Does all the stress of finding a partner get you down? Do you ever wish you could just start a family of your own, with kids that looked just like you, but without all of the trouble of finding another individual to mix sex cells with?
If you’re a human, tough shit (for now, anyway). But if you come from a number of fish, reptile, and even bird species and happen to read Gizmodo, you can forget about sex and romance. That’s right—teeny critters like hydra and bacteria aren’t the only creatures that produce asexually, and you may be surprised to learn even some big creatures like hammerhead sharks have done it.
Before we start though, a little science lesson to make explaining this stuff easier: Lots of animals have two entire sets of chromosomes, with each chromosome matched up to another. Through the process of meiosis, a germ cell with two sets of chromosomes splits into sex cells, each with only one set of chromosomes. When the male and female sex cells meet, they fuse and once again create cells with two pairs.
So if you’re alone this Valentine’s Day, take solace in the fact that somewhere out there, some creatures are just living their lives and procreating without the burden of flowers, wine, or any social interaction at all.
Scientists have observed hammerhead sharks giving birth seemingly without a male around to provide its own sex cells. But how could one be sure that the female sharks weren’t just storing sperm inside themselves from a prior sexual encounter, like this horrible crab?
Back in 2001, one of three female hammerhead sharks at a Nebraska aquarium gave birth. It would have been nearly impossible for them to store sperm, since all three were caught as juveniles three years prior. A team of scientists performed some genetic tests, and in a 2007 paper confirmed that, yep, one of the sharks had reproduced asexually.
How’d the shark do it? Sharks, too, normally go through meiosis. But through a process called parthenogenesis, some animals may still produce viable offspring on their own—perhaps those sex-cell forming cells didn’t split their chromosomes up, or two of the sex cells fused with one another. The question is, well, did the shark do this on accident or on purpose? Such is the case with other animals, too, like...
Two Komodo dragons in the United Kingdom also gave birth to viable offspring without a mate in another widely reported story back in 2006. One, Flora, laid 11 eggs (eight of which developed normally) and another, Sungai, laid 22, four of which yielded normal male dragons.
That might sound confusing—shouldn’t the child be a clone, and therefore female? That’s not how it works, though. The corresponding chromosomes in each set are not necessarily exactly she same. Meiosis also happens to shuffles the genetic data around before creating sex cells. These dragons managed to double the shuffled-around chromosomes after meiosis occurred.
But why were the offspring male? You may be familiar with the sex determination system where individuals are female unless they have a Y chromosome which gives an individual male features. In Komodo dragons, the so-called “ZW” system is flipped, where ZZs are male, and the presence of a W gives the dragon female features. So, if an egg with a single chromosome set had a Z, then managed to double its genetic material, it would end up with two Zs, and therefore hatch a male dragon. Two Ws and it would die, probably.
Also, I hate to break it to you, but this sort of proves that if Mary really did had a viable virgin birth (which isn’t possible, since mammals don’t undergo parthenogenesis), then Jesus would be female. Just saying.
But scientists have called into question whether those Komodo dragons and sharks really were occasionally reproducing asexually, or just got lucky after a meiotic accident. There are some species of lizard that really do reproduce asexually as a rule, like the all-female whiptail lizard.
How does that work? These lizards simply always produce eggs with two full sets of chromosomes. They happen to have twice the number of chromosomes as some other lizard species, reports Scientific American, and a different method of shuffling this genetic material around. This allows them to be more genetically diverse.
One meiotic error led to a meiotic requirement, and a mutant crayfish taking over Europe that you may have heard about.
Just last week, scientists published a paper about the marbled crayfish, a new all-female species that only evolved 25 years ago through a wacky mutation. A German collector kept a crayfish as pet in the 1990s, and it had lots and lots of offspring. He gave them away to his friends, and they, too, soon had hundreds of asexually reproducing crayfish on their hands. Soon, the owners were dumping their crayfish in the wild, where they are multiplying like crazy to this day.
What happened? Apparently, a female and male must have mated a while back. Meiosis could have crapped out for one of the sex cells and produced a cell with two sets. That cell still successfully merged with another, regular sex cell with only one set. This led to an animal with three sets of chromosomes. Somehow that animal survived, decided it didn’t need males anymore, and just started giving birth by making its eggs form embryos on their own.
Unlike the Komodo dragons and hammerhead sharks that are accidentally reproducing but not forming clones, marble crayfish are literally cloning themselves.
Back in the 1950s, a team of scientists at a Maryland agricultural research center discovered some turkeys reproducing asexually. As many as 30 percent of unfertilized turkey eggs may develop into baby turkeys. Few people seem to be talking about this anymore.
One thing you might think about asexual reproduction is that it’s bad for genetic fitness. Inbreeding reduces genetic diversity, for example (though that’s not as problematic as you might think), and can cause a species to accumulate unhelpful mutations. But there’s one fish that seems to be doing just fine.
Scientists just this week reported finding very little evidence of those accumulated mutations in the all-female Amazon molly species. The molly still requires sperm from a mate, but doesn’t incorporate the sperm into its egg—it just uses the sperm as a trigger to get its own development started. The mate is always from a different species.
“What preserves genome integrity in the Amazon molly is unclear,” Pedram Samani and Max Reuter author write in a commentary for Nature Ecology and Evolution. It just goes to show that some rather advanced asexually reproducing species can do just fine.
Some species can reproduce both sexually and asexually. But the offspring produced asexually are usually missing something.
Take lots of insect species, like bees. Unfertilized eggs with one set of chromosomes result in males. Fertilized eggs with two sets become female. Males with two sets can be produced in the case of the honeybee, but the workers will usually eat them.
While earthworms are all hermaphrodites, they do not reproduce asexually, contrary to popular belief. You can even watch some earthworms having sex right here. If you cut an earthworm in half, the head might grow a new tail, but the original tail will die.
So, there you go. If you’re lonely tonight, you can always consider parthenogenesis.