by Andrew Anderson (@AndersonEvolve)
To start this post, I feel the need to point out that I detest Naturalistic Fallacies (i.e., what’s natural is morally good), and the situations I will describe can be sensitive to some readers. These situations occur in nature and in no way do I think (and no one should think) that they justify disgusting attitudes and behaviors seen in our society. If you do not wish to read those descriptions, they will be in blue text so you can skip them.
One of the more interesting aspects of sex-roles and sexual selection is the concept of sexual conflict. Put simply, sexual conflict is whenever a trait or action benefits one sex over the other. There are two kinds of conflict, the first is called intralocus (within location) conflict. A rather humorous example is the thought,“why do males have nipples?” The genetic architecture for nipples, and milk delivery in general, is vital to the survival of mammals. A female without nipples would not be able to deliver milk to her offspring and would not have any successful offspring (since they didn’t make it past birth). Disentangling male and female expression of traits can take some evolutionary time, but more importantly, a mutation that stops nipple formation is very costly unless it occurs in males only.. Further, as the presence of nipples doesn’t impose a much of cost (survival or energetic) to males, they’re just there. However, milk delivery is more costly, so evolution can act with a little more intensity to separate the sexes, so males don’t have those as fully developed. The mechanism for the male/female expression is hormonal control, thus a male given a female hormone will start to express those traits. Another example is secondary sex traits. Bright, large feathers in peacocks are energetically costly and make them more vulnerable to predators, peahens would do well not to express those traits. Thus the architecture for the feathers is different for males and females, again this mostly done by hormonal control.
The second kind of conflict is interlocus (between location) conflict. This is usually when males or females engage in behaviors that are self-beneficial but potentially harmful to the other sex.
Examples of interlocus conflict can be found in insects, some species have evolved brushes to scrape out rival males’ sperm or plugs to prevent other males from mating. Clearly, the benefit to the male is the ability to reduce his mating competition without guarding the female, but these mechanisms can damage the female and prevent her from mating with a male she might deem more suitable or prevent her from being ever able to mate again even after she lays her eggs. Male water striders have evolved hooks to grab females and hold themselves in place to complete copulation. Ducks will actually force copulation with a female in a bid to put their genes into the next generation. While biologists focused on the male behaviors at first (they’re more visible and there has been a bias on male traits since historically science is male dominated), we’re learning more and more that females are engaged in an arms race themselves. Females can chemically “help” certain males’ sperm and have evolved ways to resist damage (insects), become bigger and stronger to shake males (water striders), or evolve complex vaginas to shunt away a male’s sperm who forces himself on her (ducks).
Okay, okay. You know what’s coming next if you’ve been keeping up with me… FISHES! Again, I’m going to focus on brood care. In my lab, others have shown that male pipefish (check my first post about them here) actually will provide more care to some female eggs over others. Pipefish males actually exchange resources with the eggs they are brooding (similar to mammalian pregnancy) and it has been shown that some eggs are reduced during pregnancy. This means that males can actively take resources from the eggs the female provided. Larger females produce more eggs and can completely fill a male pouch making a good return on investment for the male. Males tend to prefer larger females and will reduce a larger portion of eggs from small females, presumably to gain energy to invest in future pregnancies with large females. Thus, a small female has spent relatively more resources on reproduction only to have a low amount actually born.
One of my favorite examples outside of pipefish are the cardinalfishes (Apogonidae), which actually engage in filial cannibalism. Cardinalfishes engage in male mouthbrooding, a process which is costly to males. They cannot eat until the eggs are hatched. Since they are investing in a brood they want to maximize that investment. If a female produces a small egg mass, the male may actually consume the entire brood in one gulp, especially if he encounters another gravid female and he has not been brooding long. While this is an amazing feature, what’s even more mind-blowing is some cardinalfishes the females have evolved a response. They don’t fully develop some eggs (saving energy), but include those eggs in the masses they give to the male, so the mass seems like it has more eggs and the male would be less likely to consume it.
The world of sexual selection and fishes is wild and weird and there’s nothing quite like it!