Well, it must even out, but nine-banded armadillo’s always have four babies…and all four babies are always the same gender.
It’s quite the mystery, I think.
More than just the same gender - they’re identical quadruplets. In armadillos a single egg is fertilized, but it splits into four.
Surely it is quite common for mammals to be polygamous (if that is the right word), with one dominant male controlling a whole harem of females: horses, lions, elephant seals, gorillas, are examples. Is it not the case that more females than males are born in such species? (If not, what happens to all the excess males? Do they all get killed off by stronger rivals before getting a chance to reproduce?)
I also recall being told, when I kept stick insects as a kid, that most stick insects are female, and males are quite rare.
In general, surely in most circumstances having more females than males is a more efficient reproductive strategy. I thought it was only pair-bonding species, perhaps even just those where the male helps during child rearing, that produce equal numbers.
They sit around, watch a lot of porn and complain about their miserable sex lives on message boards.
The principle states that there is equal investment in males and females, not that the sexes be numerically equal. In polygamous species, males are generally much larger than females (so they can compete with other males). If males are larger at birth, one would expect fewer of them to be born, because one male “costs” more than a female to produce.
Even so, only a small number of males are able to reproduce. Males that don’t succeed in vanquishing another male to take over a harem may form “bachelor herds” separate from the harems controlled by dominant males. Gorilla troops may contain a silverback male, his female harem and their offspring, plus several younger subordinate males.
That’s a big if, though. I can think of at least one sexually dimorphic species off the top of my head where males and females are the same size at birth to within a rounding error, and I don’t recollect (say) bull calves being appreciably bigger than cows.
There’s a lovely article on this subject by Stephen Jay Gould. He describes a species of mite, where a female gets about 9 offspring, 8 females, one male. The Fisher principle does not apply there because the brother and his sisters mate snd they do it while still inside the mother mite’s skin.
Can we talk about angler fish now? I want to understand why some species of angler fish have several males attached to one female. I also want to know if the males make the females feel itchy all the time.
Please note that the cartoon angler fish are much prettier than real life angler fish.
Whoa. I did not know that.
I don’t think it’s some species, it’s some individuals. It’s hard for male and female anglerfish to find each other in the deep sea. If a male finds a female he may want to attach even if there’s another male already there, rather than risk never finding another female. The female gets offspring with more genetic diversity out of the deal.
We do. More males are conceived than females, and the dying starts before birth - a male fetus is more likely to miscarry. And even after being born males are more vulnerable to disease and parasites - this is true of mammals in general, not just humans. Human males are also more prone to be mentally ill or mentally disabled.
Meiotic drive, aka segregation distortion, is a type of intragenomic conflict which may produced bias sex ratios if the sex chromosomes are involved. It’s a mad, mad world out there, and it’s each haplotype for itself, so, for example, if an X chromosome can kill off some or all of the Y chromosomes, it is more likely to be passed down, resulting in a female-biased sex ratio.
OK. That makes sense. It seems to me, though, that the OP’s question has rather been lost sight of in this thread. The answer to the question asked is “Yes, lots.” To fill that out, those who know should be offering examples of species where the ratio is not 1/1. I offered a few suggestions, but I am not really an expert.
My impression though (which may be wrong) is that in actuality, a near to 1/1 ratio of births, such as we see in humans, is the exception rather than the rule (whereas the OP seemed to think it is the rule, or even universal).
So- back to the OP’s intent; do herd/harem animals like deer, lions, etc. have roughly even sex birth ratios, or is there a wildly uneven ratio favoring more females? I have heard of adolescent male horse herds (bachelor herds) but not deer, for example.
That makes sense.
I’m glad I’m not an angler fish, though.
Remember, even in harem animals, on average a male will have the same number of offspring as a female. It’s just that (in a stable population) each female will usually have two offspring*, while most males will have none and a few males have a lot. But the average for each sex has to be the same, since each baby has one mother and one father.
So, on average, a male baby and a female baby will be equally good investments (as far as reproduction goes). And we’d expect to find roughly equal birth rates (as mentioned above, you’d account for higher male infant mortality, so maybe slightly higher male birth rates, with sex ratios equal at reproductive age).
*Yes, slightly more than two, since some females will die before having any children. And much more than two when we count the ones that die before reproducing themselves, etc. But I kept things simple to make the point.
They have roughly the same number of male babies as female babies. Lots of males never get to reproduce. The bachelor males hang out around the edges of the harems waiting for a chance, lots of them never make it.
If you think about it, if you were the parent of a deer, which would make more sense to give birth to, a male or a female? If you give birth to a female then if she lives to adulthood she will likely be able to mate and give birth. If you give birth to a male, he’ll either become the master of a harem and reproduce a lot, or be a bachelor and not reproduce.
Either way, the average outcome is equal. So there is no incentive to produce a female baby or a male baby.
Now imagine that everyone else was producing female babies, because most male babies die without reproducing.
Now which is more valuable to produce, a male baby or a female baby? Obviously a male baby, because there are so few males a male baby is very likely to be able to become a herd master and reproduce a lot. So if there are more females it is more advantageous to have a male baby, if there are more males it is more advantageous to have a female baby.