4 ms·

Exclusive homosexuality is found in nearly 10% of male sheep, as well as a bunch of other species of mammals and birds. What’s interesting with the giraffes is

Exclusive homosexuality is found in nearly 10% of male sheep, as well as a bunch of other species of mammals and birds. What’s interesting with the giraffes is the really high prevalence (94%), which suggests it’s an adaptation to some form of natural selection. Even the rate of 10% would suggest an adaptive response, but the really high rate emphasizes this.

usrusr 8y ago
One way in which it could be an adaption that actually helps its bearer's genes would be as an overpopulation safety valve. Instead of the whole group starving (all of them closely related genes), pause breeding for a while? I would expect that most species that have some form of throttling reaction like that would apply the brakes to female fertility because that is also relevant for individual survival, but it should not be impossible for throttling to appear in males as well. If the giraffe evolution happened to stumble upon a particularly sensitive shortage-sensing circuit breaker in the form of a gender preference flip, then we would see it firing non stop in a time of dramatically shrinking giraffe habitat. It's been a while since giraffes could be observed outside of crisis. (or they just like it, it does not have to be an evolutionary trick)
- int_19h 8y ago
  There are a bunch of plausible theories that ethologists have come up with for evolution of homosexuality. This is one of them - and it does show up in other experiments, e.g. with rats: if you make their population denser, a higher proportion exhibits same-sex behavior. But it's not the only one. Another is that species that employ K strategy (low reproduction rates, high investment into offspring to ensure its survival) and that are social, gradually develop some kind of social safety net mechanisms for offspring, in cases where e.g. the parent dies or is sick and unable to provide proper care. One way to do that is to have some proportion of the population that does not breed on their own, thus ensuring that they are always available to rear others' offspring. Since evolution generally takes the path of least resistance, it's easier to achieve this by redirecting sexual drive than by switching it off entirely. And this can even make sense from a "selfish gene" perspective of one individual, because if you help enough of your close relatives (who share many genes with you) keep their kids alive, it may actually be more effective than betting on a few children of your own surviving to carry more shared genes. (If this theory is right, that has interesting implications for all those laws restricting adoption by same-sex parents...)
  - meruru 8y ago
    The K strategy theory looks reasonable, but I don't see how evolution could develop a population-control mechanism like the parent comment postulated. If you start with an overly-dense population that's half straight and half gay, the straight ones are still at an evolutionary advantage relative to the gay ones. It doesn't matter that they'll all die when they reach the reproduction rates of a 100% straight population, that's still where the evolutionary forces will take them.
    - int_19h 8y ago
      In case of population density, it might be epigenetic. But also, don't forget that evolution does not "care" about organisms, only about genes; and a gene can be carried across generations without manifesting itself. Suppose it is beneficial to carry the "gay gene", because it means that more of your descendants don't die, even if some of them won't procreate - producing a net gain in the propagation of your genes on the whole, ironically. That would trigger selection, once such a gene appears.
      - meruru 8y ago
        >Suppose it is beneficial to carry the "gay gene", because it means that more of your descendants don't die, even if some of them won't procreate That doesn't work because even though less of your gay descendants will starve, the straight organisms in the population will also reap the benefits and they'll still be at an evolutionary advantage.
        int_19h 8y ago
        I didn't mean to imply that all descendants would be gay, only that all would carry the gene and pass it to their descendants in turn. Most would still breed. And in your original example, the straight organisms still die in the end. So over the lifetime of multiple colonies, the ones that survive would carry more "cooperative" genes, no? I mean, isn't this basically how biological altruism develops in general?
jessaustin 8y ago
...really high prevalence (94%), which suggests it’s an adaptation... I don't have anything to back this up scientifically, but ISTM this phenomenon doesn't have to be adaptive in and of itself, since it could be a side effect of something else that is adaptive. (Human example: getting sick from sickle cell is not adaptive, but resistance to malaria is.) TFA describes female giraffes as only occasionally fertile, and somewhat aloof even then. Perhaps it takes a real horn-dog to persist given all the rejection. Well that's nice once every two years, but what is a horn-dog to do the rest of the time? What do human horn-dogs do when placed in environments without women, like prison? It's interesting that the inherent ungainliness of giraffes allows females to reject coitus as they choose. Of course this is just an idea, which, even if it describes some of the male giraffes, may not describe all of them. I don't mean to suggest that homosexual giraffe sex is in any respect less important than heterosexual giraffe sex.

usrusr 8y ago

One way in which it could be an adaption that actually helps its bearer's genes would be as an overpopulation safety valve. Instead of the whole group starving (all of them closely related genes), pause breeding for a while? I would expect that most species that have some form of throttling reaction like that would apply the brakes to female fertility because that is also relevant for individual survival, but it should not be impossible for throttling to appear in males as well. If the giraffe evolution happened to stumble upon a particularly sensitive shortage-sensing circuit breaker in the form of a gender preference flip, then we would see it firing non stop in a time of dramatically shrinking giraffe habitat. It's been a while since giraffes could be observed outside of crisis. (or they just like it, it does not have to be an evolutionary trick)

int_19h 8y ago
There are a bunch of plausible theories that ethologists have come up with for evolution of homosexuality. This is one of them - and it does show up in other experiments, e.g. with rats: if you make their population denser, a higher proportion exhibits same-sex behavior. But it's not the only one. Another is that species that employ K strategy (low reproduction rates, high investment into offspring to ensure its survival) and that are social, gradually develop some kind of social safety net mechanisms for offspring, in cases where e.g. the parent dies or is sick and unable to provide proper care. One way to do that is to have some proportion of the population that does not breed on their own, thus ensuring that they are always available to rear others' offspring. Since evolution generally takes the path of least resistance, it's easier to achieve this by redirecting sexual drive than by switching it off entirely. And this can even make sense from a "selfish gene" perspective of one individual, because if you help enough of your close relatives (who share many genes with you) keep their kids alive, it may actually be more effective than betting on a few children of your own surviving to carry more shared genes. (If this theory is right, that has interesting implications for all those laws restricting adoption by same-sex parents...)
- meruru 8y ago
  The K strategy theory looks reasonable, but I don't see how evolution could develop a population-control mechanism like the parent comment postulated. If you start with an overly-dense population that's half straight and half gay, the straight ones are still at an evolutionary advantage relative to the gay ones. It doesn't matter that they'll all die when they reach the reproduction rates of a 100% straight population, that's still where the evolutionary forces will take them.
  - int_19h 8y ago
    In case of population density, it might be epigenetic. But also, don't forget that evolution does not "care" about organisms, only about genes; and a gene can be carried across generations without manifesting itself. Suppose it is beneficial to carry the "gay gene", because it means that more of your descendants don't die, even if some of them won't procreate - producing a net gain in the propagation of your genes on the whole, ironically. That would trigger selection, once such a gene appears.
    - meruru 8y ago
      >Suppose it is beneficial to carry the "gay gene", because it means that more of your descendants don't die, even if some of them won't procreate That doesn't work because even though less of your gay descendants will starve, the straight organisms in the population will also reap the benefits and they'll still be at an evolutionary advantage.
      - int_19h 8y ago
        I didn't mean to imply that all descendants would be gay, only that all would carry the gene and pass it to their descendants in turn. Most would still breed. And in your original example, the straight organisms still die in the end. So over the lifetime of multiple colonies, the ones that survive would carry more "cooperative" genes, no? I mean, isn't this basically how biological altruism develops in general?

jessaustin 8y ago

...really high prevalence (94%), which suggests it’s an adaptation... I don't have anything to back this up scientifically, but ISTM this phenomenon doesn't have to be adaptive in and of itself, since it could be a side effect of something else that is adaptive. (Human example: getting sick from sickle cell is not adaptive, but resistance to malaria is.) TFA describes female giraffes as only occasionally fertile, and somewhat aloof even then. Perhaps it takes a real horn-dog to persist given all the rejection. Well that's nice once every two years, but what is a horn-dog to do the rest of the time? What do human horn-dogs do when placed in environments without women, like prison? It's interesting that the inherent ungainliness of giraffes allows females to reject coitus as they choose. Of course this is just an idea, which, even if it describes some of the male giraffes, may not describe all of them. I don't mean to suggest that homosexual giraffe sex is in any respect less important than heterosexual giraffe sex.