Science Sunday: The Diversity of Animal Sex
As many of you are aware of, sex is kind of a big deal for humans. To the surprise of absolutely no one it’s a big deal for other species too, if not for the same reasons. Sex is for many animals the only way to get offspring. So the different members of species will participate in sexual reproduction, trying to ensure the existence of their kind. Some will even die for it.
I have narrowed this down to animals, because going through the different reproductive cycles of plants would turn this into a rather large book. But fear not, as there are still many examples of different ways to reproduce in the animal kingdom that we can look into.
To start off we can look at one of the simpler organisms, namely the earthworm. Earthworms are quite fascinating little invertebrates, more so because they are hermaphrodites, meaning that all earthworms have both male and female sex organs. Contrary to what many might believe, this does not signify that earthworms usually reproduce with themselves. As the reproductive organs are on different segments on their bodies, the earthworms will instead lie head to tail towards each other to copulate. After the eggs are fertilized, the earthworm will leave them in a slimy cocoon to mature, considering it’s job as a parent done. This is a pretty efficient way to reproduce, as both of the earthworms get fertilized during a mating.
For some species with a more complicated physiology, the reproduction can tend to be more elaborate, as it is with many birds. For those birds that do not form life-long bonds, there’s often an arduous courtship process for the male. Male birds will try to build a big and sturdy nest to attract a female, or they might try to gain her goodwill by giving her food. Some birds even put on big displays to challenge other males and to attract mates. The peacock is a great example of a bird that is running around with a lot of unnecessary plumage in order to be as impressive as possible. When two birds are satisfied about their choice of mate, they will of course copulate. The act itself differs a bit from what you would expect, as many birds do not have reproductive organs similar to those of mammals. Instead, both the males and the females have a cloaca, where they excrete feces, urine and sperm/eggs. So to copulate, the male will usually balance on top of the female until their cloacas touch, which is called a “cloacal kiss” (that’s all I’m going to say about that term). After the internal fertilization is done, the eggs will develop for a while, before the female lays them and start the brooding period.
Most fish species have largely dropped everything connected to internal fertilization, and instead they depend on the cooperation between the male and the female to get the eggs fertilized externally. Female salmons will, for example, carefully make small nests in the gravel in the shallow rivers which they return to. A female and a male will then release their eggs and sperm at the same time in the nest, initiating the fertilization of up to ten thousand eggs. They repeat this several times, and each time covering the old nests with gravel to protect their offspring. However, the story ends in a sad manner for the new parents, as they expend all their energy on the reproduction as they migrate up the river that they once hatched from. In the end, all of them die, and their offspring swim towards the ocean a few months later, starting the process anew.
It’s not just for salmon that sex can get an unhappy ending. For spiders, it usually ends badly for the male. First of all, spiders do have an internal fertilization, but it is indirect. This means that there’s and intermediate step where the male first ejaculates on a small web, before he picks up the sperm. When the male finds a female spider of the same species, he will try to fertilize her. The problem for male spiders is that they are usually a lot smaller than their female counterparts. Females will often mistake males for prey, and the male then ends his life as lunch. Spiders therefore often have courtship rituales in order to recognize each other, which makes the female a bit less inclined to eat her mate. This is often for naught for the male spider in the end, as the female has a tendency to eat the male afterwards anyway. His sacrifice does at least not go to waste, as it provides important nutrition for the female, and a small evolutionary advantage to their offspring.
This is of course just a small sampling of different weird ways that animals can procreate. The major point is that they are all different strategies which work well for the different species. The diversity of strategies also points back at a long evolutionary process, as each species has weighed the amount of child-rearing against the number of offspring and many, many other factors. The physiology has no doubt played a big role too, being both a limiting and enabling element. In the end, there are very few things which are as fascinating as the diversity that evolution can produce.