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  • [ ♪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 sexesother than their reproductive parts, of coursebiologists

  • call itsexual 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 femaleswhich might seem a bit unfair.

  • Like, girls can fight over awesome guys, too, Nature.

  • And don't worrythey 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 birdswhich 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.

  • Femaleswho have relatively dull feathersgo 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 chickswho got their

  • color from their dadlived 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, tooprobably 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 sourcelike, 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 outand 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 orchidsmuch 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 parentsand 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 expectbig.

  • 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 processboth 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 comparisonlike, 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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  • [ ♪OUTRO ]

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B1 male female pollen bee sexual elephant

For These 7 Species, Sex Changes Everything

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    林宜悉 posted on 2020/03/30