Diet and Spatial Pattern of Foraging in Ectatomma opaciventre ...

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610 Sociobiology Vol. 58, No. 3, 2011 carried to the nest in their mandibles. The observations were made in the dry season from 14:00 to 18:00, which corresponds to the period of the highest foraging activity for this species in the afternoon (Pie 2004). The total time of observation was 72 hours. The food collected was stored in 70% alcohol for further identification. RESULTS Foraging pathways The workers of E. opaciventre foraged on average at a maximum distance of 2.18m ± 1.6m (SD) (range: 0.40-5.14 m), spending on average 28.1 ± 15.6 min (SD) (range: 8- 66) outside their nest. The maximum foraging distance from the nest and the trip duration are presented in Table 1. Workers of nests 1, 2, and 3 showed no significant difference in the distance or time of foraging (p> 0.05). The Spearman’s rank correlation coefficient for the variables time and distance was r = 0.848 (p
Tofolo, V.C. et al. — Spatial Pattern of Foraging in E. opaciventre The foraging trails presented a sinuous pattern from the beginning to the end, with directional fidelity (Fig. 2). The pathways from nest 1 were concentrated around the nest along the north-south axis; in nest 2, they were distributed southeast and northwest to the nest; and in nest 3, they were west-oriented. Thus, the foragers were spatially distributed in the foraging area so as to avoid overlap with the trails from neighboring colonies. These areas were explored repeatedly, demonstrating the preference of colonies for specific regions in the foraging arena. In the 3 colonies, most trajectories were close to the nest, with few pathways more than two meters away from it. Diet and foraging effort During the observation period, few foragers left the nest in search of food resources. The workers of E. opaciventre left the nest on average 5.4 ± 2.7 (SD) times per foraging hour, returning almost at the same rate, 4.9 ± 2.5 (SD) times. Of the 371 foraging trips recorded, only 9.16% ended with the workers returning with prey. Liquid food was not observed in the mandibles of the foragers, only solids. The description of the prey captured is shown in Table 2. Over half of the prey items collected (52.78%) belonged to Formicidae, including an individual of the same genus, E. brunneum. All the prey were collected individually except a chrysalis of Lepidoptera for which there was a recruitment strategy known as social facilitation. In this event, an individual recruited a nestmate to collect the prey item without using a scent trail. As the prey was too large and fixed to the substrate by silk threads, the forager quickly returned to the nest and then left, followed by another ant. The recruited ant followed the recruiter to the food source and both cut the silk threads to collect and transport the food item to the nest. DISCUSSION The diet of E. opaciventre was narrow, probably because the nests were located in a degraded area and the research was carried out in the dry season. In the Brazilian savannah (Cerrado), Pie (2004) observed that, in 50% of the cases, the diet of E. opaciventre is composed of termites, and over 20% of the other prey consists of other ants, mainly those of the genus Atta. Wheeler (1986) analyzed the diet of E. tuberculatum in Panama, also using the method of collecting prey items that workers carried to the nest. Insects 611
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Diet and Spatial Pattern of Foraging in Ectatomma opaciventre ...

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