Aggressive behavior is widespread in animals, although its use differs from species to species. Despite its wide use throughout the animal kingdom, aggression does come with costs which must be weighed against the benefits of a successful aggressive interaction. Aggressive behavior is widely studied in animals that have specialized weapons (e.g., claws, antlers, teeth) designed specifically for these interactions. Research on animals without specialized aggression weapons, however, is less prominent. Despite the lack of research on this topic, aggression in animals without weaponry is common. Animals often use alternative aggression tactics such as kicking, wrestling, or lunging at an opponent, as well as species specific intimidation tactics.
When imagining an aggressive animal, the first thing that comes to mind may be an elk, a bear, or another large “intimidating” animal. However, aggression appears in animals that we do not consider to be combative animals, such as a fruit fly. In fruit flies (Drosophila melanogaster) aggression is prevalent, particularly in relation to food and mating rituals. Aggression is usually observed among males and includes threatening wing raises, lunges, pursuit of other males, and boxing or wrestling. Male fruit flies who engage in more aggressive behavior are more likely to have mating success in both high and low density colonies, due to either their ability to attract and defend females for mating, or their ability to draw attention away from other competing males. Because successful aggressive interactions lead to higher mating success for dominant males, aggression has a great impact on the fitness (reproductive success and survival) of an individual fly.
Despite the benefits procured through aggressive interactions, heightened aggression is often selected for in only the most fit individuals because of the high maintenance costs it requires. In their recent research on fruit flies, Guo and Dukas (2019) hypothesized that although male fruit flies do not have any specialized features to aid them in their aggressive pursuits, there must be some sort of cost related to dominance and heightened aggression. They predicted that the costs of increased aggression in fruit flies would manifest in both physical and physiological ways.
To test these predictions, Guo and Dukas (2019) set up several experiments with aggression treatments for the fruit flies. The setup of each experiment was identical, besides a small change in the treatment given or the conditions flies were exposed to after their aggressive interactions. Each experiment also included a control group (a group who never experienced the aggression treatment), which served as a baseline for survivorship and damage. The basic setup began with a group of wild type male flies (the focal flies) and a group of selectively bred hyper-aggressive male flies. The flies were separated into individual vials. On each treatment day (four days in total), one focal male and one hyper-aggressive male were put together in an “arena” with a mixture of food in the center, to provoke aggressive behavior. After 8 hours of exposure to aggression, the males were separated and the hyper-aggressive male was discarded. The focal male was placed back into its individual vial (Figure 1.). Experiment #1 and Experiment #2 were identical up to this point.
After the 4-day experiment, each focal male was then inspected for any wing damage they may have incurred during aggression. Now, all that was left to measure was the survival of the flies exposed to aggression, compared with those who did not experience aggression. Each focal male was put into another vial to test their survivorship after aggressive interactions. From here, the researchers either deprived the flies of food (Experiment #1) or deprived the flies of food and water (Experiment #2). By doing this, they provided all of the flies with comparable post-experimental conditions, which allowed them to attribute any differences in survivorship to the aggression treatments rather than differences in food intake. The activity level of the flies was monitored until the flies died.
The results of this experiment revealed that there are significant costs of aggression in fruit flies. The group had originally predicted that there would be significant physical and physiological costs of aggression. Inspecting possible wing damage allowed the researchers to test the “physical costs” aspect of their aggression exposure hypothesis. This test revealed no significant differences in the amounts of wing damage in the control group and the treatment groups. This finding indicates that it is possible that physical damage may not be as prevalent in animals without weapons, particularly fruit flies. However, there were significant differences in the survivorship of males exposed to aggression and those who were not. In both experiments, the researchers saw a significant decline in survival for males who had been exposed to aggression. Because survival is a key factor in the overall fitness of an individual, a decline in survival indicates a reduction in fitness. Although Guo and Dukas (2019) did not explore the underlying mechanisms behind this difference in survivorship between treatment males and control males, their results indicate that there are significant costs of aggression, even in animals without weapons. Aggressive behavior can increase the fitness of an individual by increasing the likelihood of successful mating or feeding opportunities, but individuals who are involved in aggressive interactions likely decrease their survival rates by partaking in these activities.
References:
Guo X, Dukas R. 2020. The cost of aggression in an animal without weapons. Ethology 126: 24-31. https://doi.org/10.1111/eth.12956
