Sexual conflicts along gradients of density and predation risk: insights from an egg-trading fish

Abstract

In principle, the intensity of sexual conflict is best measured as a loss of fitness associated with the expression of conflict-related traits. But because the relevant traits may be difficult to manipulate and fitness difficult to assess, proxy variables linked to conflict intensity may provide important tools for empirical measurement. Here we identify two common types of sexual conflict-one within mating pairs over the less expensive male role, and one between mating pairs and intruders seeking to obtain fertilizations-and consider how they vary in intensity along gradients of population density and predation risk. To do this, we develop and analyze a model of mating dynamics in the chalk bass, an egg-trading simultaneous hermaphrodite that lives on Caribbean coral reefs. In this species, within-pair sexual conflict leads each female-role partner to provide in each mating episode only a subset (parcel) of its egg clutch to its mate for fertilization. Pair-intruder sexual conflict (i.e., sperm competition) increases the proportion of the gonad allocated to male function. In the model, more parceling and greater male allocation both resulted in lower fitness at the ESS, our measure of conflict intensity. Male allocation increased along the density gradient but decreased along the predation-risk gradient, reflecting shifts in intrusion frequency. Parcel number sharply increased and then decreased more gradually along a gradient of increasing local density, initially responding to increased availability of alternative mates across low densities and then to diminishing clutch size toward higher densities. Parcel number decreased with predation risk as each mating episode became more dangerous. Conflict intensities were usually greatest at intermediate positions along the two environmental gradients, and each conflict ameliorated the intensity of the other. Overall, parceling and sex allocation may be good though imperfect proxies for intensities of within-pair and pair-intruder sexual conflicts among chalk bass.

Direct reciprocity stabilizes simultaneous hermaphroditism at high mating rates: A model of sex allocation with egg trading

“…As a result, streakers may not compete for paternity on an equal footing (Crowley and Hart ; Hart et al. ). We account for this by incorporating a paternity “discount” for streakers relative to the male‐role mate.…”

“…Streakers are often physically more distant from released eggs than the mating pair and may be the target of aggression from the individual mating in the male role (Oliver 1997). As a result, streakers may not compete for paternity on an equal footing (Crowley and Hart 2007;Hart et al 2011). We account for this by incorporating a paternity "discount" for streakers relative to the male-role mate.…”

“…By offering eggs only to partners who reciprocate, traders increase their reproductive success in the male role, effectively using their eggs as "bargaining chips" in exchange for fertilization opportunities. Egg trading thus partially resolves the sexual conflict that arises from a general preference for the male mating role in many hermaphroditic species (Anthes et al 2006;Hart et al 2011;Schärer et al 2014). This mating role preference is an expected consequence of anisogamy.…”