I don’t have a total answer but I can make some predictions/contribute to the model:
The more dimorphism between sexes in a species or niche, the less we should expect to see (simultaneous) hermaphrodism, because success as either sex requires costly investment. I think you’re making a mistake dismissing this because physical dominance as rare- sex via physical dominance qua physical dominance is rare, it is much more often serving as a costly signal of resource abundance, which is exactly what bird songs are doing.
Males are only deadweight loss if a population is producing fewer offspring than the environment can support. For a species at carrying capacity, twice as many offspring means twice the death rate. Therefor we’d expect to see hermaphrodism in k-strategists (which we don’t, I’d guess because being a k-strategist already requires a number of costly investments) and in r-strategists that are frequently nearly wiped out locally, or finding new locations (I think this is true on the margin, although not overwhelmingly).
sex via physical dominance qua physical dominance is rare, it is much more often serving as a costly signal of resource abundance
This makes sense in pair-bonded species where the resources of the males help the females. But it seems like there are plenty of species where the male contributes very little, and so in those species the question is: why is the equilibrium of signalling resource abundance favoured over the equilibrium of signalling hermaphroditic fertility?
For a species at carrying capacity, twice as many offspring means twice the death rate.
Yes, but for any individual, twice as many offspring means that your genes make up a greater proportion of the next generation. So I think that this doesn’t change the pressure towards hermaphrodism on an individual level (although maybe it makes group selection for it weaker).
I think we’re in the uncanny valley where you’re asking questions the literature genuinely doesn’t address well, but also missing some stuff it has already covered, in ways that make it hard to work out through LW comments. This is an area of interest of mine and I’d love to discuss in person at some point.
I don’t have a total answer but I can make some predictions/contribute to the model:
The more dimorphism between sexes in a species or niche, the less we should expect to see (simultaneous) hermaphrodism, because success as either sex requires costly investment. I think you’re making a mistake dismissing this because physical dominance as rare- sex via physical dominance qua physical dominance is rare, it is much more often serving as a costly signal of resource abundance, which is exactly what bird songs are doing.
Males are only deadweight loss if a population is producing fewer offspring than the environment can support. For a species at carrying capacity, twice as many offspring means twice the death rate. Therefor we’d expect to see hermaphrodism in k-strategists (which we don’t, I’d guess because being a k-strategist already requires a number of costly investments) and in r-strategists that are frequently nearly wiped out locally, or finding new locations (I think this is true on the margin, although not overwhelmingly).
This makes sense in pair-bonded species where the resources of the males help the females. But it seems like there are plenty of species where the male contributes very little, and so in those species the question is: why is the equilibrium of signalling resource abundance favoured over the equilibrium of signalling hermaphroditic fertility?
Yes, but for any individual, twice as many offspring means that your genes make up a greater proportion of the next generation. So I think that this doesn’t change the pressure towards hermaphrodism on an individual level (although maybe it makes group selection for it weaker).
I think we’re in the uncanny valley where you’re asking questions the literature genuinely doesn’t address well, but also missing some stuff it has already covered, in ways that make it hard to work out through LW comments. This is an area of interest of mine and I’d love to discuss in person at some point.
Makes sense, let’s chat directly!