Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males. Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.
Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus Journal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x
Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127