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Males in group-breeding salamander species sometimes deposit spermatophores on top of other spermatophores during mating. Several adaptive hypotheses have been proposed to explain this behavior: a tactic used by males, making sperm from other males unavailable to females thereby reducing the intensity of sperm competition, or that females more easily find and obtain sperm from stacked spermatophores which are elevated from the substrate. I tested the following hypotheses in the smallmouthed salamander, Ambystoma texanum: (1) males adjust spermatophore capping behavior in the presence of a competitor; (2) males stack on top of spermatophores belonging to competitors more often than expected at random; (3) females pick up sperm from stacked spermatophores relative to single spermatophores more often than expected at random; and (4) females distinguish spermatophores they obtain sperm from with respect to the identity of the depositing male. I found that males stack spermatophores more often in the presence of a competitor and that they are more likely to stack their spermatophores onto those belonging to their competitor. I found no evidence that females obtain sperm preferentially from stacked spermatophores or that females discriminate between spermatophores deposited by different males. I found males behaviorally interacted more frequently with their competitors than with females during mating trials in the lab, suggesting a mechanism males may use to preferentially stack their spermatophores on those belonging to their competitor. My results are consistent with the hypothesis that male spermatophore stacking is used to prevent sperm from rivals being available to females.

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