Sperm competition occurs when a female copulates with two or more males and the sperm of those males compete within the female's reproductive tract to fertilize her eggs. The frequent occurrence of sperm competition has forced males of many species to develop different strategies to overcome the sperm of competing males. A prevalent strategy is for males to increase their sperm investment (total number of sperm allocated by a male to a particular female) after detecting a risk of sperm competition. It has been shown that the proportion of sperm that one male contributes to the sperm pool of a female is correlated with the proportion of offspring sired by that male. Therefore, by increasing his sperm investment a male may bias a potential sperm competition in his favour. Here we show that male meadow voles, Microtus pennsylvanicus, increase their sperm investment when they mate in the presence of another male's odours. Such an increase in sperm investment does not occur by augmenting the frequency of ejaculations, but by increasing the amount of sperm in a similar number of ejaculations.

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FERK~N, M. H., and SEAMON, J. 0 . 1987. Odor preference and social behavior in meadow voles, Microtus pennsylvanicus: seasonal differences. Can. J. Zool. 65: 293 1 -2937. Olfactory cues play a major role in inter-and intra-sexual agonistic behavior for Microtus pennsylvanicus. We show that seasonal changes in odor preference and behavioral interactions coincide with varying patterns of social organization. Odor preference trials were conducted in a Y maze and social behavior was measured in dyadic encounters in a neutral arena. During the breeding season, paired encounters between females contained more agonistic acts than did encounters between males, or those between males and females. Breeding males preferred the odor of females to the odor of coyspecific males. In contrast, breeding females preferred their own odor and the odor of males to the odor of other females. The data support female exclusiveness during the breeding season. During the nonbreeding season, paired encounters between females contained fewer agonistic acts than did encounters between males or those between males and females. Nonbreeding females preferred the odor of female conspecifics to their own odor or the odor of male conspecifics. These results are consistent with group nesting and a relaxation of female territoriality during the nonbreeding season. Nonbreeding males did not exhibit a preference for their own odor, the odor of other males, or the odor of female conspecifics. Our results indicate that nonbreeding males are solitary, and overwintering groups are female-biased. Odor preferences may underlie seasonal changes in social organization. F E R K~N , M. H., et SEAMON, J. 0 . 1987. Odor preference and social behavior in meadow voles, Microtus pennsylvanicus: seasonal differences. Can. J. Zool. 65 : 293 1-2937.Les signaux olfactifs jouent un r61e important dans les comportements agonistiques intersexuels et intrasexuels chez Microtus pennsylvanicus. Nos donnCes indiquent que des changements saisonniers dans les prCfCrences d'odeurs et les interactions de comportement coi'ncident avec une variation dans l'organisation sociale. Des epreuves de choix d'odeurs ont Ct C realiskes dans un labyrinthe en Y et les comportements sociaux observees dans une arene neutre lors de rencontres deux par deux. Durant la saison de reproduction, il y a plus de manifestations d'agressivite au cours des rencontres entre femelles que dans les rencontres entre miles ou les rencontres mile-femelle. Les miles reproducteurs prCfkrent l'odeur de femelles a celles d'autres miles. En revanche, les femelles reproductrices choisissent leur odeur propre ou celle de miles de preference a celle d'autres femelles. C'est la un indice de l'isolement des femelles durant la saison de reproduction. Hors de la saison de reproduction, les rencontres entre femelles sont moins agressives que celles entre miles ou celles entre un mile et une femelle. A cette pCriode, les femelles prkferent l'odeur d'autres femelles a leur propre odeur ou a celle de miles. Ces rCsultats concor...

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