Why don't you die after you have sex?S

Did you or do you or will you enjoy your long stretch of reproductive years? According to Cole's Paradox, you shouldn't. Many species need just a tiny increase of fecundity in order to justify one shot at sex, followed by death. Find out why you "should" die after you copulate, and why you don't.

Semelparity and Iteroparity

All your life, without knowing it, you have been iteroparous. And that's a good thing, too, because anything else would mean immediate death after a single shot at mating. Iteroparous animals reproduce many times over the course of their life, or at least take a crack at it. Semelparous creatures, on the other hand, have one chance to mate, give it their all, and then die afterwards. The most famous examples are spawning salmon, who swim upstream, dodging bears all the way, in order to seriously affect the water quality in the stream where they were hatched and then die immediately afterwards. The female giant pacific octopus protects and grooms her eggs, and as they hatch, she blows out a stream of water to help them on their way while she dies. There are even a few male marsupials who will get a huge adrenaline spike just as mating season comes on, wreck their own immune system by mating constantly, and then die horribly.

For obvious reasons, this system favors the animals that can abandon their young rather than those that have to stick around and raise them. Semelparity is not the default state for animals. It requires a huge and well-timed spike in energy, the ability to ignore even the most basic survival needs in favor of mating, and the production of offspring that can fend for themselves. It seems that every instance of semelparity evolved from iteroparity. Given the requirements for a successful instance of semelparity, it's a wonder it evolved at all.

Cole's Paradox and Semelparity

Or so it would seem. Lamont Cole, a biologist who spent the early 1950s crunching numbers, published a model of population growth based on reproduction that startled quite a few people. There should be a lot more semelparity in the world, he claimed. He was looking at reproductive rates as a way of figuring out why some animals lived a long time while other animals lived only a short time. He found that species that were annuals, and lived only one year, needed to produce only one single more offspring per "litter" than species that were perennials in order to maintain the same population growth. So a tiny increase in litter size favored semelparity. Why, then, was it so rare?

Why don't you die after you have sex?S

There are reasons why humans don't go semelparous. Someone has to raise the baby, and until there are three sexes, male, female, and parent, we will have to put a lot more work into raising our offspring than the salmon put into raising theirs. But how did we evolve in the first place? If semelparity is favored, why would there even be ancestral primates that needed multiple years to raise their offspring? Why aren't we all like the unfortunate Australian marsupials? This schism, between the seeming advantages of semelparity and its relative rarity, became known as Cole's Paradox.

Better Modeling Means We Live!

Cole's Paradox, like many paradoxes that are refuted by a quick look at the evidence, couldn't stand for long. People were quick to point out flaws, and build better models. For example, Cole didn't look at juvenile versus adult mortality, he just assumed that everyone survived until their appointed time to die. In the real world, there's a rate fertile adult mortality and a rate of juvenile mortality, and the difference between those two rates has a lot of influence on whether semelparity is an option. If a species has an abysmal rate of adult mortality, but juvenile mortality is relatively low, it's best to turn things over to a new generation than suck up resources.

Why don't you die after you have sex?S

The same goes for environmental factors. Reproduction almost always requires risk and energy — whether it's salmon swimming for miles to get their eggs to a nice quiet place in a stream where they can safely hatch or desert marsupials who have a short season of plenty, during which they have a little extra time on their hands. If it takes 90% of a species' energy to produce young, using 100% of that energy to produce a bigger litter is a better strategy than conserving 10% and having a small litter.

And, of course, there's the relative likelihood of wiping out thousands of salmon versus the likelihood of killing off five lion cubs, or of one set of humans not conceiving for a year or two. In the end, there are some animals that are shaped by their environment, their reproductive systems, and their inherent traits into a long-lived species with potentially high failure rates for reproduction, and there are some that become short-lived species that can really pump out a winning litter. We happen to be the former. I don't want to be too speciesist, but I think we can all be grateful for that.

[Via Semelparity and Iteroparity, Why are Life Histories So Variable, NCBI]

Images: top to bottom: Pressmaster/Shutterstock, Sekar B/Shutterstock, Mogens Trolle/Shutterstock.