Resource quality afkcts the agonistic behaviour of territorial salamanders

Previous studies suggest that territories of red-backed salamanders, Plethodon cinereus, function as exclusive feeding areas, because prey are limited in availability during rainless periods. To test this idea, males were paired in laboratory chambers under three feeding conditions: (1) termites, a high-quality food resource, (2) ants, a low-quality resource and (3) ants plus termites. Termites are a superior food because they pass through the digestive tract significantly faster and yield significantly higher digestion efficiencies than do ants. Both territorial residents and intruders showed significantly more aggressive behaviour when higher-quality food had previously been fed to the resident than when lower-quality food had been fed to the resident. These data provide evidence that P. cinereus defends feeding territories and that the quality of a territory may be a function of the value of available food resources. Territoriality is the non-random spacing of sitetenacious organisms as a result of agonistic behaviour employed by an individual (resident) that keeps others (intruders) out of a certain site. An organism involved in such a contest should act in ways that maximize fitness by weighing the costs of defence against the benefits of obtaining the contested resource (Maynard Smith & Parker 1976; Parker & Rubenstein 1981). The quality of the limiting resource should be one of the most important factors in assessing the benefits of owning a territory. For example, if the quality of prey in an area is too poor for the individual to maintain a net positive energy budget, then that area is not worth defending as a feeding territory. While it has been shown that food resource quality benefits the inhabitant’s fitness in terms of access to mates (Miller et al. 1969; Walls et al. 1989) and habitat quality affects agonistic behaviour (Itzkowitz 1979), there has been less documentation of the influence of food quality on agonistic behaviour (but see Gill & Wolf 1975; Powers 1989). We hypothesized that the quality of a food resource is one of the dominant factors affecting agonistic behaviour for terrestrial salamanders. We studied the redbacked salamander, Plethodon cinereus, because its aggressive defence of sites is well documented in the laboratory (Jaeger 1981, 1984; Jaeger et al. 1982) and in the natural habitat (Gergits & Jaeger 1990; Mathis 1990). 0003-3472/95/010071+09 $08.00/O c The red-backed salamander is completely terrestrial, found under rocks and logs, and in leaf litter on forest floors of eastern North America. Both males and females appear to establish territories (Jaeger et al. 1982) in defence of limited prey against conspecific and congeneric competitors (Gergits 1982; Jaeger et al. 1982) during courting and non-courting seasons (Mathis 1989). Although red-backed salamanders forage on a wide variety of invertebrate prey (Jaeger 1972), prey are limited in availability between rainfalls (Jaeger 1980, 1990), and individuals maintain territories during such periods (Mathis 1989). A salamander would benefit from attempting to obtain a territory with high-quality food resources for two reasons: (1) on a daily basis, most ‘floating’ salamanders (those lacking territories) maintain lower energy budgets than territorial owners (Jaeger et al. 1981; Mathis 1991), and (2) gravid females prefer to associate with males having high-quality prey (termites) in their faeces, and thus in their territories (Walls et al. 1989; Jaeger & Wise 1991). Thus, we expected a salamander’s agonistic behaviour to be affected by the quality of food available in a territory. We predicted that prey quality is in part a function of digestion efficiency and gut-passage time for salamanders. A prey item that passes through a salamander’s digestive tract faster than another type of prey item would allow a salamander to begin foraging again sooner. For 1995 The Association for the Study of Animal Behaviour