EFFECTS OF NATURAL VARIATION IN SEX-RATIO AND HABITAT STRUCTURE ON MATE-GUARDING DECISIONS IN AMPHIPODS (CRUSTACEA)

We employed field-based studies, with complementary laboratory-based s tudies, to investigate social and environmental influences on tactical mate-guarding decisions in amphipods (Crustacea). Firstly, we investi gated variation in precopulatory mate-guarding duration in Gammarus du ebeni celticus in relation to the social structure of natural populati ons. Variation in population density of up to two orders of magnitude had no effect on precopula duration, whereas guarding durations increa sed as the sex ratios of the populations became more male biased. That is, males have some ability to assess the probability of other males taking females into precopula and are prepared to guard for longer as this threat of male: male competition increases. A field demonstration of tactical shifts in reproductive behaviour in response to pertinent social conditions is thus provided. Secondly, the 'habitat segregatio n' hypothesis, which proposes that positive size-assortative pairing i n amphipod populations arises due to variation in the use of micro-hab itats, was tested in natural held populations and under laboratory con ditions in Echinogammarus marinus. This was necessary in order to dist inguish any purely environmental determinants of size-assortment from the role of active decisions by males concerning mate choice and male: male competition. The hypothesis was rejected on the grounds that siz e-assortative pairing arises under both heterogeneous and homogeneous environmental conditions. Further, in both study species, male and fem ale body size were positively correlated with precopula duration. Thus , indirect competition for access to large, fecund females, based on t he timing of male entry into precopula, together with direct aggressio n, provides the explanation for size-assortative pairing in amphipods.