Social monogamy without biparental care: Empirical tests of the territorial cooperation and mate -guarding hypotheses

Monogamous associations should be rare, because individuals of one or both sexes should usually benefit in fitness by being polygamous. However, partner-exclusive behavior has evolved in a range of taxa. Multiple ecological and physiological factors probably contribute to the evolution of this type of social monogamy. I considered two factors, the potential benefits of joint territoriality and selection for mate-guarding behavior, as they relate to social monogamy in the snapping shrimp Alpheus angulatus, a species with no paternal care of offspring. My data reveal likely benefits to both males and females through territorial cooperation. Males invest significantly less time in burrow construction, and females may invest less in territorial defense, as they have a lower risk of eviction when paired than when solitary. These data suggest the possibility of a role for territorial cooperation in the evolution of social monogamy in snapping shrimp.;I also considered the potential role of mate-guarding behavior in the social structure of this species. My data reveal that males of A. angulatus are able to predict the timing of female sexual receptivity, prefer to associate with pre-receptive females, and invest differentially in burrow defense with respect to their female partners' sexual status. These behaviors are consistent with both theoretical predictions of the behavior of mate-guarding male crustaceans. My data also suggest that males of A. angulatus may alter their pairing behavior in response to differences in the availability of females. Males in experimentally female-biased sex ratios were more likely to abandon recently mated females than were males in equivalent sex ratios. Males did not appear to respond to differences in population density by changing their pairing behavior. These data suggest that the relative availability of females may affect the decisions of guarding males, especially with respect to the length of time that males spend guarding individual females.;Overall, my data suggest that both factors, territorial cooperation and mate guarding, are likely to influence the fitness of individuals of A. angulatus, and consequently, that these factors may have played a role in the evolution of social behavior in this taxon.