Sexual Dimorphism: How and Why Does One Sex Develop Bigger Than the Other?

You may use the word macho, but this kept its meaning only with most mammals, many birds and crocodiles, as in many species females are larger than males (such as most insects, amphibians, fish, snakes), while the two can have the same size in other species (like some bird species).

Scientists have been puzzled by how the sexual size dimorphism is developed.

If both genders grow with the same velocity, then the bigger one should have a larger growth period or, alternatively, should grow faster.

A research team composed of 13 researchers from 10 countries studied the issue utilizing comparative data on 155 species of insects and spiders (arthropods) belonging to 7 major groups.

The output of their research pointed that, generally, growth speed difference between the sexes is a more important factor than the growth time difference in developing size dimorphism in arthropods.

But the growth periods also presented a variation depending on the taxonomic group: males and females have similar growth periods in beetles (Coleptera) and water striders (a type of bugs, Heteroptera), males show longer growth periods than females (two types of flies, Diptera), or males possess shorter growth periods than females (so-called protandry), even if this was not always proportionally linked to the size difference between the genders (spiders, butterflies Lepidoptera, and Hymenoptera, which compasses bees, ants and wasps).

Because sex dimorphism in most arthropod groups is translated to females larger than males, they have to grow faster, a development pattern totally different than that found on well studied primates and birds, in which the main factor that triggered sexual dimorphism was the difference in the growth period between the genders.

The biologists emitted three possible explanations for this mechanism of producing sexual dimorphism evolved in female arthropods. One of the most interesting explanations sustains that despite the fact that generating (small) sperm compared to generating (large) eggs consumes less body energies, developing male gonads and genitalia could result more difficult than developing female gonads and genitalia and males would require more time to mature having bigger bodies.

Photo credit: Matthias W. Foellmer. A female (huge) and male (tiny) spider