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Some territorial species have been shown to react more aggressively toward unfamiliar conspecifics than established neighbors, a behavior referred to as the "dear enemy" phenomenon. Laboratory experiments were conducted to determine whether males of a territorial African cichlid, Neolamprologus pulcher, can discriminate between size-matched familiar and unfamiliar male neighbors and whether they respond more aggressively toward unfamiliar males. It was found that males spent significantly more time near the territorial boundary that they shared with unfamiliar neighbors and directed significantly more aggressive behavior toward these individuals.

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We investigated determinants of territory size in White-winged Trumpeters (Psophia leucoptera), frugivorous, cooperatively breeding birds that defend large, permanent, all-purpose territories in lowland Amazonian rain forest. During a 7-mo period, we measured changes in food density at a site in Manu National Park, Peru, and conducted full-day focal samples on individuals that allowed us to record daily food intake of territorial birds. We used data from laboratory analyses on energy content of fruit species eaten by trumpeters to calculate a phenology of energy content for food available on territories of focal groups, and to calculate daily energy intake of focal trumpeters. Additionally, we used lab measurements of basal metabolic rate of trumpeters to estimate daily energy requirements of wild trumpeters.Both census and focal sample data suggested that food availability on trumpeter territories varied widely, sometimes greatly exceeding and sometimes falling short of the amount that we calculated was necessary to meet average daily energy requirements of groups. To determine whether trumpeter territory size was related to food density, we conducted fruit removals. Every day, we removed an amount of fruit from the territory that was equivalent to the amount of fruit ingested daily by the focal group during a control period preceding the removal experiment. We conducted removal experiments during periods when food density on territories appeared to be abundant and also when food abundance appeared to be low. Trumpeter food consumption did not change when fruit was removed during periods of resource abundance; however, when experimental removals were conducted during periods of food scarcity, the total energy value of food consumed decreased significantly. These results suggest that food abundance on trumpeter territories sometimes exceeds and sometimes is close to or below the amount needed to meet energy needs of the groups. Based on our data, we propose that territory size of White-winged Trumpeter groups reflects some minimum size needed to provide the group with a baseline level of food intake during seasonal periods of decreased food abundance.