Gender inequality (in plant populations). What causes unequal numbers of males and females?

A female and male mandrake
Mandrakes from Jacob Meydenbach Hortus sanitatis, 1491

Populations of dioecious flowering plants (which have male and female individuals) often depart from the expected male:female ratio of 1:1. The causes of skewed sex ratios are complex and still poorly understood. As with many species that have two sexes, females must invest more resources in reproduction. In the case of flowering plants, this is the extra cost associated with flowering and subsequent fruit production. Consequently, females often delay flowering and are more susceptible to environmental stresses, suffering higher mortality rates compared with males. These factors can lead to a greater than equal representation of males in the population.

A recent AOB study attempts to untangle these complex causes of biased gender ratios. This meta-analysis of existing studies of dioecious flowering plant species found a large amount of variation in sex ratios between populations within a species. There was also much variation between species, although several patterns have emerged.

Due to delayed and less frequent flowering by females, younger populations tend to have a greater bias towards males. This was demonstrated elegantly by looking at fire-adapted species, which recolonise an area following a fire, so that the age of the population can be exactly known. Males are also more frequent in populations at higher altitudes, reflecting greater mortality of females as the environment becomes more stressful. Chance historical effects can also play an important role in skewing the sex ratio. For example, in clonal species, where a few individuals colonise an area and largely reproduce vegetatively, chance biases in the representation of the sexes can be preserved over long periods.

You can read the study, free, at the Annals of Botany.

Ecological context and metapopulation dynamics affect sex-ratio variation among dioecious plant populations (2013) Ann Bot 111(5): 917-923. doi:10.1093/aob/mct040