The urine of male tilapia fish contains information about the male’s dominance. Males use this information to communicate their social dominance to other males. This acts as a competition deterrent amongst males. Male fish showed decreased aggressive responses when exposed to urine from dominant male fish.
The researchers in this study took tilapia from captive populations, and reared them in social group circumstances. During this time, they observed the behavior of the male fish regarding their dominance. Male fish were rated according to the level of dominant behavior they displayed. The researchers collected urine samples from the dominant male fish. Some of these were used in the experiment, some were analyzed for their chemical composition.
For the experiments themselves, the researchers put male fish into a tank and allowed them to acclimate for several days. Then, an opaque wall was removed to reveal a mirror, which simulated another male fish. The researchers decided to use mirrors as a way to ensure that the stimulus was only visual, not sound or another smell. It also means that the fish were not responding to size differences between themselves and the ‘other male’. When the mirror was revealed, urine from a dominant male was introduced into the tank, then the behavior of the male was observed. The researchers also tested different mixes of chemicals found in the urine to see if there was a difference.
The study found that male Tilapia that were exposed to the urine of dominant males were much less aggressive towards their reflections than males that were not exposed to urine. The study found something interesting in testing combinations of chemicals found in urine. In testing different chemicals extracted from the urine, researchers found no single chemical elicited the strongest response in lowering aggression. Researchers hypothesized that there might be a multicomponent effect, meaning that different parts of the urine contribute in different ways to the signal as a whole, making the effect greater than the sum of its parts. This is important, as it was documented that urine might be a signal between males, but that only one chemical component had been identified in a previous study.
There are a couple of limitations to this study. Researchers discussed how mirrors may not be the best way to test for aggression response, given that the testing male must always be the first one to make a move. It also does not allow for different positioning and posturing in aggressive responses, which is a part of that behavior, only allowing for head on interactions. This may have contributed to the lower number of males that showed aggression at all. Despite these drawbacks, this is a reasonably robust study in showing the ways by which male Tilapia signal to each other, meaning that we can fairly confidently draw conclusions from what they found.
Signal ecology is a complicated field, and the area of chemical communication is relatively unstudied compared to things like sound and vision. The more we know about the communication systems of these fish, the better we can understand them. Fish are often negatively impacted by chemicals released by humans into waterways and aquatic habitats, and if we are to prevent further losses, knowing about how fish respond to chemical input is valuable. Additionally, given Tilapia’s value in aquaculture, it is a valuable organism to study.
Keller Costa, T., Saraiva, J., Hubbard, P.C., Barata, E.N., Canario, A.V.M., 2016. A multi component pheromone in the urine of dominant male tilapia (Oreochromis mossambicus) reduces aggression in rivals, Journal of Chemical Ecology, 42, 173-182.