Both physiological and environmental constraints can be important selective factors on behaviors. The results of the present study showed that fecundity is significantly reduced if copulation is delayed for as short a time as 6 ± 6h in the amphipod crustaceanGammarus palustris(Bousfield, 1969). This led to the prediction that selection would favor behaviors which minimize the likelihood of delays. In amphipods of this genus, amplexus occurs before the female molts and copulation occurs within minutes afterwards. The results of field observations support the prediction; it is estimated that 99% of the females are in amplexus when they molt. However, while the physiological constraints that were demonstrated show the advantages of pairing before copulation, it is speculated that amplexus as the specific pairing behavior evolved in response to the selective pressures of the free‐roaming habits ofG. palustris.