Like why aren’t lions total roid monsters if only the strongest male gets to breed the females?
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Because “strongest” isn’t the whole story. “Best able to survive on little food”, “best camo”, “fastest”, and a bunch of other things go into it. Evolution cares about ability to pass genes on and have those genes continue to pass on in future generations, not the complete optimization of any one trait.
Evolution is the process of surviving in an environment. Survival is based on the ability to do all the things an animal needs to do, like eat, sleep, run from predators or attack prey, and reproduce. Being “strong” is only possible when most of these conditions are met. An animal has to be alive to be strong, has to be eating and sleeping well to be strong, and has to have the genes for it also. But physical strength is not a common selection factor in mating systems.
That being said, the animals that have reproductive strategies that select for strength are freaks of nature. Elephant seals have a system where the largest, strongest males fight for dominance and claim all the females in that area for mating. The dominant bull elephant seal is always an absolute beast. Polar bears and other northern bears also tend to determine mating ranges based on strength, so they are giant murder machines as well. But even relatively small creatures, like the beetles in the rainforest that fight and throw each other off logs to win mates, are amazingly strong for their size.
The trouble they have is that their strength comes with high demands, and when the environment diminishes in quality, it makes it very difficult for any individual to reach it’s maximum strength. What we see selected for then is smaller individuals of the species in general. Being very strong usually is selected for when there is a high concentration of resources, such as cow elephant seals on the breeding beach, or few rich territories of food like for the polar bears. Otherwise, that strength is just a burden to maintain most of the year with no rewards in breeding frequency. At some point of a declining environment, say one with less food, the females just start breeding with survivors, not selectively breeding with the strong.
You’ve started with a leading premise.
It’s not the “strongest” male, it’s the fittest.
Strength is one of many factors that are called “honest signals”, but all signals are essentially traits that are used to imply a creature is fit.
A well fed, physically strong male likely has better genes than a sick or dying one. A peacock with more robust feathers has more calories to spare than a sparse one. A loud insect must be better at hiding than a quiet one.
But a peacock that puts too much energy into feathers and gets caught and eaten before it mates is also unsuccessful, which puts a soft cap on the process of natural selection.
To go back to your example: a “roided up” super lion is unlikely to be able to sustain itself through a lean year with poor hunting or low food and, again, may die before it mates.
Lots of things go into being “fittest”, and fittest doesn’t mean “strongest”. Huge muscles require obscene amounts of calories to maintain, wheras there’s strong but not “roided” (as you say). A lean animal without excess bulk generally has better stamina and doesn’t use as many calories to move around. It’s kind of like asking why if tanks are the strongest kind of vehicle, why doesn’t everyone drive tanks? well because the gas mileage is terrible, they’re slow, and the vast majority of people have no need for something with that much armor. It’s not that anyone thinks a sedan could take out a tank, it’s the fact a tank is overkill and impracticle and slow. Same thing with animals – bigger isn’t necessarily better.
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There’s a trade-off between muscle gain and other things, like endurance and flexibility. Those things matter a lot too for top predators. So too much muscle means you lose other advantages. And in battles over mates and territory, brute strength might not be sufficient and endurance/flexibility/quick reflexes matter too.
But, “runaway sexual selection” IS absolutely a thing. Think about peacock tails and how ridiculous they are, making the males targets for predation, and the tails make it hard for the males to run/fly. It’s possible for females to select strongly for traits that actually cause eventual problems for male survival. It all comes down to whether or not the individuals can still do well enough to reproduce and pass on their genes, that’s all that matters in the game of Evolution.
As a biologist with a focus on ecology/evolution, I’d like to add that I DON’T think OP is confusing physical strength with evolutionary fitness. From what I can tell, they are asking why the males don’t keep getting stronger and stronger in cases where animals fight for mates/territory. The issue is not knowing the trade-offs that go into fitness, rather than fitness itself as a concept. I’ve provided my answer in another comment.
Sometimes, we see two things happening together and assume that one caused the other. Sometimes we’re right. So we might see dominant animals as being the most fit and therefore becoming dominant, but the inverse might be the case. By sheer happenstance, a younger male may find itself in a position where it unseats a more aggressive, but older animal to become the dominant male. He now gets a better share of food, and can sculpt his pride by killing off younger males before they can challenge him effectively.
So while it appears that dominant males are superior because of their genetics, selective pressures might not actually be operating on those genetics, because luck and timing aren’t in your genes. Over time, yes, the ‘fitter’ male’s genetic lineage might survive, but being stronger is not always the best strategy for every species in every environment. It’s likely a complex factor of multiple traits that allow male lions to survive on the periphery of a pride without challenging the dominant male too early, or too late. Social intelligence probably has a lot more impact for lions than the sheer strength of the males, especially seeing as it’s the female lions that do the brunt of hunting to ensure the survival of the group.
Moreover, male sex chromosomes in mammals have a much smaller genetic footprint than female chromosomes. In species where a dominant male mates with many females, almost all of the genetic variation passes along the female line. This is what really drives natural selection: Genetic pressure on variation.
Lions are incredibly strong animals and absolute beasts. They are an apex predator, meaning no other animal hunts them. There is no need for them to be any stronger because they are the top of their food chain.
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