Biologically speaking, why is it that animals – all of them, as far as I know- reproduce males and females of their species at roughly the same rate?
If someone wanted to produce as many hamsters as possible given a fixed amount of fertile hamsters, food, water, and space, he would most likely choose just one or two males and make the rest females. This arrangement would likely produce the greatest number of offspring because one male could easily impregnate dozens of females.
The concept is so simple that even a six-year-old could understand and apply it. And yet, nature doesn’t do it this way.
Why do animal species produce so many males? Why can’t species evolve the ability to produce more females than males to maximize their reproductive success??
That may be so for the species, but it’s not so for the individual. Where the sex ratio is unequal with fewer males, the payoff in terms of offspring from producing more males is higher than for females. A mutation favouring more male offspring would pay.
This is not the case for all creatures, ants being an example due to sterile workers IIRC.
Your assumption is flawed. There are a number of animal species where one sex greatly outnumbers the other. Hive insects such as ants come most readily to mind. The workers, which comprise the vast majority of the population, are infertile females. Among the fertile population, the males outnumber the females.
I should add that besides the well-known cases of hermaphroditism and asexual reproduction, there was also a recent thread in GQ about animal species that have more than two genders. All these cases are exceptions to your assumption that all animal species have two genders in equal numbers.
One reason is that during conception, the woman donates two X chromosomes and the man donates an x and a y. So there is a 50/50 chance of having either a male (xy) or a female (xx).
Another reason is that it helps with genetic variation. Having only two males means that all offspring in a group are related.
Sorry, correct that, during conception the woman donates ONE of her x chromosomes; the man donates EITHER an X OR a Y. The only two combinations available are xx (female) or xy (male). Hence 50/50 shot.
The fact that exactly two outcomes are possible does not imply 50/50 odds.
The essential point is that sperm are close to half X and half Y. If there were some way for a mutation to alter this and if such a mutation resulted in greater numbers of offspring that the 50/50 mix does, presumably we would see examples.
The evolutionary reason is simple. Suppose you were a member of a species that was 99% females and 1% males, and that the population of the species is in equilibrium, it is neither increasing or decreasing. If you are a female, you have on average one offspring. But if you are male, you have on have on average 100 offspring. That means that males have 100 times more reproductive success than females. Any parent who gives birth to a male wins the lottery. If the gender of your child is under genetic control (which it usually is), then any organism that has a trait that makes it more likely to produce males will be overrepresented in the next generation. Within a few generations everyone is going to be descended from those male-producing individuals. And suddenly the gender ratio is back to 50-50. If you reverse the equation and make females rarer, selective pressure will increase the proportion of females.
But also, in many if not most species the limiting factor for reproduction is not the number of females. Typically the females are capable of raising many times more offspring than can possibly survive. Also in many species there is no parental care, or the males provide parental care, so the investment females make is not as great.
Oh, and the reason there are so many more cows than bulls is that humans kill and eat all but a few bulls. You have to let the cows grow up to produce more calfs, but you only keep the very best quality bulls. But wild cattle have roughly equal gender ratios, even though herds typically have only one mature male. The left-over males form bachelor herds and are excluded from reproduction by the alpha, until and unless a bachelor become strong enough to challenge an alpha, displace him from his harem, and become an alpha himself.
I don’t believe he’s asking how it is that humans currently produce roughly 50/50 males/females, but why it came to be that way.
Also, Hawthorne, I don’t follow your logic, could you elaborate?
In other words, natural selection - at the level of individuals - will generally produce a 50:50 sex ratio. If males are rare, the genetic contribution of each one to the next generation will be large. Therefore, females that produce a lot of males will pass more of their own genes to the next generation, and thus be selectively favored. If females are rare, then the opposite will be true. For many organisms, this ends up producing a ratio close to 50:50.
Note that this doesn’t apply strictly to the numbers of each sex produced, but to the parental investment in each sex. For example, if male offspring are larger than females, and require more energy to produce, one would predict their would be fewer of them. What is being equalized is not numbers but energetic investment.
However, Fisher’s Theory only applies to large, randomly mating populations. Hamilton (1967) extended the analysis to other kinds of mating systems:
Emacknight, the question refers to all animals, not just humans.
For those species where it is applicable (which would be most of them), the ratio of female:male is approximately one-to-one over time; that is, there will be times when more males are born and times when more females are born. For some animals, sex determination is affected by temperature, parasitic infection, mating behaviour (e.g., incest), and availability of resources. Some species are capable of switching between “male” and “female”, while others are purely hermaphroditic.
There are many many many other methods of sex determination out there. Lots of them don’t have a 50/50 bias built in.
For example, there’s a species of marine worm where the male develops into sort of a little parasite on the female’s body. Sex is determined by where the larva lands. If it lands on a female, it becomes a male. If it lands anywhere else, it becomes a female, waiting for a male to show up.
I’d suspect that with a little research, we could find a whole lot of other exceptions to your rule.
Lemur866 has given a good explanation of why the sex ratio is close to equality in diploid animals. Strictly speaking this argument (which was devised by the late great RA Fisher) applies to equal parental investment, but when male and female children cost about the same to produce the result is close enough.
But the sex ratio is not always 1:1. Hamilton [“The genetical evolution of social behaviour” ‘Journal of Theoretical Biology’ 7, pp 1-52 (1964)] showed that in eusocial haplodiploid animals the proliferation of the reproductives’ genes is maximised by a 1:1 ratio, but the proliferation of the workers’ genes is maximised by a 1:3 (males:females) ratio of investment. Trivers and Hare [“Haplodiploidy and the evolution of the social insects” ‘Science’ 191 pp249-63 (1976)] found that in most species of ants the sex ratio of reproductive offspring of a colony is indeed 1:3. But later work (for which I cannot find a citation) has found that among the slave-taking ants and the species that kill the queens and take over the colonies of other species, the ratio is 1:1.
The sex ratio in honeybees does not conform either to 1:1 of to 1:3. This is probably because a drone by himself and a young queen with a swarm (or a young queen who keeps the nest while her mother goes off in a swarm) represent very different parental investments.
Haplodiploidy is the key her, not eusociality alone. Naked mole-rats have a 1:1 sex ratio. I don’t know about termites.