Science With Friends: on sexes and reproduction

What do scientists talk about when they get together? All sorts of random things. Welcome to the first installment of Science With Friends, a new series where friends write about science. I hope it is both entertaining and informative. Enjoy!

[Image: Praveer Sharma]
Female (left) and male (right) black-necked stilts showing moderate sexual dimorphism. (Image: Praveer Sharma)
Why do men and women exist? Why are there not just…people? Peace, dear reader—I’m not channelling Yoko Ono’s Twitter feed here, but addressing the biological question of why so many creatures (but not all!) are divided into male and female.

The simplest way to reproduce is to just make an identical copy of yourself. This is mainly how bacteria, the oldest and most numerous organisms on the planet, get it done. But plenty of more complex beings, such as plants and animals, can reproduce the same way, through cuttings, budding or other similar means. This asexual sort of reproduction has many advantages—there’s no need to wander around searching for a mate and all one’s tried-and-true genes get passed on, instead of taking a chance and ending up mixed and matched with some rando’s janky DNA.

Asexual reproduction does not result in absolutely unvarying organisms—there’s inherent randomness in the biochemical processes governing life and errant radiation or chemicals can also come by and scramble things. These generate genetic variation, on which the evolutionary processes of selection and drift can act. But this way is slow and conditions, whether they are climatic or pathogenic, can change fast. When a new critter comes along that wants to hitch a ride on you/feed on you/liquefy you from the inside out, it would helpful to have the tools to deal with it sooner rather than later. This is where grabbing some DNA from another individual of your species can come in handy—they just might have what you need to fend this threat off. This is sexual reproduction. Continue reading


Why female house finches prefer redheads over blonds

Walk into any clothing store and you’ll see that the women’s section is more colourful (and bigger!) than the men’s section. Not so in the bird world. In most bird species, males are more colourfully and elaborately dressed than females. This type of sexual dimorphism in which males and females look dramatically different is frequently driven by sexual selection. That is, female preferences for bright colours and ornate decorations have pushed each successive generation of males to evolve more and more flamboyant plumage. But why do female birds prefer brightly coloured males in the first place?

One idea is that ornamental traits like coat colour are indicators that can provide useful information about important survival traits. For example, the theory of parasite-mediated sexual selection proposes that the quality of a male’s ornamental display signals how well he can resist infection by parasites. To test this theory, many researchers are turning to the house finch, a common bird found across North America. Males have variable red-to-yellow colouration on their head, breast and rump whereas female finches are a rather boring greyish brown in colour. The bright red males enjoy more popularity among the females than their more drab yellow rivals. Many factors contribute to male colouration, including nutrition and parasite exposure during feather growth.

Male house finches vary in their colouration from red to yellow. (Image: Diane Pierce, National Geographic)
Male house finches vary in their colouration from red to yellow. (Image: Diane Pierce, National Geographic)

Speaking of parasites, let’s talk about the complex relationship between house finch feather colour and parasite infection. Several studies, including this one in 2004, showed that male house finches infected with the bacteria Mycoplasma gallisepticum develop more yellow and less bright feathers than uninfected males fed the same diet. Yellow males also seem to be at a disadvantage when it comes to surviving an infection. In the mid-to late-1990s, an M. gallisepticum epidemic hit the house finch population in the eastern United States. Surveys conducted before and after the epidemic found that red males survived better than yellow males, which drove the eastern house finch populations to become more homogenously red than populations in the rest of the country. In laboratory experiments, red males from unexposed populations resolved symptoms of M. gallisepticum infection faster than yellow males. So what exactly is going on? How does feather colour affect parasite infection and vice versa? Continue reading