Beyond X and Y: The strange world of sex strategy in the animal kingdom

For the most part our biological sex is decided by the combination of our genes. As a species we have two types of sex chromosomes—X and Y—which pair together as XX for women and XY for men. But across the vast spectrum of life, the essential processes of sex and reproduction can get a whole lot stranger still. From gender-flipping limpets to multisexual mushrooms, here are a few outlandish examples straight from the world of the birds and the bees.

Turtles: Cool guys and hot girls

A few degrees change in temperature may be barely noticeable to us but for some reptiles it’s the difference between being born as a boy or a girl. For leatherback turtles, a warm nest hatches females and a cool nest produces males, with intermediate temperatures producing a mixture. Intriguingly, it’s the other way around for crocodiles and alligators.

How or why this set-up evolved isn’t entirely certain, but it’s often the larger sex that needs the warmest surroundings. Size matters to female turtles so they can produce as many eggs as possible, ensuring at least some make it to adulthood. In crocs meanwhile, males fight over females, so being big and beefy gives them the edge in battle and mating rights.

With climate change worsening, rising temperatures could upset the balance between males and female reptiles that develop this way. Too many of one sex relative to the other could be disastrous for the majestic leatherback—yet another reason for us to take better care of this beautiful planet.

Multisexual mushrooms

Mushrooms may be more closely related to humans than plants, but they certainly don’t do things like us. Species like ours, with two breeding sexes, effectively cut out half of the gene pool’s potential mates—if you’re a male, then your only chance of reproducing is by shacking up with a female, and vice versa. Instead, why not have thousands of different sexes to choose from? This is the way of the split gill fungus, Schizophyllum commune.

For the split gill fungus, each individual’s sex is determined by two pheromone molecules which act like a personal two-part password. Each ‘word’ can be one out of a total selection of hundreds. Before mating, the mushrooms check each other’s passwords. If the combinations match up, the mushrooms are too closely related and reproduction will not take place. If however the markers are different—which is far more likely—then it’s time for these fungi to have a bit of fun.

Penis-jousting flatworms

From their name you might think that Flatworms sound like desperately boring creatures, but many are really quite beautiful to look at. However, what’s less stunning and more horrific is their mating strategy. Flatworms are hermaphrodites, meaning they can produce both sperm and eggs. When some hermaphrodite species like earthworms mate, there’s an agreeable and fair swapping of sperm: share and share alike. With the flatworm however, it’s a case of screw or be screwed.

Each flatworm really wants to be the male for each sexual encounter because sperm are so much cheaper to make than eggs and there’s no responsibility of care. They barely have a brain so discussion is out of the question, but they’re well equipped with penises to joust with—imagine an hour-long game of thumb war—but if you lose, you get pregnant. The mating is also pretty violent, with each side stabbing holes in the other in their vicious quest to ‘be the daddy’.

Hivemind of the honeybee

Queen bees have total control over the sex of each offspring by simply deciding whether to fertilise a given egg or not. Unfertilised eggs hatch into male drones, no sperm necessary. Only the female workers and future queens have a father, using sperm stored up inside the queen from her first and only flight before she founded her colony.

This bizarre system works due to the odd genetics of social insects like bees, ants and termites. Males have half the number of chromosomes of females and don’t have separate sex chromosomes like we do. The males only have one parent—the queen—which inspired fierce loyalty within them.

In another genetic quirk, females from the same queen are much more genetically similar than we are to our siblings. The close-knit genetics of the hive keeps it working together smoothly, rather like the cells in our own body.

Male to female slipper limpets

When it comes to Slipper limpets boys will be boys, but not for very long. Also known as the fornicating slipper snail, these invasive Atlantic snails are often found in stacks up to ten creatures high. These animal towers are usually made up of large older females at the base and topped off by one or two smaller males. And, as their name might give away, the stacks are all about fornication.

Each limpet starts off life as a male, travelling around and looking for a mate. After finding a female on a rock, he settles down on her and mates. After breeding, the male undergoes a 60-day sex change and then waits for a new male to come along and extend the stack, making them both sequential hermaphrodites and serial daters.