Why Bird Penises Are So Weird

This video was sponsored by 23adnMe...And
hi, Im Emily from MinuteEarth. The Argentine lake ducks spiraling penis
extends more than 16 inches. The seagull doesnt have a penis at all. For two animals with so much else in common,
this is a pretty big spectrum  in fact, birds exhibit the biggest range in relative
penis size of any class of vertebrates.

And there are a couple key reasons why. For one, male birds are uniquely set up to
pass helpful sexual traits directly to their sons  traits like showy, mate-attracting
tail feathers or genitalia of certain dimensions. Animals generally hand these kinds of traits
down through their sex chromosomes  but the sex chromosomes male mammals pass on to
their sons is pretty tiny and has relatively few genes. Male birds, on the other hand, have way bigger
sex chromosomes with lots of genes, and therefore have higher chances of passing a mutation
like one for extra-sexy tail feathers  directly to their sons, who in turn can
attract more mates and potentially pass a mutation for extra-extra sexy tail feathers
to their sons, and so on down the line.

In short, male birds comparatively huge
sex chromosomes are the reason much more exaggerated male traits have evolved in birds than in
mammals. Like the lake ducks reeeeally long penis. Male ducks often mate with unwilling females
by force, and it happens that the males with the longest and twistiest genitals are the
most successful, which means the longest-and-twistiest-genital genes keep getting passed down from father
to son. On the other hand, in bird species where less
well-endowed males do better - maybe because females prefer them, or because big genitals
make flight tough - males can end up with small nubs, or even no penises at all.

One other reason the bird penis spectrum is
so big is that its really easy to make a bird penis shrink. A single mutation on the birds large sex
chromosome triggers production of a protein that basically erases the developing embyro's
penis. All it then takes to make the entire species
penis-less is for this mutation to get passed down the line from male to male. But given how the chromosomal arrangement
of birds allows more male traits to change more easily, even penis-less-ness isnt
permanent: the chachalaca, a wild cousin of the chicken, went from having a penis, to
not having one, to growing one again.

Speaking of chromosomes, this video was sponsored
by 23andMe, which has a special interest in the chromosomes of our own species. 23AndMe not only lets you learn about your
own DNA story - including ancestry, personal traits and health insights - it can also help
you learn which one of your parents you have to thank for some of your genetic traits. To get testing kits for you and your family
- and to support our channel - go to 23andMe.Com/MinuteEarth..