Tactile stimuli trigger group effects in cockroach aggregations
Résumé
In many gregarious species, social interactions among group members have such a potent influence on the physiology and/or the behaviour of individuals that isolation has dramatic consequences for their development and survival. Although 'group effects' have been described in many insect species, the mechanisms involved in these processes remain poorly documented. Our aim was to shed light on group effects in cockroaches by investigating the nature of the social stimuli triggering differences in developmental rates between grouped and isolated individuals. Group effects were absent in nonaggregating species (Blattella lituricollis, Blattella biligata), but present in species that form social aggregations (Blattella germanica, Symploce pallens). The presence of conspecifics increased rates of both nymphal development and oothecae production in B. germanica. Tactile stimuli were sufficient to trigger developmental group effects in this gregarious cockroach. Blattella germanica nymphs reared with tactile stimuli, either from a rotating feather or from insects of other species, grew faster than isolated nymphs. Although the role of tactile stimuli has been understudied, they could be involved in group effects in many insect species. Living in groups is widespread and provides different kinds of benefits such as decreased predation risks, decreased energetic costs of movement, increased foraging efficiency or increased encounters with potential mates (Krause & Ruxton 2002). Although a large variety of animals aggregate, the mechanisms responsible for group formation and complexity of communication differ between species (Parrish & Hamner 1997). Depending on these characteristics, conspecifics can have such a potent influence on the physiology and/or behaviour of individuals that social isolation has dramatic consequences for their development and survival (e.g. Harlow 1965; Wilson 1971; House 2001; Zeigler & Marler 2004; Perelló a et al. 2006). Two main categories of aggregations emerge from the many definitions that have been given (Parrish et al. 1997). (1) In nonsocial aggregations, individuals are attracted by a given source but do not interact socially and they disperse when the source is consumed or vanishes. In this case, large increases in density at a source can induce microenvironmental modifications that happen to favour development. For example, under crowded conditions some dipteran and lepidopteran nymphs benefit from better regulation of body temperature and of water evaporation than singletons and thus their developmental rates increase (Clark & Faeth 1997; Slone & Gruner 2007). These differences between grouped and isolated individuals have been termed 'mass effects' and are generally observed in large groups of animals (Grassé 1946). (2) In contrast, social aggregations are formed and maintained by mutual attraction among their socially interacting members. In this case, differences in physiological , morphological or behavioural traits between grouped and isolated individuals are mainly attributable to social interactions and have been called 'group effects' (Grassé 1946). They differ from mass effects in that they are caused by the perception of social stimuli emanating from conspecifics and can be observed as soon as only two individuals are grouped (Grassé 1946). Group effects have been described in many insects including orthopterans, aphids, coleopterans and blattodeans; their best-documented consequences pertain to modifications of developmental rates or group-induced phenotypic plasticity. Grouping often accelerates nymphal development (Chauvin 1946; Long 1953), although it can delay imaginal moulting in a few