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Go to Brilliant.org/SciShow to learn more. [ ♪INTRO ]. Animals and plants come in an amazing variety of shapes, sizes and colors.

And generally speaking, biologists can use these traits to sort out different species. But sometimes, members of the same species look vastly different. Like, some are way bigger, have totally different colors, or even live as parasites on others — yes, I'm looking at you, male anglerfish.

In fact, this often happens with sexes, because having different gonads can mean individuals lead very different lives. When differences occur between sexes — other than their reproductive parts, of course — biologists call it “sexual dimorphism”. And it can get pretty extreme.

One of the most obvious ways that sexes can differ is their size. In our species, for example, people who produce sperm are, on average, a bit larger. But we've got nothing on Southern Elephant Seals.

Males of this species are between four and 10 times heavier than females. And it's not that male seals are getting their workouts in more often. Their size is the result of sexual selection.

That's when the traits that give individuals an advantage over others are solely related to reproduction. And it often happens in mating systems like the elephant seal's, where one male breeds with many females, because most males end up partnerless. Only the males on the extreme end of the key traits that let them win against other males get to mate.

And then their extreme genes get passed to the next generation. Over time, these traits can get exaggerated to the point of absurdity, a phenomenon scientists call runaway selection. And it's why the males are so big.

Males control harems of females that they breed with, and harem leaders are decided through vicious battles. So, they're usually the heaviest seals around, since their size helps them overpower the competition. And their victories in battle score them fatherhood of almost 90% of all elephant seal pups.

Though, there's a species of cichlid from Africa's Lake Tanganyika that's even more impressively sized. In these fish, males are sometimes 12 times heavier than females. But it's not because they duke it out over mates.

Male cichlids compete for females by hauling and defending impressively large snail shells which they eventually use as nests. And after mating, the females go inside the shells to spawn and look after their brood. That means there are selective pressures related to size for both sexes, and that's why they're so divergent.

Not only is there pressure for males to be buff, there's also pressure for females to be dainty so they fit into the shells. Now, a lot of sexually dimorphic traits exist because males of that species compete for the attention of females — which might seem a bit unfair. Like, girls can fight over awesome guys, too, Nature.

And don't worry — they do. But, males are often the ones competing for mates because females sink a lot of energy into making egg cells, which are orders of magnitude larger than the males' sperm. And this imbalance in investment can also lead to what used to be called reverse sexual dimorphism: where females are bigger than their partners.

Though, that makes it sound like it's rare or something, and it's not. Just look at pretty much any insect or spider, lots of fish, and most reptiles. But it's rarer in terrestrial mammals and birds — which is why the large female moas that dominated New Zealand until about 500 years ago stand out.

In these giant flightless birds, females were sometimes nearly 7 times heavier than males! Females of the largest species weighed as much as 240 kilograms, while their mates were a mere 34 to 85 kilograms. That's probably because it took a female moa about a decade to reach the age where she could breed.

So when a female did eventually have a chick, she really wanted to make sure it survived. And unlike with other birds, she didn't have to stay dainty to take to the skies. So, the larger her body was, the more energy she could store, and therefore, the more she could put into each offspring.

Females are often understandably choosy about who they let fertilize their expensive eggs. And for the most part, if males want to pass on their genes, they have to impress a female somehow. For elephant seals, that means being king of the sand.

But for many birds, it means looking absolutely fabulous. Compare a male Mandarin duck to a female one, for example, and you'll see what I mean. And beautiful male colors aren't just for show.

Often, they tell females if a male has good genes or will be a good dad. In other words, they're honest signals of mate quality. Take great tits, for instance.

THE BIRD, obviously. Females — who have relatively dull feathers — go for males with bright yellow and blue feathers and more ultraviolet sheen to their tails. And that's probably because those colorful males produce hardier kids.

A 1993 study published in Nature showed that brightly-colored male chicks — who got their color from their dad — lived longer than duller male chicks, even if they were raised by a duller male. So if a great tit has a bright splendid plumage, it indicates he'll pass on some awesome genes. And that's not all.

See, some of a male's color comes from his diet. And a 2001 study found that chicks raised by brighter males grew bigger than chicks raised by dull males, regardless of whether their biological father was bright or not. So those bright colors also tell females that he's good at finding nutritious food for himself and their future young.

Sexual dimorphism is often talked about as an animal thing, but some plants separate their sexes, too — probably because keeping them apart can help prevent inbreeding. Take catasetum orchids, for example. Unlike other orchids, where flowers have both male and female parts, catasetum plants make either male or female flowers.

Male flowers are bright, ornate, and have petals that flare out in a star shape, whereas female flowers are usually green, subdued, and hooded. And having such different-looking flowers helps promote their pollination by bees. See, they're pollinated exclusively by euglossine bees that seek out the orchids' flowery scent.

When a bee lands on the male flower's lip, it triggers the flower to shoot a large, sticky pollen mass right at the bee. Just like, bam! Which is a pretty unpleasant experience, so the bee generally goes off in search of a less treacherous nectar source — like, a female catasetum flower.

Female flowers hang upside down and the bee has to wiggle inside to get at the nectar. So when it comes time for the bee to leave, it has to squeeze its way out — and the large pollen mass gets left behind in the process. After receiving pollen, female flowers swell closed.

So unlike with a lot of plants, each female flower only gets pollen from one male. That sets up the competition among males needed to drive sexual selection. Male flowers that attract the most bees will pollinate the most females.

And studies have shown that its the males' bright colors that bees love, because they grab the insects' attention. And that means being beautiful is what counts for male orchids — much like being bigger matters to male elephant seals. Of course, not all investments in reproduction come from the literal act of making offspring.

It can also take a lot of work to be a good parent. Just ask three-spined sticklebacks. In these fish, males construct elaborate nests for their future young made of vegetation and a glue secreted from their kidneys.

Then, they court females that swim by. And if a male can convince a female to mate with him, she'll lay her eggs in his nest then swim off, saddling him with the job of parenting. If he wants his young to survive, he has to fan water over the eggs almost constantly so they get enough oxygen.

And he has to protect them from hungry mouths and other, thieving males. All of this work is probably why the males' brains are, on average, 23 % bigger than those of females, even when you control for their slightly larger overall size. Basically, they need more brainpower to keep up with all those complex parenting tasks.

And researchers see evidence for this when they test their smarts in the lab. Like, male sticklebacks do better than females on cognitive tests where they have to control their impulses, suggesting their large brains give them better inhibitory control. That's important because males have to resist the urge to eat their own young during all that time caring for them.

And that's not easy, given they would make tasty snacks and are similar in size to the fish's usual prey. But also, inhibitory control is essential for a lot of cognitive processes, since it means an organism is better at focusing on the task at hand. And you can't solve problems or make good decisions if your thought train is constantly being derailed.

Interestingly, scientists have found that female sticklebacks prefer males with better inhibitory control. So it seems like bigger brains help the males be better parents — and that makes them all the more attractive to females. Sometimes, you have to make a pretty big sacrifice in the name of reproduction.

That's why female rusty tussock moths have lost their most essentially moth-y trait: flight. Male moths look like what you'd expect — big. brown wings. furry antennae. But females just kind of look like a fluffy bean.

And that's because they take better care of their offspring than most other moths. Once a female has emerged from her cocoon, she'll stay put and release pheromones to attract a male. Then, after mating, she'll lay her eggs right there on her old cocoon and cover them with a protective secretion.

This is quite a taxing process — both making the eggs in the first place and protecting them with that special sauce. And it turns out that females who used more of their energy this way had more offspring survive than those who put energy into wings. So over time, they just sort of stopped making them.

That might sound kind of sad, but don't feel too bad for these flightless females. They probably did get to fly at one point. You see, larvae of this species can spin a silk thread to use air currents to float to a new location, a trick called ballooning.

So even though they're leaf-bound as adults, in their youth, they likely rode the breeze to find their place in the world. I mentioned male anglerfish at the beginning of this episode because they're one of the most extreme cases of sexual dimorphism out there. Males are teeny, and they attach themselves to females, eventually becoming little more than a parasitic nub.

But they're not the most extreme example. That honor probably goes to green spoonworms. The difference between female and male green spoon worms is so great that you can't even see the males.

The green, blobby worms you might spot in shallow ocean waters are all females. The males are tiny in comparison — like, one 8/100 the size. If humans were like this, then guys like me would be smaller than a grain of rice.

And they're not only super small, they actually live inside a holding cell in the female's body called the androecium. Their only job is to fertilize the female's eggs. That's it.

Males don't even eat and digest food! They just absorb what they need from the female. This parasitic, co-dependent relationship is because of the environment they live in.

See, spoon worms are pretty shy and live in burrows that they don't leave much. Basically, they just use their long mouth parts to reach out and grab food once in a while. And prime burrow real estate is scarce.

So, instead of which genes they get determining their sex, spoon worms do something unique. In short, whether a spoon worm becomes a male or female depends on who else is around. If, after floating around a bit, a larva lands on empty ocean floor, it becomes a female.

And then, once fully grown, she'll start making a toxin called bonellin which turns her green. Any floating larvae that come in contact with this toxin become male. Then, they can get sucked up by the female and make their way into her man-tomb.

This strategy means no female misses out on a house, and no male misses out on the chance to mate. Which is actually a pretty fair deal. If you can get over the whole parasite-that-lives-inside-your-partner...thing.

So it's no coincidence that different sexes of the same species can be totally different sizes, shapes, or colors. It's all nature's way of dealing with the battles within and between sexes. If there's any lesson in all these extremes, it's that, when it comes to evolution, there's no one answer to solving life's challenges.

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