Social monogamy has evolved independently in many taxa, and often involves biparental care of the young. Where it does not, mate guarding and shared territoriality have been invoked as causal factors. We evaluated mate guarding and shared resource defence (a common shelter) as factors that could have led to social monogamy in the snapping shrimp, Alpheus heterochelis. This species is found in male–female pairs that defend a common shelter together. Female receptivity lasts only for a few hours immediately after her periodic moult. Their monogamous pair bond may represent mate guarding or joint defence of a territory. Monogamy in A. heterochelis seems most importantly driven by the cryptic nature of the female's moult cycle. We found that males did not discriminate among females at different intermoult stages for pairing, nor did they modulate their defence of mate and shelter (vs. the risk in finding a new shelter and mate) according to female moult stage. This, together with the short period of female receptivity before her single copulation per cycle, make extended mate guarding the most efficient method for a male to secure a mating opportunity. Comparing eviction rates of paired and unpaired shelter residents by conspecific intruders provided no evidence of enhanced resource defence that would confer a selective advantage to a pair. Male presence during the moult is beneficial for the female, as searching for a male during her soft‐bodied receptive phase would put her at mortal risk. Our results show empirically for the first time that guarding may be beneficial, even if males are not able to assess the female's reproductive stage. This extends the theoretical framework for understanding the evolution of social monogamy in taxa without biparental care of young.

The big-clawed snapping shrimp, Alpheus heterochelis, is found in size-matched male-female pairs. A common assumption is that a large size advantage in males, an extended pair bond, and a positive correlation between female size and fecundity promotes size-assortative pairing. Since all of these conditions apply to A. heterochelis, we investigated size-assortative pairing in the laboratory by designing experiments in which the pairs were size matched or mismatched using three size categories; large, medium and small. We found that snapping shrimps prefer to pair according to size and that such pairing is stable. We also found that, contrary to the initial assumptions, female-female competition and female choice for large males plays the main role in the pair formation process. Smaller, and hence less competitively successful, females will pair with smaller males, but in a male-biased environment, will preferentially pair with larger males. Large males provide the best protection from shelter eviction, which is crucial for female A. heterochelis, as their reproduction is closely associated with moult, and therefore requires shelter and a male present for successful reproduction.

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