Evolutionary Conflict: Sperm Wars, Phantom Inseminations

Resource Availability Drives Mating Role Selection in a Simultaneous Hermaphrodite Aplysia californica

In simultaneous hermaphrodites, a clear conflict exists between sperm donor and sperm recipient roles, and how such conflict is mediated remains up for debate. This study observed and recorded mating role selection as a function of resource availability in the simultaneous hermaphrodite Aplysia californica. When food was plentiful, animals assumed both sperm donor and recipient roles at relatively even frequency. However, when half of the mating pairs were placed on restricted diets, food-limited animals assumed the sperm donor role at significantly (P < 0.05) greater frequency than their ad libitum partners; nevertheless, the frequency of successful mating events remained the same. The mass and frequency of eggs laid were also significantly (P > 0.05) correlated with parental food intake. These results demonstrate how mating strategies can change within a mating season, as a result of shifting environmental conditions, and call for a diverse framework to address these issues in simultaneous hermaphroditic mating systems.

Evidence for age-dependent mating strategies in the simultaneous hermaphrodite snail, Lymnaea stagnalis (L.)

In many mating systems female reproductive capacity is a limiting resource over which males will compete. As a consequence, males and females have usually different fitness optimization strategies which may give rise to sexual conflict. Since simultaneous hermaphrodites have, in theory, the option to mate as male or as female at any time, conflict will occur if partners insist in taking the same role. Several lines of evidence exists that body size influences gender choice. However, growth in many invertebrates is indeterminate and therefore age is generally a covariant of size. We therefore investigated the effect of age on mating choices in the simultaneous hermaphrodite Lymnaea stagnalis. Using fully sexually mature animals sampled from three different age groups we show that copulation frequency declines with age. Specifically, in age-matched couples the frequency of primary and reciprocal copulations declines with age. Furthermore, the younger partner tends to mate as male with greater probability in couples of unequal age. Size was never a factor in the sex role preference of Lymnaea. Thus, young Lymnaea always attempt to copulate as male independent of the age of their partner, whereas senior snails act primarily as female. The sex role choices of middle-aged snails appear to depend on their partner's age. In addition, we demonstrate that the likelihood that an animal will copulate as male is not correlated with prostate gland size but correlates with the level of afferent electrical activity recorded in the nerve originating in the prostate gland. Together, our results indicate the existence of an age- and not size-dependent mating system in Lymnaea.

The tragedy of the commons in evolutionary biology

Garrett Hardin's tragedy of the commons is an analogy that shows how individuals driven by self-interest can end up destroying the resource upon which they all depend. The proposed solutions for humans rely on highly advanced skills such as negotiation, which raises the question of how non-human organisms manage to resolve similar tragedies. In recent years, this question has promoted evolutionary biologists to apply the tragedy of the commons to a wide range of biological systems. Here, we provide tools to categorize different types of tragedy and review different mechanisms, including kinship, policing and diminishing returns that can resolve conflicts that could otherwise end in tragedy. A central open question, however, is how often biological systems are able to resolve these scenarios rather than drive themselves extinct through individual-level selection favouring self-interested behaviours.