I think this must be a common fantasy, seeing how often this situation crops up in fiction - due to war/natural disaster/quirk of nature, one sex now greatly outnumbers the other. Changes in social norms and living arrangements must be made, blahblahblah. I think it was mentioned in one of David Eddings’ books
Anyway, I then heard that this situation is self-correcting, because if one sex outnumbered the other, any mutation that produced more of the rarer sex will spread quickly, bringing the ratio back to 1. So, is this always the case? Any species with more births of a certain sex? All I can think of are ants and bees. And don’t some frogs’ sex change with temperature? Any more “advanced” animals?
This is expressed in Fisher’s Principle, that states that sex ratios will tend to 1:1 because each sex contributes equally genetically to the next generation and thus is equally valuable to a parent. (More precisely, there will be equal parental investment in each sex. For example, if males are more “expensive” to raise to adulthood, then fewer males may be produced compared to females.)
In social insects the principle doesn’t apply to sterile workers. Reproductives will be produced at a 1:1 ratio, taking into account any differences in how much energy it takes to produce each sex.
Many fish are sequential hermaphrodites. All members of some species start out as one sex, then change to the other at a certain size, or if they are the dominant individual in a group. Which sex changes to the other depends on the relative reproductive advantages of large females vs large males in that particular species.
Sorry, but this is not true for bees- several hundred drone (male) bees may be produced in a year, but only a few (maybe 10 or so) queens.
Incidently, if the queen is lost, worker bees will actually start laying viable eggs. These will all be male, as the workers haven’t mated, so they’re not truly sterile either.
In the case of dogs and many other mammals the sex of the child is heavily influenced by the relation to what part of her cycle she was in when she mated. A very early breeding of dogs will usually produce all females. If she produces all females it will increase the compettion and lead back to a proper ratio.
Yes, I should have specified honeybees. Not really convinced by that proposal though, I’ve come across it before, and it does look a bit like “I’m going to prod data until it does what I want”…
Still, there are several species of invertebrate that are entirely, or almost entirely, female- for example some stick insects have no known males, and some have a sex ratio of thousands to one, and daphnia normally reproduce parthenogenically, and only produce some males in severe conditions (after mating they lay eggs which can remain dormant until conditions improve).
Basically any species which can reproduce by parthenogenesis can wind up with very unequal sex ratios, even if that isn’t necessarily great for long term species survival.
IIRC, there’s a mutation in insects that causes only males to be hatched; it’s carried in male eggs and poisons female eggs. It’s a single gene mutation so it spontaneously appears on a regular basis, and spreads itself efficiently by eliminating all competition, despite its ultimately self defeating nature. It follows a cycle of appearance via mutation-> rapid spread-> population collapse when there aren’t enough females.
There’s also a parthenogenic wasp species that hatches only females because of the influence of a bacterium; eliminate the bacterium and they can produce males. I recall that one because a scientist quipped “penicillin can cure virgin birth.”
Males tend to have a higher mortality rate for a variety of reasons, including being more prone to be killed in accidents, fights, warfare, etc. What counts with regard to evolution is the sex ratio upon reaching adulthood rather than that at birth.
This is often true in domesticated animals, but not naturally, rather as a result of human intervention.
Often these animals are bred via artificial insemination, and the transported semen is treated to increase the likelihood of one gender.
For example, in pleasure horses, mares are preferred. (Except in racehorses, but AI isn’t legal for racehorses.) In cattle, beef breeds prefer steers, while dairy breeds prefer heifers (obviously). For laying chickens, you need hens, but meat chickens are mostly male.
I was offering an explanation of why there might be more males at birth. If one sex preferentially experiences greater mortality before reproductive age, this can produce selection for there to be an unbalanced sex ratio at birth in favor of that sex.
Those articles offer other explanations as well. Not just survival. In times of stress more males are born not only, according to the stated hypothesis, because males are less likely to survive, but because the investment pays better returns in stressful times:
