Fed up with attention they don’t need, some female snails have learned to smell like guys.
Unlike other species of periwinkle snail, Littorina saxatilis ladies can’t be identified from chemical cues in their slime trail.
Deprived of their standard mate-finding trick, male L. saxatilis chase trails indiscriminately, learning only at the last moment whether they’ve lucked out.
This confused singles scene has likely been fueled by evolutionary pressures that “select for mechanisms in the female to avoid both male harassment and excessive matings,” wrote researchers led by Kerstin Johannesson, a marine ecologist at Sweden’s University of Gothenburg, in a paper published in Public Library of Science ONE.
Johannesson’s group had noticed that male Littorina littorea snails, a closely-related periwinkle species that shares L. saxatilis‘ coastal Swedish habitat, usually follow the mucous trails of females, while L. saxatilis males demonstrate no such preference.
Because male and female periwinkles look similar from the outside, the trail-following mixup means that “observed frequencies of male-male pairings in the wild [are] exceptionally high in this species,” wrote the researchers.
Two explanations made sense to Johanneson’s team. Either the females were disguising themselves, or male L. saxatilis were just aromatically clueless.
To investigate these propositions, the researchers studied trail-tracking behaviors in two more periwinkle species, L. obtusata and L. fabalis.
Once again, males followed females, underscoring the ubiquity of sex-specific trails in periwinkles. And when L. saxatilis males found trails left by other periwinkles, they generally followed the females. Their sense of smell seems to work just fine. Instead, it seems that a sex-specific “mucous cue is absent in female L. saxatilis,” wrote the researchers. “They have either permanently lost this cue or have the ability to optionally remove the cue.”
The females’ hiding is likely due to the burdens of reproduction.
During mating, when the weight of two snails is supported by just one, and waves drag on double the usual surface area, it’s much easier for snails to fall into the sea.
This risk is further magnified by the extraordinary population densities of L. saxatilis, which are more than 100 times greater than in the other periwinkles.
As a result, “expected male-female encounter rates would be two orders of magnitude greater in this species,” wrote Johanneson’s team. For every unwanted encounter experienced by the average periwinkle, an L. saxatilis will have one thousand.
This is, of course, as much a problem for males as for females. But whereas females improve the chances of passing on their genes by lavishing care on their finite supply of eggs, males enhance their legacy by mating as often as possible.
“Adding to the number of lifetime matings is their only way to increase lifetime fitness,” wrote the researchers. “Hence males would be prepared to take larger risk compared to females and strive to mate as frequently as possible despite these costs.”
In response, the females go into hiding.
Image: 1) Littorina saxatilis./Björn Canbäck, University of Gothenburg. 2) Copulating L. saxatilis./Patrik Larsson.
Citation: “Indiscriminate Males: Mating Behaviour of a Marine Snail Compromised by a Sexual Conflict?” By Kerstin Johannesson, Sara H. Saltin, Iris Duranovic, Jon N. Havenhand, Per R. Jonsson. Public Library of Science One, August 9, 2010.
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