In the animal kingdom, being the alpha male in the group offers many benefits. One of the main benefits is having the first choice of mating partners. In many species, females prefer the dominant males to mate and have offspring with. However, it’s still not completely understood how females know who the dominant male is.
In 2010, Italian researchers, Laura Aquiloni and Francesca Gherardi tried answering this question in invertebrates, specifically female crayfish (Procambarus clarkii). It had been previously shown that female crayfish took part in social eavesdropping, where they watched male crayfish fighting. Afterwards, the females would chose the winner of the fight to mate with. It was also speculated that female crayfish may determine the winner based on smell since male crayfish release pheromones during a fight that can signal dominance. To further understand this phenomena Aquiloni and Gherardi explored if both vision and smell are used by females to determine the winner of a fight and how that information is interpreted.
The researchers created five experimental conditions to test their questions: treatment C, treatment V, treatment CV, treatment RCV and the control treatment. In treatment C, the female crayfish was placed in an opaque box with holes that only allowed it to smell but not see. In treatment V, the female was placed in a transparent box that did not have holes that allowed the female crayfish to see but not smell. In treatment CV, the crayfish was placed in a transparent box with holes that allowed it to see and smell. In the control treatment, the crayfish was placed in a opaque box that did not have holes. In treatment RCV, a female from the treatment CV was introduced to two males that the female had never smelled or seen before.
The experiment was carried out in two phases that each lasted 30 minutes: the fight phase and the choice phase. During the fight phase, three crayfish were placed in the tank. The female crayfish was placed in a container that corresponded to its treatment while two males were allowed to roam free in the tank. After the two male crayfish interacted and fought, the experiment would move on to the choice phase.
During the choice phase, the two males were secured at opposite ends of the tank with string and only the female crayfish that had been placed in the container earlier had been allowed to roam freely. The researchers collected data on who was the first male visited, how many times the male was visited, how long were the visits, and number of contacts between the male and female crayfish.
The researchers found that female crayfish that were able to see and smell the male crayfish fighting visited the winner first, made more visits to the winner, spent more time in the winner’s proximity, and made more contacts compared to the loser. Compared to the other treatments, only the CV treatment, that allowed female crayfish to see and smell the males during a fight, showed significant difference between the duration of visits, number of contacts, and first male visits.
This data shows that both visual and olfactory cues are needed by female crayfish to determine who is the dominant male. The researchers suggested that having both visual and olfactory cues allow female crayfish to chose a suitable mating partner in their natural crowded environments. Even more so, it implied that female crayfish are able to recognize individual males based on the dominance they displayed earlier.
This study clears up some of the misconceptions surrounding mating choice in crayfish. Previously, it was believed that female crayfish may chose dominant males based off of a particular trait, such as pheromones or claw size. However, this study provides evidence that not only do female crayfish view male crayfish as individuals and use visual and olfactory cues to determine dominance but are also capable of remembering which crayfish displayed dominant behavior.
References
Aquiloni, L., & Gherardi, F. (2010). Crayfish females eavesdrop on fighting males and use smell and sight to recognize the identity of the winner. Animal Behaviour, 79(2), 265-269.
