According to this 2009 paper, seahorses aren’t actually an exception, and the males are indeed the more choosy one, at least in this one experiment. (They’re an “exception” to Bateman’s principle in the sense they have the smaller gamete, but this is explained away by their greater parental investment.)
In general, yes, parental investment can “outweigh” gamete size in some situations, and typically this ends up happening in cases where investment in offspring isn’t as strongly physiologically sex-linked as it is in most mammals which allows different strategies to evolve more readily.
For instance, in birds, the egg is quite large, but because incubation is quite time consuming (as is feeding) and this can be shared between parents, you end up with more bi-parental care, as a male can increase his reproductive success by staying and incubating. Because there are many species of bird with bi-parental care, this opened up the pathway for evolution of the jacana, where the male does all the parental care, and the male is rate limited by how many eggs he can incubate and is more choosy.
In mammals, since most of the energetic investment is in gestation and lactation, both of which only females can do, and you end up with bi-parental care being more rare than in birds. One notable exception being humans, which have an exceptionally long childhood that extends long after weaning.
Insects in general and fruit flies in particular were a particularly bad species to detect Bateman’s principle in, because they’re r selected rather than K selected. In K selected species, genetic quality of the mate matters much more because of how few offspring there are; in species where the goal is to produce as many offspring as possible, genetic quality and therefore mate quality and ergo choosiness has a much smaller impact.
According to this 2009 paper, seahorses aren’t actually an exception, and the males are indeed the more choosy one, at least in this one experiment. (They’re an “exception” to Bateman’s principle in the sense they have the smaller gamete, but this is explained away by their greater parental investment.)
In general, yes, parental investment can “outweigh” gamete size in some situations, and typically this ends up happening in cases where investment in offspring isn’t as strongly physiologically sex-linked as it is in most mammals which allows different strategies to evolve more readily.
For instance, in birds, the egg is quite large, but because incubation is quite time consuming (as is feeding) and this can be shared between parents, you end up with more bi-parental care, as a male can increase his reproductive success by staying and incubating. Because there are many species of bird with bi-parental care, this opened up the pathway for evolution of the jacana, where the male does all the parental care, and the male is rate limited by how many eggs he can incubate and is more choosy.
In mammals, since most of the energetic investment is in gestation and lactation, both of which only females can do, and you end up with bi-parental care being more rare than in birds. One notable exception being humans, which have an exceptionally long childhood that extends long after weaning.
Insects in general and fruit flies in particular were a particularly bad species to detect Bateman’s principle in, because they’re r selected rather than K selected. In K selected species, genetic quality of the mate matters much more because of how few offspring there are; in species where the goal is to produce as many offspring as possible, genetic quality and therefore mate quality and ergo choosiness has a much smaller impact.
I’ve read that male humans are capable of lactation but it takes very weird circumstances (possibly involving drugs) to make it actually happen.