Phylogénie Et Evolution Du Comportement Social Chez Les Blattes ...

Ev o l u t i o n d u c o m p o r t e m e n t s o c i a lcovered with a clean filter paper during the first half of the dark period (using red light –Koelher et al., 1987). A transparent plastic cover, (10 x10 cm) was immediately placed abovethem. This cover was high enough (0.2 cm) to permit cockroach larvae to move freely, butlow enough to induce permanent dorsal contact (preventing artefactual aggregation caused bythigmotactic stress). After 15 minutes, their position was scanned using a video tape-recorder(8 mm, Canon Hi) placed up to the experimental device. The nearest neighbor distance (NND)was measured for each larva at this moment. Five replicates for each group were done. Themedian (m) of the nearest neighbor distances was compared between the four species by amultisample median test (Zar, 1999: 200-201), followed by a Tukey-type multiple comparisontest (q – Levy, 1979).This protocol has been slightly modified for the burrowing species P.b. for which fewspecimens were available. Only five individuals were placed in the arena which was modifiedaccording to P.b. way of life (gallery burrowing) to avoid thigmotactic stress as alreadyexplained for the four other species. The modification consisted in the addition of a layer oftransparent marbles. Then, these observations cannot be strictly compared with the previousones but bring informative data about tolerance to crowding for this species.Dyadic interactionsDyadic interactions were characterized with ad libitum sampling (Altmann,1974; Martin and Bateson, 1986) of the behavioral events for each species in groups of 6cockroaches including both males and females. They were placed in the same device aspreviously, except that the plastic cover (13 x 9 cm) was high enough (1.5 cm) to permit themto exhibit more diverse behaviors. The density was high enough to promote interactions andto cause the species to display their richest repertoire (Breed and Byers, 1979). One hour later,a continuous recording observation period of 15 minutes began. However a first experimentshowed that this protocol was not adapted to P.b. Indeed, after one hour, all the individualswere aggregated in a corner of the arena, showing thigmotactic stress. For this reason,observations of P.b. started five minutes after placing individuals in the arena.320