Sex ratio patterns and trade-off between sexual and asexual reproduction in the brown hydra

Investigators of biases in sex ratios have sporadically focused on a group of animals that are able to engage in both asexual and sexual reproduction. Sex ratio in these animals often is different from the fundamental 1:1. The sex ratio in the study populations of Hydra oligactis followed an unusual pattern: from male dominance, through an equilibrium, to female dominance at the end of the reproductive period. A delay in the time of switching from asexual to sexual reproduction between males and females appears to be the general reason for the bias in the sex ratio. A sex-specific cost of sexuality illustrated by the different responses of male or female individuals to the conditions inducing sexual reproduction appears to exist. In a laboratory experiment, differences in the percentage of sexually induced males and females were greater for females than males between treatments with sharp and mild temperature decreases. Females appeared to benefit by waiting for an intense temperature cue before beginning to mature, whereas males matured earlier and in response to milder cues. Mature females had high mortality and little capacity to revert from sexual to asexual reproduction, which indicated a higher trade-off between these 2 modes of reproduction compared to males.