The Argentine lake duck's spiraling penis extends more than 16 inches.

The seagull doesn't 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 duck's 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 it's really easy to make

a bird penis shrink.

A single mutation on the bird's 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 isn't 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.