The Hidden Patterns of Sexual Evolution
This is the 8th article in the Game Theory of Sex series:
What Makes Men and Women Different explores core biological differences.
Other Ways Men Have Evolved to Have More Children focuses on men.
Where Is The Game Theory of Sex Series Going? explains the aims of the series.
How Have Women Evolved to Attract Men? focuses on women.
Article 4 covers a bunch of questions, like short-term vs long-term mating strategies, why we have sex while women are not fertile, why we love, and why we enjoy sex.
The Shape of Us explores what types of body shapes are attractive in men and women.
Bestial Kingdom dives into the animal kingdom to draw lessons for humans.
In the last article, we talked about dozens of weird sexual adaptations in animals. What can they tell us about humans? What are the hidden rules of sexuality that shape the bodies and lives of animals—including us?
As a rule of thumb, it makes sense for females to be as big as possible, because that allows them to have more resources to lay more eggs and support their offspring better. We shouldn’t be surprised to see animals where the female is bigger.
So when the males are bigger than the females, something else is going on. What environments cause that?
One of the key considerations is whether the animals live in 2D or 3D.
2D vs 3D
When an animal lives in three dimensions, like in the sea, the air, or forests, it’s very difficult for the male to control the female, because she can easily escape and hide. As a result, the female has ample choice in selecting her mate or cheat, and the male has less incentive to coerce the female over time, or to best his enemies through force. The result is much more female choice, and much less male-male competition than for animals living in two dimensions, such as on land.1
That’s why female and male fishes and birds are usually similar in size, and the males are much more colorful than the females: Because it’s hard to control the females, they are the ones to choose their mates. Males need to differentiate themselves from other males in a way that females find attractive.
Since males need to differentiate themselves in ways that are compelling to the female, they might evolve to invest more in the offspring. Frequently, the result is monogamy in fish and birds. Long-term partnerships form, usually for the mating season. Monogamy for life exists, but is rare. That makes sense: If you mate only with one partner, what happens if that partner is not a great match genetically? What if he is not adapted to a future threat? The more partners you have, the more genetic diversity for the offspring. These animals are monogamous2 to grow offspring together, and then diversify their partners.
This is also why there’s a fair amount of cheating, even in monogamous animals (defined as those forming long-term partnerships): It’s one thing to get a mate to help defend you and the nest and bring food, but something completely different to get his sperm, or that of a Chad. It’s also why a lot of sperm competition exists in these animals: Since males can’t control the female, and the female cheats, males need to find ways to increase the probability that they are the fathers.3
Compare this with animals who live in 2D—that is, on a surface. There, a male can control the movement of potential female partners or male rivals much more easily. This means it’s easier to protect or coerce females, and fend off rivals. Here, males have an advantage in becoming bigger: The stronger they are, the easier they can fight off other males and win.
Because males are bigger than females in such environments, and because females can’t easily escape, males in 2D environments are in a much better position to force, harass, or intimidate females into sex.
The result is that in 2D environments (land), males tend to be bigger than females, more aggressive sexually, develop more weapons, and don’t need to look especially pretty.
With stronger and more aggressive males, females might find it beneficial to join harems to reduce harassment from other males. If this is true, harems should be more common on land than in the sea or air.
The exceptions here are illuminating. Take the elephant seal that we’ve discussed earlier. It’s an aquatic animal, isn’t it? But where does it breed? On small stretches of beach! So the females can be extremely concentrated, which means they’re very controllable physically, and hence males have evolved to be dramatically bigger.
The male Lamprologus Callipterus fish can be up to 60x bigger than the female. There’s a reason for that.
Females lay their eggs in snail shells. For that, they need to be small. Males, meanwhile, are the ones collecting the shells, so the stronger they are, the more and bigger shells they can carry, allowing them more mating opportunities.
The males put all the shells close together to defend them more easily, and at this point it becomes nearly a single point, a 0D harem.
If we summarize this theory, animals in a 3D environment will tend to have:
➕colorful males, with colors and feathers
➕long-term partners, for the season
➕male investment in the offspring
➕cheating
In 2D, instead, they’ll tend to have:
➕stronger males, with more muscle and weapons
➕aggressive males
➕forced copulations, harassment, and intimidation by males against females
➕harems
➖cheating
Density of Individuals and Their Predators
Imagine that you’re in an environment with lots of predators. If you roam around, odds are you’re going to get killed, so if you find a nice place to hide, you’ll survive for longer. What happens in a situation like this?
The female, who is more valuable, will tend to stay put. She will find a good spot and remain sedentary there. This is especially good, because it allows her to feed from her immediate environment and bulk up, which she needs to lay eggs. Females might invest some time to grow more before reaching sexual maturity—ie, before focusing on kids.
What about the males though? They need to roam around to find sedentary female mates. But because there are lots of predators, there are not a lot of mates around. The density of mates is low. So the males need to search for longer for potential mates. That’s extremely dangerous with so many predators around!
What’s a good solution?
First, become sexually mature as fast as possible, to increase the odds that they find a mate before they’re eaten.
Second, be as small as possible: It helps mature sexually faster, but it also makes it harder for predators to see and eat them.
Third, develop cannibalism. With such small males, it’s easy for the female to eat them. The males aren’t likely to survive much longer anyway, because they’d need to rove again and before they find another female, odds are they will have been eaten by a predator. So it might be better to give that protein to the female—and hence offspring. So sexual cannibalism is logical in that situation.
Since it’s so hard to find females, there’s little male-male competition, so little incentive for males to be bigger to fight other males.
Also, in these circumstances, it might be beneficial for more males to be born than females. This is what we see in a lot of these species.4
If you think about it, this is a bit like spermatozoids and eggs, but at a bigger scale: The egg is big and expensive to make, because it contains not just the nucleus, but many other cell contents that are needed for the animal to grow.5 The spermatozoon primarily contributes just the genetic material. So the egg is bigger and doesn’t move as much, while the spermatozoon is small and does the moving. You can have a ton of small spermatozoa, because they’re cheap to make and you can waste a lot, as long as a few make it to the egg.
So that’s a situation where you have a ton of predators and little individual density. Conversely, if there are not a lot of predators around, and if there are enough resources, the density of individuals of a species might be very high. In that case, there will be a lot more competition between males to mate with females. The males will be more likely to grow bigger and stronger, and to develop sexually coercive behaviors.
Life Expectancy
If a species doesn’t live very long, it will tend to be smaller, reach sexual maturity earlier, and be as aggressive as possible in finding a mate. The male might do anything he can to mate with a female. That can even include dying in the process, as we’ve seen in spiders and mantises, or FC6 (including those that just hatched from an egg), like in fruit flies.
Social
Some species evolve to be social. This can change the entire mechanics we’ve seen so far, because now individuals are not alone. They can ally with other individuals to achieve their needs. We saw gang FC in mallards, but also matriarchal alliances in hyenas, and harems. We also saw in a previous article that human men have probably banded together to kill more dominant men, and coalitions of females have been seen in several primates, and exist in humans too.
Depending on the social dynamic, this will influence how the sexes evolve. In the case of a species with a harem, the male will likely become bigger. In the case of a species that collaborates for hunting like hyenas, chimpanzees, or humans, there might be pressure to limit the size of males, because those who are too dominating will be killed by alliances of males.
It's interesting to think about the human combination of being social and having evolved in a 2D environment. The 2D environment fostered development of instincts for sexual coercion. But we have also evolved strong social behaviors to counter them. Now, our social nature has prevailed, and we are replacing competitive selection with pure mate choice. This is obviously better for most people, including most women and non-aggressive men, but not for more aggressive men.
2D vs 3D, density of individuals and predators, length of life expectancy, degree of social behaviors… A lot of these are speculative theories for now. Very few scientists have explored them. Some of them are probably true, and there are probably many such rules that influence sexual differences. Their combinations in various environments, as well as some idiosyncrasies, have produced the wild diversity and complexity we see today.
What is surprising is that this type of rule even exists! We tend to look at animals and ourselves as just the offspring of idiosyncratic evolution, when in fact there might be strong, fundamental forces driving it all and we don’t even realize it. We know surprisingly little about all these rules.
This means that the animal kingdom is shaped by hidden laws that we don’t yet fully grasp. I bet they exist.
The key insight around 2D vs 3D sexual behavior comes from Beauty and the beast: mechanisms of sexual selection in humans, David A. Puts.
Usually for a season, sometimes more.
It’s also why rape might exist in some of these species: It’s hard for a male to always keep tabs with the female, but it’s easy for any male (or group of males) to zero-in on a female they notice and take advantage of her.
This theory is laid in this paper.
Is this one of the reasons why human females don’t produce eggs after birth?
Forced copulation
You asked why newly hatched birds are helpless (referred to as altricial) as opposed to other birds like mallards whose young are able to fend for themselves soon after hatching (precocial). Mother bird has to invest a lot of protein in an egg to support enough development of the fetuses so that they can walk and feed on their own soon after birth. In seed, fruit, and nectar eating birds it may be difficult for mother to find enough protein to invest in the egg for that much development. Precocial young in birds, mammals, and other groups have the advantage of being able to flee predators soon after hatching, whereas altricial young are helpless and depend on hiding to avoid predation. This is probably why ground nesting species like ducks and ostriches have precocial young... they need to be mobile to try to escape ground predators. Ungulates also have precocial young, probably for the same reason of escaping predators. Think antelopes on the Serengetti plain in Africa, with big cats, hyenas, and hunting dogs all looking for a meal. Primates are unusual among mammals in having helpless young and perhaps this is due to a physicial limit on how big a head can fit through the mother's birth canal.
Another interesting contribution to this series. Once again weighing in as a biologist, I think the 2D vs 3D perspective is an over simplification. There are many exceptions. Birds with flight have a 3D life history, but one finds a wide range of mating systems from life-time monogamy to extreme polygyny. A more important factor is the distribution of resources essential to breeding and a male's ability to defend resources. When resources like good nest sites or food are unevenly distributed, then a dominant male may be able to defend high quality territories and attract multiple females to mate polygynously, as in red-winged blackbirds. When nesting and feeding resources are more uniformly distributed, as in tropical rainforests, then monogamy is the more likely mating system since male contributions to rearing chicks is more important. Even in monogamous species, however, both sexes can increase the genetic diversity of their offspring through sneaky extra pair copulation. And then there are lek mating systems, where males gather in communal display areas to signal, and females visit only to obtain sperm from one male and then leave to rear offspring entirely on their own. Leks can be 2D, as in frogs and grouse, or 3D as in birds of paradise and some bats. Hard to come up with dimensional contributions to leks. Add in scramble competition mating systems when large numbers of males and females congregate for a single seasonal mating event, as in gnats and wood frogs - no rules here, it's every frog or gnat for itself. The more I've learned about mating in the myriad of different animal groups over my career, the less confident I am in coming up with "rules" to teach students.