Mate-quitting rules for male thirteen-lined ground squirrels

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Abstract
Male thirteen-lined ground squirrels (Spermophilus Iridecemlineatus) typically copulate several times with a female before leaving her to search for other mates. The males routinely experience sperm competition, which features a first-male advantage. We examine several possible approaches to determining their optimal mating investment. From evolutionarily stable strategy sperm competition models, males are predicted to have equivalent sperm transfer strategies when mating first or second. Analyses of two potential correlates of sperm transfer (duration of longest copulation, cumulative time copulating) revealed no significant differences across mating order, and no significant variation according to the males' access to information about their mating order and sperm competition prospects. Thus, as assessed by copulatory behavior, sperm investments by first and nonfirst males appear to be similar. We next explored the criteria males use for departing from females. Field data on mating duration, measured from the male's first mount to departure from the female, showed that mating duration is not related to an individual male's paternity gains, thereby prohibiting a marginal value approach for deriving the males' optimal mating investment. Furthermore, of several potential rules of thumb males could use, no support is evident for those based on increasing time costs, nor do males appear to remain with the female until they have copulated a fixed number of times. Instead, male departures seem to be triggered by attaining a copulation that meets or exceeds an absolute threshold of about 9 min. The departure criterion therefore is based on the behavioral variable that has previously been implicated as a crucial determinant of male fertilization success

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