Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to aboveversus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.

Summary1. Species of the dioecious South African plant genus Leucadendron vary strikingly in their degree of sexual dimorphism, with males of dimorphic species typically possessing more and thinner branches and smaller leaves than the corresponding females. 2. Many Leucadendron species are serotinous, with females of serotinous species maintaining seeds in transpiring cones for a number of years. Since cones must be kept alive, serotinous females presumably incur higher costs in terms of water loss than non-serotinous females and, particularly, males. Given that branching architecture is known to affect water conductance, with less resistance in less highly branched plants, we predicted a positive correlation between the degree of serotiny in a species and sexual dimorphism in branching. 3. We tested the effect of serotiny and several other factors on the degree of sexual dimorphism in Leucadendron by conducting a comparative analysis of variation across 49 species using phylogenetic regression. 4. There was a strong positive association between the degree of serotiny and sexual dimorphism. This association is consistent with the idea that the costs of maintaining cones may select females to be less highly branched than males. 5. Synthesis. Serotiny is a strategy of maternal care that contributes to offspring survival after seed maturation. We found that, in Leucadendron, the evolution of increased serotiny corresponds with the evolution of more marked sexual dimorphism. This study exposes a hitherto unidentified association between sexual dimorphism and maternal care in plants and points to a possible additional reproductive burden carried by serotinous plants compared with those that release their seeds at maturity.

A swollen right eye in a childMark S Harris foundation year 2 doctor, ear, nose, and throat surgery 1 , Gaurav Chawdhary specialist trainee, ear, nose, and throat surgery 2

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