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

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614 Sociobiology Vol. 58, No. 3, 2011 and roots) in the diet of E. brunneum (=quadridens). A preference for ants was also observed in E. permagnum, which showed a diet consisting almost exclusively of workers and winged individuals of Pheidole and Camponotus (Paiva & Brandão 1989), and in Pachycondyla striata, which includes in its diet foragers of Odontomachus, Camponotus, Pheidole, Solenopsis, Atta, and even other Pachycondyla (Medeiros & Oliveira 2009). The proportion of ants preyed on by E. ruidum was observed to be similar both in the dry (18.8%) and in the rainy (17.2%) season. Although its diet consisted of 19 items, almost 30% of them corresponded to liquids rich in carbohydrates, such as extrafloral nectar, honeydew, and fruit pulp (Lachaud 1990). Similarly, in E. tuberculatum, liquid foods represented more than 35% of its diet (Valenzuela-González et al. 1995). Collecting resources of animal origin requires longer foraging distances; capturing and transporting them is more time consuming compared with liquid foods (Harkness & Harkness 1976; Fourcassié & Oliveira 2002). This “preference” for other ants is not likely to be due to selectivity by the foragers, but to the low diversity of food items available in the environment. This can be proved in the laboratory, since workers of E. opaciventre easily accept other food items, such as mealworms, which are not found in their natural environment. The predatory behavior observed in E. opaciventre was very similar to that found by Pie (2004) in the Brazilian savanna and by Tofolo and Giannotti (2009) in the laboratory: prey aggressiveness determined the type of approach used by the workers. As a consequence of the absence of recruitment previously described for this species, the size of the food item collected was dependent on the capacity of the worker to carry it without help, one at a time. This behavior is characterized as social facilitation, that is, the return of a forager to the nest that may increase activity inside the nest, leading other collectors to leave and search for food sources even in the absence of a chemical trail or any other directional information, as observed in Pachycondyla goeldii (Orivel 2000) and E. ruidum (Lachaud 1985). In the degraded area studied, the foragers of E. opaciventre did not stay more than 5.14 m away from the nest. A similar behavior was observed by Medeiros and Oliveira (2009) in numerous colonies of Pachycondyla striata. Fourcassié and Oliveira (2002) reported that, in Dinoponera quadriceps, this distance was more than twice as long, reaching 13 m. Pie (2004) also
Tofolo, V.C. et al. — Spatial Pattern of Foraging in E. opaciventre recorded site fidelity in the area explored by workers of E. opaciventre, that is, the forager tended to have a preference for a specific foraging pathway, which made it a specialist in the individual exploration of sub-areas, becoming able to return to the nest quickly. The same pattern was observed in Pachycondyla apicalis (Fresneau 1985; Deneubourg et al. 1987), D. quadriceps (Azevedo 2009), D. gigantea (Fourcassié & Oliveira 2002), Cataglyphis exhibits, and C. bicolor (Buchkremer & Reinold 2008). Fresneau (1985) proposed a simple mechanism to explain such fidelity: during the first trip, new foragers select a portion of the foraging area, which is confirmed by the capture of the first prey. Through a simple learning process, the ants keep searching in the same area during the following trips. If foragers from the same colony meet in an overlapping area, both make intense antennal contact before leaving for different pathways. This mechanism induces the forager to decrease search for food in areas used by other foragers, increasing the effectiveness of foraging on a colony-basis. Gordon (1995) believes that following the same direction enables foragers to explore a specific fan-shaped area, so as to deplete the resources in one direction before moving to the adjacent area. This way, when they return to the initial position, the resources are available again. This was also observed in P. striata (Medeiros & Oliveira 2009), in the desert ant Veromessor pergandei (Rissing & Wheeler 1976), and in the army ant Eciton burchelii (Franks & Fletcher 1983). Seemingly, the individual foraging pathways of E. opaciventre are spatially oriented so as to avoid overlap with those from other nests, thus minimizing competition with individuals of other species. Considering that collectors do not use chemical markers on their foraging trails, researchers believe that landmark cues contributed primarily to the orientation of these ants in the field, as observed in D. quadriceps (Azevedo 2009), D. gigantea (Fourcassié et al. 1999), and Paraponera clavata (Deneubourg et al. 1987, Baader 1996). There are no studies proving magnetic orientation in E. opaciventre. Dinoponera quadriceps also presented a positive correlation between maximum distance to the nest and duration of foraging trip, with a sinuous movement pattern and directional preference (Azevedo 2009). According to Bonser et al. (1998), the time spent in the foraging area depends on the nest distance, and further areas must be explored less frequently and for longer 615
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