Animal contests in nature

Next time you stand on a seashore and look carefully with your ‘zoologist's eyes’, you may be surprised at the high diversity of animal phyla that are present, even within a single intertidal rock pool. If you are patient and can stay still for a few minutes, another surprise in store is the preponderance of aggressive behaviour demonstrated by the intertidal fauna. Depending on which part of the world your rocky shore is in, you might observe some of the following: male Azorean blennies fighting over the nests that they need in order to attract females; pre-copula pairs of shore crabs with inter-male aggression over the ownership of recently moulted females, as these females are only receptive to sperm during a brief post-moult period; common European hermit crabs rapping in an attempt to evict an opponent from its gastropod shell; and, if you really have a lot of time on your hands, you might notice slow-moving sea anemones striking one another with special tentacles called acrorhagi, during disputes over space. Of course, aggressive behaviour is not restricted to intertidal marine animals. Take a walk in the woods and you could witness aggression over the ownership of territory; this is one of the reasons why male birds sing, why male butterflies perform many of their aerial displays and why armies of female worker wood ants try to kill individuals from a different colony. These examples illustrate two important points about aggression: first, animals will fight over a range of resources, when the ability to access those resources is a major constraint on fitness. In many cases this involves conflict over access to mates, as in the case of shore crabs. However, other resources such as territory, food and shelter are also contested, and influence the fitness of females as well as males. The second point is that aggressive behaviour is extremely widespread among animal taxa: these examples alone are drawn from three different phyla: chordates, arthropods and cnidarians.