Guest Post by Stephen Heap, PhD Candidate University of Melbourne
Brown (1964): ‘The evolution of diversity in avian territorial systems’, The Wilson Bulletin 76, 160-169
Colonial breeding gannets (Morus bassanus), wide-ranging wolf (Canis lupis) packs and courting poison arrow frogs (Dendrobates pumilio) each have a vastly different style of territory? Can they all be explained by the same concepts? Jerram Brown demonstrated that they could way back in 1964, and his ideas have influenced our understanding of territorial behaviour ever since.
Many animals are territorial at some stage of their life, whereby they act to exclude other individuals from using a specific area. Territories can be used for a variety of reasons, depending on the species in question: from breeding to feeding to courtship or all of the above. Furthermore, the size and shape of territories, as well as the behaviours that are expressed within them can vary between different animals. Up until 1964, people were unable to provide any consistent way of thinking to explain this variation. Each type of territory seemed so unique and there seemed to be no way to explain them all with reference to the same processes. One of the more robust theories suggested that territoriality was a mechanism of population control, in which a group of animals would be territorial to impose a constraint on survival and reproduction to avoid having a population crash. However, any modern student of evolution will realise that this mechanism doesn’t exactly fit in with the concept of the selfish gene, which states that every individual acts to maximise its own fitness even if this comes at the cost of reduced population fitness.
The eventual breakthrough came from the mind of Jerram Brown, who applied an economic framework to territoriality. Simply put, competition for limited resources (such as food or mates) can lead to the evolution of aggressiveness because aggressive individuals can gain priority access to the required resources. However, aggressiveness can be costly in time and energy, so it is adaptive to be only aggressive in certain circumstances. When space translates to food, mates or other resources, then site-related aggressiveness can be selected for. Whether or not it is adaptive to defend an area is determined by the benefits of having access to resources and the costs of preventing others from gaining that access. Brown termed this trade-off economic defendability and it now serves as the central concept of understanding territorial behaviour. For example, colonial bird territories encompass a nest-site and little else. The benefit provided by the territory is access to a suitable site for laying eggs, which only requires a few square feet. Thus, any effort expended towards excluding others from further away would be wasted. In contrast, birds of prey can have territories that extend for miles. In these cases, it is beneficial for individuals to chase away competitors in order to monopolise a hunting ground. Similarly, frogs, insects or birds that attract mates with acoustic signals require an area that is as free from interference as possible, and will thus keep others from using the same space. Overall, the type of territorial behaviour that evolves in a species is determined by the resources being competed for and the degree to which these resources are economically defendable in space.
The concept of economic defendability is still used to explain the evolution of territorial behaviour to this day. Early research expanded the concept from its initial application in bird systems to territoriality in general. The past decades have then principally focussed on exploring how the size of territories respond to manipulations in resource availability and pressure from territorial intruders. For example, adding food resources to a territory can result in the territory shrinking, because the resident can more efficiently provide for itself with a smaller area. However, these studies are finding a great deal of variation in how territories respond to manipulations and we currently lack a theory that can consistently explain this variation. Thus, history has come full circle and researchers interested in territoriality now find themselves in the same position as those before the concept of economic defendability. Current lines of enquiry include studying the conflicts between territory owners and those seeking to establish their own territory, the manner in which an owner balances defence of its territory with exploiting its resources, the influence of the physical landscape and the interactions that take place between territory owners. Now we need someone to bring it all together and provide an improved way of thinking, just as Jerram Brown did all those years ago.