MATHEWS, L.M.: Evolution of social monogamy in snapping shrimp

Social monogamy in the absence of biparental care is a topic of interest, as it is predicted to be rare by parental investment theory, yet it occurs in a broad range of taxa. Most or all caridean shrimp of the genus Alpheus occur in heterosexual pairs, a social organization that may have evolved as a form of extended mate guarding of females or territorial cooperation between partners. The mate-guarding hypothesis predicts that males should be able to detect female sexual receptivity, and should prefer to associate with females closer to receptivity. An experiment to test for distance communication of gender and sexual status revealed that male A. angulatus are repelled by water containing chemical cues from intermolt conspecifics (male or female) but are attracted to water containing chemical cues from premolt (pre-receptive) females. A mate-choice experiment showed that this is translated into active mate-choice decisions: males paired with premolt females significantly more than with intermolt females. The territorial cooperation hypothesis predicts that individuals in pairs benefit through shared territorial defense, by having a significantly lower risk of eviction from the burrow when faced with an intruder. Laboratory experiments showed that females in pairs are less likely to be evicted than solitary females (p=0.034), but there was no difference in the eviction rates for paired and solitary males. Furthermore, males were significantly more likely to abandon the burrow if their mates were low-value (far from receptivity) than if their mates were high-value (p=0.004). These data together suggest that male and female A. angulatus are under different selective pressures leading to social monogamy: males engage in extended mate-guarding, which is tolerated by females because of the advantages of shared territorial defense.