One of the most enduring questions of humanity is “what is the meaning of life?”.

Well, spoiler alert, we won’t be answering all of your existential questions today. But as far as evolution is concerned, the most important thing that animals can do is make more of these.

No no, not just the babies. Genes! Without passing down our DNA through sexual reproduction there would be no adorable spring baby bears, no beetles, no us!

Sexual reproduction is part of the very definition of what it means to be an animal. Even though life has found other more efficient methods of reproducing -- like cloning which just takes one individual -- most living animals, and the last shared ancestor of all animals, pass on their genetic make-up with sexual reproduction. I’m Rae Wynn-Grant, and this is Crash Course Zoology.

Let’s lay down some ground rules, because sexual reproduction is complicated. All animals have some sort of biological sex, which is harder to define than you’d think because it's a combination of our DNA, hormones, and physical characteristics. Unlike other animals, humans can also have a separate concept called gender, which involves how some people view themselves in the context of our society and culture.

Today our focus will be on non-human animals. We’ll use “male” for animals that produce reproductive cells, or gametes, that don’t take a lot of energy to make like sperm, and “female” for animals that produce high-energy-investment gametes like eggs. And sexual reproduction is a process that happens when male and female gametes fuse to form offspring, combining the genetic material of both parents.

Non-human animals whose biological sex characteristics don’t fit the male/female binary are hermaphroditic. We’ll talk about hermaphroditic animals to refer to the many non-human animals that can produce both types of gametes throughout their lives, like earthworms, land snails, or our old friend C. elegans… the nematode. Binaries, like “yes or no” or “male or female”, are actually pretty rare in nature.

So let's start by sorting through the puzzle pieces of biological sex, which varies across animals. It's a combination of a lot of biological signals and structures that make sperm, eggs, or both, or neither. Genetically, various biological instructions made of DNA called genes influence animals’ biological sex.

And in many species, these genes (along with plenty of other non-sex genes) are found on allosomes, or one particular pair of chromosomes, which are big chunks of DNA. Like some animals -- including humans -- have an XY sex determination system. We don't all share the exact same X and Y chromosomes, but similar allosome combinations produce similar biological sex characteristics.

So in the XY system, individuals that get two of the same allosomes produce high-investment gametes and are called female. And individuals with two different allosomes produce low-investment gametes and are called male. Other animals evolved a ZW sex determination system.

Here, biologists picked different letters because the patterns work differently. Animals with two different allosomes produce eggs and are called females, and males producing sperm have two Zs. And -- to be clear -- not all Z or W-chromosomes are the same across animal species either, just like not all Xs and Ys are the same.

ZW, XY, and other chromosome systems evolved independently multiple times from different non-sex chromosomes throughout animal evolution. Like the X0 sex determination system, which shows up in our friends C. elegans the nematode and C. elegans the tarantula. This system only has one type of allosome, and biological sex is determined by how many copies the animal gets.

Males get just one X, and individuals with XX are female or hermaphroditic depending on the species. Very efficient! But like I said, genetics is only one piece of the biological sex puzzle.

For some animals, the environment they happen to be in as they develop plays a bigger role than genetics. The most famous example is temperature-dependent sex determination, which shows up in some reptiles and fish. Under normal conditions, some parts of the nest are cooler than others and a mix of males and females hatch.

But if the nest is too hot or cold -- even by just a few degrees -- we get only one sex. All of the animals we’ve talked about so far -- whether their biological sex is determined by their genetics or their environment -- stay one sex. But many animals are serially hermaphroditic and switch which type of gamete they produce based on cues in their environment.

Let’s live a day in the life of one of the most famous examples! Allow me to introduce the clownfish. Our clownfish is the largest male and leads his small group with his mate, the female fish.

Cued by the moon, these two are the only ones to reproduce, laying their eggs on a flat surface near their anemone. The father takes good care of his eggs, fanning them with his fins and chasing off predators. Like all other clownfish, our clownfish began life as an undifferentiated juvenile, who are neither truly male nor female because they don’t make gametes, and don’t mate with the leader fish.

He spent his early years tending to his anemone home. But even big, dominant clownfish don’t live forever, and every once and a while, the largest female gets eaten by a predator or dies of natural causes. Losing their only reproductively active female would be a disaster for the fitness of the colony… but our clownfish has a creative solution!

He grows rapidly and his biology changes, taking her place as the largest female. The largest juvenile then moves up, and becomes a sexually mature male, and mates with the new female leader. The new breeding pair takes over the colony and starts having their own eggs.

Finding Nemo would’ve been a very different movie if it was 100% scientifically accurate. Have a nice day clownfish! So animals can end up producing different gametes based on their genetics, their environment, and even what their social group needs to survive.

But the challenges of sexual reproduction are just getting started. To get their gametes together, animals still have to find mates and do the whole sperm-and-egg-combining thing. To help them find mates, lots of insects have special sensory organs.

Like the male silk moth has huge, bushy, super-sensitive antennae that can sense chemical signals from female moths from nearly 3 miles away! Other animals, like deep sea anglerfish who live in the vast, empty deep sea, are very clingy once they find a mate. The male latches onto his mate, and doesn’t let go, eventually fusing with her body and providing her with a reliable supply of sperm.

Once an animal knows where its potential mate is, the next step is to get there and actually combine gametes. Which for some animals, like those with a rooted-in-place lifestyle, is a lot more complicated than it looks. One strategy is to let the gametes do most of the travelling.

Like once a year when the moon and water temperature are just right, entire colonies of coral reefs release their eggs and sperm into the ocean, creating an underwater blizzard of gametes! Once animals needed to mate on land, many evolved a system where their gametes meet inside one of the individuals since they couldn't count on their gametes floating in water anymore. Which requires the animal producing the low-investment gametes like sperm to have an intromittent organ to deliver those gametes.

Like spiders use their pedipalps, their chemo-sensitive front limbs. Although...their testes containing the sperm are all the way back in their abdomen, so they have to first put their sperm on the ground and then pick it up again with their palps before being ready to mate. Besides the actual logistics, many animals also have to contend with convincing a member of the opposite sex to be their mate.

This is where sexually selected traits, or traits that affect an individual’s ability to mate, come into play. Like antlers to fight off rivals or brightly colored ornaments or an impressive song to attract a mate. Many of these traits show up only in males because typically the sex that has to invest more time and energy into reproducing gets to choose the mate -- and often that’s females since eggs take more energy to make.

So in many species, females choose mates, but it’s the opposite in seahorses, some spiders, and others where only males protect the eggs. That’s assuming offspring are the goal. For offspring to be produced, both high and low-investment gametes are needed, but that’s not the only mate pairing we see in nature.

Same-sex sexual behavior happens all across the animal kingdom, and has been observed in 1000 species and counting. In fact, it could be an evolutionary adaptation passed down from a common ancestor. Like red flour beetles engage in same-sex behavior where two males will couple with each other.

And we think this behavior helps the beetles get rid of old, low quality sperm. Lots of social animals, like dolphins and bonobos, engage in same-sex sexual behavior too, which may help maintain bonds within their group. Whatever gametes you have, the whole process of sexual reproduction is complicated, messy, inefficient, and even dangerous, so we might wonder why we haven’t evolved something better.

Actually, some animals have evolved to reproduce asexually, where sex isn’t needed to make offspring. But, these species are way outnumbered by sexually reproducing animals, which suggests that sex is (evolutionarily) better. We're still exploring why that is, but there are lots of interesting hypotheses, including two big ideas all about shuffling genes around.

Since sexual reproduction results in offspring with a mostly random combination of genes from their parents, it increases the genetic diversity, or the number of unique combinations of genes, in a population. This can prevent harmful gene mutations from becoming widespread, and might lead to potentially helpful combinations of other mutations. But there’s also the Red Queen hypothesis.

Mixing and matching genes through sexual reproduction might be super important for evolving defenses against external factors like parasites, viruses, and disease-causing bacteria. But since these threats are also constantly evolving, animals are locked into sexual reproduction just to keep up. And at the end of the day, the work of sexual reproduction isn’t really over.

Many animals put a lot of time and energy raising their fused gametes into fully developed offspring by feeding, protecting, and even teaching their babies until they can take care of themselves… sometimes with substantial costs. But all that effort is worth it to pass their genes down to the next generation. Sexual reproduction is one of the four key parts of being an animal, so it’s maybe not so surprising that animals do all sorts of interesting things to make it happen.

Next episode, we’ll keep with the theme of animals interacting with other animals by covering how they work together in colonies and societies to do things they couldn’t do alone. Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE. It is shot on the Team Sandoval Pierce stage and made with the help of all these nice people.

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