Why size matters in the plant world too

Over 60 years ago, evolutionary biologist Bernhard Rensch calculated that males are typically the larger sex in big-bodied species such as humans, whereas females outdo them in small-bodied species such as spiders. Now it turns out that many plants obey Rensch's rule too.

Most plants produce both male and female sex organs, but around 7 per cent are dioecious, meaning individuals are purely male or female. Kevin Burns and Patrick Kavanagh at Victoria University of Wellington in New Zealand measured the leaf and stem sizes of 297 plants from 38 dioecious plant species in herbarium collections of the National Museum of New Zealand and discovered that they follow the sex-size rule.

Why? "Females need to hold the seeds and the fruit," Burns says, adding that female stems also must be large enough to display the fruit and support the animals that spread the pollen or seeds. If metabolism, predators or climate promote the evolution of smaller plants, however, males can shrink because their gametes are smaller.

The sticky bit, says Burns, is why males produce larger leaves in bigger species. Martin Burd at Monash University in Melbourne, Australia, says Rensch's rule can be explained in animals because males compete for females and often the larger fellows win.

"It's possible that successful male plants produce larger flowers or more flowers to attract more pollinator visits," says Burd. Bigger stems and leaves would therefore be needed support the floral display, but this needs to be tested, he adds.

Size might not be the only such gambit adopted by plants and animals to successfully reproduce. Burd has preliminary evidence that both birds and plants evolved similar tactics – of cramming flowers with ovules or increasing egg number per nest – to capitalise on unpredictable changes in their food supply.