Aggregation and Nourishment

It has been suggested that one of the main functions of gregarious behavior in cockroaches is to signal to unas-sociated individuals the proximity of food and water (Wi-leyto et al., 1984). The addition of extra animals to a group, however, results in both added competition for food and higher travel costs (Chapman et al., 1995). Cockroaches in aggregations are central place foragers; they travel from a central location to forage elsewhere, then return to shelter. Short-range foraging is the rule in B. germanica, and food patches placed near shelters are depleted before patches placed farther away (Rivault and Cloarec, 1991; Rierson, 1995). When overcrowded, however, individuals are known to move more than 10 m (Owens and Bennett, 1983). Large, persistent aggregations no doubt depend on constant renewal of food resources in the vicinity of the harborage, such as dirty dishes left in the sink at every meal or the regular deposition of guano by bats.

In gregarious cockroaches, social facilitation in meeting nutritional requirements may occur within two contexts: (1) in locating and ingesting food away from the harborage, and (2) in the use of food originating from conspecifics within the harborage. Individuals of B. germanica forage individually but often converge on the same sites (Rivault and Cloarec, 1991), suggesting that there may be a social component to food finding. Trail pheromones (Chapter 9) may facilitate movement from the harborage to renewable food sources (a garbage can, for example). In habitats where food is unpredictable, ephemeral, or patchily distributed, a different form of social facilitation may occur. Cockroaches leave behind at feeding sites a variety of residues in the form of saliva, glandular deposits, and fecal pellets. Feeding sites that are "marked" by these residues may be more attractive than unmarked food patches because, whether or not foraging cockroaches are present, the food has been made "visible" by the traffic of conspecifics. If so, cockroaches exhibit the simplest form of food-related grouping behavior: local enhancement—the act of cueing on conspecifics for food information (Mock et al., 1988). Attraction to residues by cockroaches would be the chemical equivalent of the visual attraction of birds to feeding flocks, or the acoustic attraction of bats to the echolocation calls of conspecifics (Richner and Heeb, 1995). Cockroaches show a number of similarities to rats, which are nocturnal, omnivorous, central place foragers that leave chemical cues in the form of urine and fecal pellets on resources (food patches, nest sites) used by other rats. These residues provide a mechanism for social learning and are used in a variety of contexts (Galef, 1988; Laland and Plotkin, 1991).

The benefits of cueing on foraging conspecifics can be considerable for young nymphs, who do much better de-velopmentally on the same food source if an adult is present. The adults seem to "condition" the food in some way, either by moistening it, breaking it into smaller pieces, or making initial excavations into a tough food item. Both Blattella and Supella have been observed depositing saliva on food (C. Schal, pers. comm. to WJB), and the development of B. germanica nymphs fed whole dog food pellets was slower by approximately 43% than nymphs that were fed the same food, but pulverized (Cooper and Schal, 1992).

Nutritional advantages of associating with conspecifics may also occur within the harborage. The exuvia, corpses, feces, exudates, oothecal cases, embryonic membranes, and unviable eggs produced by individuals in an aggregation as they progress through their lives are fed upon by other members of the group (reviewed by Nalepa, 1994) (Table 4.6). The presence of this proteinaceous food in the harborage may be of particular value to females and to young nymphs, as it is these stages that have the high est nitrogen requirements. Juveniles in particular may benefit from a ready source of high-quality food for several reasons. First, young insects have relatively small reserves, a high metabolism, and nutritional requirements that differ from those of adults (Slansky and Scriber, 1985; Rollo, 1986). Second, young cockroaches are inefficient in their foraging behavior, and typically do not forage far from shelter (Cloarec and Rivault, 1991; Chapter 4). Third, as noted above, young nymphs have difficulty processing physically hard food. High-quality, easily processed food that originates from conspecifics in their immediate vicinity may allow the young to pass more quickly through the stages during which they are most vulnerable.

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