Assessments and Decisions During Mate Choice in Gammarus Pulex (Amphipoda)

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Abstract
Male Gammarus pulex preferentially enter precopulatory mate guarding with large females close to moulting. The interactions of males and females were observed in an attempt to elucidate the process by which males assess females and, further, to determine how such information gathering influences mating decisions. Males which encountered single females, which had recently been experimentally separated from other males, grabbed and held those females in a perpendicular hold prior to establishing the precopulatory hold. The higher the reproductive quality of the female the more rapid was the male decision to move into the perpendicular hold but this was followed by an increased level of abdomen flexing and antennae touching. Once precopula was established, higher quality females received more stroking with the free gnathopods. When two females were encountered, some males simply took the first contacted and this resulted in random pairing. Other males, however, demonstrated the ability to sequentially sample as well as simultaneously compare females and, if this occurred, they almost invariably took the higher quality female. Discrimination between two females was more rapid as the difference in their qualities increased whereas males engaged in more grabbing and holding when females were of similar moult stages. The data show that assessment of female size and moult stage occurs early in the interaction, whereas subsequent activities such as flexing and stroking are probably involved in courtship, possibly serving to pacify females.