behind the rapid evolution of male genital morphology among species.
“Most within-species studies have focused on sexual selection on male genital traits owing to events during or after copulation that increase a male’s share of paternity,” the abstract says.
“Very little attention has been given to whether genitalia are visual signals that cause males to vary in their attractiveness to females and are therefore under pre-copulatory sexual selection.”
To look into this dearth of knowledge, the researchers “reduced” male mosquitofish genitalia to varying lengths, as shown in the photos on the right, then tested the reactions of the females.
“On average, female eastern mosquitofish Gambusia holbrooki spent more time in association with males who received only a slight reduction in the length of the intromittent organ (gonopodium) than males that received a greater reduction,” the researchers observed.
“This preference was, however, only expressed when females chose between two large males; for small males, there was no effect of genital size on female association time.”
Male mosquitofish do not court females, but rely on forced matings, according to a news statement about the research released by Biology Letters. “This means association preferences likely lead to mating biases. Thus, it appears size really does matter for these little fish.”
Photos courtesy Australian National University