Sexual encounters of pond snails can result in just one of them acting as a male and the other like a female, or in subsequent reciprocation. But, the snails could disagree, what if both mating partner insist on providing, but not receive sperm? This is called gender conflict, and the resolution depends on one of the individuals compromising and adopting its less preferred role, at least initially.
Petra Hermann and collaborators studied the effect of age on sex role preferences in the great pond snails, Limnaea stagnalis, disentangling it from the effect of size. They reared three batches of snails that had known ages at the time of the experiments (young, Y, middle age M, and senior, S) and matched them by size.
First, they looked at the effect of age on mating interactions in age-matched couples. The chances of copulation decreased strongly with age, with young snails copulating with much higher frequency (80%) than senior snails (30%). Encounters between young snails normally ended in a reciprocal intromission (the individual acting as a female initially, then acting as a male), middle aged and senior snails, in contrast, had mostly unilateral sperm exchanges.
Then they set encounters between snails of different ages. When pairing young and middle aged snails, also size-matched, about 25% ended in no copulation, and unilateral encounters were more common than reciprocal. The primary role in the copulation (the snail acting as a male first) was adopted by the young snail in about 80% of the encounters, indicating that the male role is the coveted one for the young snail, and that the fact that reciprocation is rare indicates that the middle ages snail is content acting as a female in unilateral encounters. There is an age related shift in sexual role preferences from male to female.
Fig. 6. Sexual interactions between young and middle-aged Lymnaea.
(A) Most couples of a young and a middle-aged snail (YM) performed a
copulation. The majority of these interactions were unilateral, i.e. the snails
do not reverse roles after finishing the first copulation. Note that in the
cases in which the middle-aged animals acted as primary male all young
partners reciprocated. By contrast, role reversal occurred in only a small
minority (18%) in the couples in which the middle-aged animals acted as
female. (B) Younger snails act significantly more often as primary male than
their middle-aged partner. (modified from Hermann et al 2009)
Animals in the early phase of the species’ life cycle tend to assure that they act as male (either primary or secondary), independent of the age of their partner (Fig. 7A). Senior animals, by contrast, tend to act preferentially as females. The sexual behaviour of middle-aged animals depends on the age of their partner. Combined with similarly aged partners they either act as male or female. Combined with a younger partner they tend to act as female only. Combined with an older partner, they tend to execute both gender roles and will act as males with a similar probability as younger animalsInterestingly, the snails resolved conflict by engaging in reciprocation. When both agreed on a role the encounters tend to be unidirectional. Note that as these snails store sperm and the partners were not virgins, the lack of interest in copulation of the older snails might stem from the fact that they might have accumulated enough sperm to fertilise their eggs. So although the authors removed the confounding effect of age from their experiments, they failed to account for the effect of sperm storage.
Hermann PM, Genereux B, & Wildering WC (2009). Evidence for age-dependent mating strategies in the simultaneous hermaphrodite snail, Lymnaea stagnalis (L.). The Journal of experimental biology, 212 (19), 3164-73 PMID: 19749110