Intralocus conflict in evolvability of parenting traits - it's way more catchy than "Men are from Mars, Women are from Venus"

Oh, I mention gender differences in parenting and everyone gets their panties in a bunch. Cries like women. Bangs their chests. Clearly there are a lot of weighty preconceptions that come to the fore when we talk about the differences in parenting between men and women. The difference between men and women is the trite fuel of bad comedy and self help books (he's just not that into you) but it's hard to deny that there are innate differences. Well, what's innate and what's learned is a big question. Nature versus nurture, that old chestnut.

Parental investment varies a lot in the animal world. From fish that lay eggs and swim away, to seahorses in which the male invests a lot of time and care, harem-maintaining mammals, and monogamous birds. Again, from an evolutionary perspective, a lot of it boils down to the only reliable difference between males and females, in plants, animals, and insects (notably not always fungi - there can be hundreds of mating types in fungi, but I digress). Females produce larger gametes - the egg. That's it. Only difference. But from that, so much follows. Males therefore have an easier time producing gametes, hence produce, and spread, more gametes than females. Females invest more from the very start, so automatically have more reason to continue investing in this one egg. From this initial imbalance, there is a selective pressure for males to invest less, and females to invest more. But let's not take this to mean that males have it easy: even in harem maintaining species, males have to invest a great deal in fighting, attracting, and guarding females. Yet it still remains a truism biologically and in human societies: males benefit from spreading their seed far and wide and females benefit from tying them down to take care of their offspring. Bring home the bacon. Thus males are cast as rogues and women as the ball and chain.

CLEARLY this doesn't explain everything, because if life were that simple then the animal kingdom (and Calgary) would be full of harems. I was MOST disappointed, as I did research for this post, that a great model system for investigating parental investment is burying beetles. That's right. Beetles that burrow into a carcass to lay their eggs. Who knew that carrion-eating bottom feeding insects were instructive to everyone, not just Wall Street executives? Intellectually, burying beetles are quite interesting because they're the only non-social insect in which both the males and females invest care in their young. But in reality they're disgusting, their offspring are maggots, and they live in rotting dead animals. And try googling "sex determination in burying beetles" and finding anything to read that does not contain pictures. And if you know me, you know I'm TERRIFIED of bugs. I'm ashamed to admit that I have invited people over (begged?) for the sole purpose of killing a silverfish in the bathroom. Anyway, I did manage to learn, before shuddering, turning off the computer and grabbing a beer, that male and female burying beetles both care for their offspring by regurgitating rotten food and spreading the carcass with anal secretions to keep it fresh enough for the maggots to eat. It's okay, I'll wait. Go get a beer. But here's where it gets interesting. If you take one or the other partner away, the remaining parent is equally capable of performing these tasks and the offspring are fine.

So are the parents in this case completely interchangeable? Truly equally helping each other out in a completely cooperative manner? Evolution would suggest that this wouldn't be a stable situation. And sure enough, the females to tend to specialize in direct care more when both parents are present. So the authors of a particular study (Walling et al. 2008. The quantitative genetics of sex differences in parenting. PNAS, 105:47. Spoiler alert: there are pictures.) investigated whether this situation would be stable. Say if perhaps the situation changed and the species were forced to evolve such that either males or females became the predominant caregiver. This is a new and interesting type of question: basically the authors were curious as to how evolvable these traits were. And surprisingly, male direct care was less evolvable than female direct care. In other words, lack of direct investment by males wasn't just the evolutionarily smarter thing to do, it was the only option available genetically. If selection increased for male direct care, there wasn't a lot that would change in the species, because the gene for direct care in males was incapable of increasing its influence. The reason for this is the title of this post - intralocus conflict. It's complicated, and not that interesting. The take home message is that some solutions aren't necessarily the smartest evolutionary strategy: they might just be the most convenient for your genetics.

As evolutionary biologists we spend a lot of time coming up with explanations for why things are the way they are that credit selection for everything. It's nice to be reminded every now and then that selection doesn't determine everything. Thus no one gets to use "I'm evolutionarily inclined to spread my gametes" as an excuse for infidelity. And the whole question of evolvability adds another layer to the old question of nature vs. nurture. It asks us: just how written in stone is the "nature" portion of ourselves?