Aresearch study quantifies the fitness of individuals that defend territories of different size and finds that individuals that defend a territory of 10 m2 have the highest fitness. this is an example of:

The research outcome that individuals that defend a territory of 10 m2 have the highest fitness is an example of ultimate explanation of territorial behavior.
 The term territorial behavior denotes the methods by which an animal, or group of animals, defend its territory from incursions in certain territorial boundaries. The territorial boundaries define the area in which the animals live and  on which they depend for resources.