Territoriality and Behavioral Regulation of Population Density

Territoriality is a complex phenomenon, which includes gains in fitness due to resource monopolization in the territory and the cost of defense that increases with territory size and with the number of floaters challenging territory owners. The theoretical models of territoriality allow for several outcomes depending on the value of environmental parameters: population density is regulated either by limited number of individuals that are territorial and the exclusion of nonterritorial individuals from reproduction or by changes of territory sizes. Territories are of different qualities and usually the best ones are the first occupied. An open question is why some individuals accept territories of much lower quality than others do, and why some accept the status of floaters. There are several hypotheses explaining it, all of them based on the concept of evolutionarily stable strategies. First, the evolutionarily stable bourgeois strategy predicts that an owner should win a game over a challenger; however, this theory allows also the so-called paradoxical bourgeois strategy in which a challenger wins. Second, stronger individuals usually win and the territory owners can be stronger either before they obtained territories or because ownership makes them stronger. Third, sometimes keeping a worse territory with poorer reproductive output is better than being a floater without a possibility to reproduce. Fourth, floaters still have a chance to reproduce in the next season and they may gain some advantages: higher probability of survival than territory holders, which have to defend territories and pay cost of the reproductive effort. A younger or a weaker individual may obtain higher lifetime reproductive success by refraining from reproduction in 1 year and reproducing later, stronger and more experienced. Theoretically, it is possible that within a single season individuals differ in their ability to survive and reproduce, but their lifetime reproductive success is very similar. Large differences within one season prevent population extinctions at the time of temporary resource shortages, and generally make the population more stable.

According to the Darwinian theory of natural selection, organisms should maximize their reproductive output, bringing about population increase to the point of resource shortage. If there is sufficient amount of resources but some individuals do not reproduce and the population size is at a level lower than possible, it seems to be against the theory. Such a phenomenon can result from the territorial behavior but it also occurs in some populations without a clearly expressed territorial-ity. For example, population of prairie deermice brought from the field was 5 times smaller than the same species kept for 30 years in captivity. Since the food, water, and nesting sites were supplied ad libitum, the population increase has been prevented by the more aggressive behavior of wild deermice, as compared to those in captivity. Laboratory mice populations are also able to stabilize their number much below the level available by the amount of food, water, and nesting sites. However, any changes of the cages, in which these populations are kept, for the new cages, even smaller than the former ones, result in temporary population increases to much higher density. It seems to be due to the destruction of social hierarchy that prevented population increase, social hierarchy, which is somehow related to a spatial distribution of individuals within cages.

Conclusively, competitive behavior may express itself by formation of territories and by social hierarchy, not by direct competition for resources, and it allows for lower population size than physiologically possible. Such behavior does not require group selection and it can be explained within the framework of Darwinian individual selection.

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