My wife and I have two lovely daughters: Charlotte is two and a half, and Veronica is seven weeks. And we are tired. We think of ourselves as being on the losing end of a tag-team wrestling match--particularly at about seven in the morning, after Veronica has gone through a few hours of pre-dawn nursing, squirming, groaning, crying, spewing, and nursing. Just when she has faded off into angelic sleep, Charlotte wakes up from a long restful night and wants to eat Cheerios, do some jumping jacks, and type on my laptop pretty much all at the same time. It's like the Destroyer giving the Crusher the high-five as one goes out of the ring and the other comes in to deliver the final flying scissor kick.

I've looked for some enlightenment about this daily bruising from evolutionary biologists. For them, these golden years are all about energy and information. In order for a child to thrive--and, ultimately, to pass on its parents' genes--it needs a lot of energy to grow. Getting enough milk in the first year or two of life makes a huge difference to a baby’s health. But a mother can't just nurse her baby on some rigid schedule--four ounces at noon, and then four at midnight--because a baby's hunger is influenced by everything from the weather to its mother's own changing health. She needs a sign, and her baby is happy to give her one, in the form of a cry.

The parental brain is finely tuned to a baby's cry; in the middle of the night it brings us stumbling over to see what's the matter. We’re pretty normal as animals go in this respect—when a bird comes to its nest and hears the sound of hungry squawks, it automatically rushes off to catch more bugs. Cuckoo birds take advantage of their slavish dedication to these squawks. They lay an egg in the nest of another bird, such as a reed warbler, and when the new cuckoo hatches it kicks out the reed warbler chicks. Yet the reed warbler parents feed the cuckoo that killed their family. Why? Because the cuckoo can mimic the sound of a nest full of reed warblers.

In the 1970s, the biologist Robert Trivers had an unsettling realization: a mother's own child is a bit like the parasitic cuckoo. She and her child only share half of their genes, which means that their evolutionary interests aren’t the same. A baby has the best shot at surviving to adulthood and having babies of its own if it gets as much food, protection, attention, and so on from its mother as possible. And anything that a baby can do to get all this may boost its odds of success. In the womb, for example, a fetus sends out signals that increase the flow of nutrients from its mother's blood vessels.

But what's good for the baby is not entirely good for the mother, evolutionarily speaking. The best strategy for a mother to pass on her genes may be to spread her energies out evenly to all her children. Bearing and raising children is hard work, particularly for humans, and if a mother works too hard fostering one child, she may have fewer resources for her next one. Her genes will have a better chance of getting passed down if she can keep the manipulations of any individual baby in check. Mothers, for example, seem to slow down the growth of their babies in the womb. As a result, the average baby is not born at the optimal weight for avoiding an early death. It's a little on the light side. Only an evolutionary tug of war can explain that gap.

Once out of the womb, baby still struggles with mother. The baby still needs milk, warmth, and protection. Its mother, on the other hand, may have a different unconscious agenda. If she wants to have another child, she needs to switch her baby eventually from high-energy milk to low-energy food. (Nursing lowers the chances of getting pregnant.) The conflict gets even tougher if the baby is weak or the mother is struggling to survive herself. It may be better to cut her losses and hope the next baby has better luck.

A baby is not helpless, though. After all, it has a direct line into its mother's head. Babies may manipulate their mothers into offering them more care with signals like crying. According to one theory, crying is a kind of "honest advertising" to convince a mother a baby's worth the effort. Crying, after all, doesn't come for free--it may actually double a baby's metabolism. So by crying, a baby may be saying, I can afford to waste this energy because I'm such a strong kid. Crying-as-advertising might solve the mystery of colic—the inconsolable wails of some children who otherwise seem perfectly healthy. They may just be trying particularly hard to impress their parents. (Here's a post about how the colors of autumn leaves may also be honest advertising, sent from trees to the insects that eat them.)

The tantrums and clinginess of older babies may just be new variations on this basic strategy. As mothers slowly try to wean their kids, the kids respond by getting in as much nursing and attention as they can. The more the child can nurse, the longer it will take for its mother to have another child.

Studies on our primate cousins back up these theories. It turns out that infant monkeys make about ten times more contacts with their mothers than vice versa, and that the mothers push away the babies as they get older. They even start ignoring their babies' distress calls--because often these calls turn out to be false alarms. (My personal favorite is the observation that young monkeys and apes sometimes jump on adults during sex. One chimp that was adopted by a married couple apparently jumped on them as well.)

But there's a flip side to this hypothesis: if it's the product of evolution, it must be partly the result of genes. In the February issue of the journal Behavioral Ecology and Sociobiology, Dario Maestripieri of the University of Chicago reports his elegant study of the genes behind the mother-child struggle. At a colony of rhesus macaque monkeys, he found 10 babies who were all born within a day of each other. He shuffled them among their mothers, and the foster mothers raised the babies as if they were their own. Maestripieri then watched how they got along as the babies grew older and the mothers prepared to have another baby.

Not surprisingly, these foster families got into more conflicts as the mothers approached their next opportunity to mate. But Maestripieri also found that some babies became pushier than others, while some mothers brushed them off more than others. And when he compared the foster children with their biological mothers, he found a genetic link between them. The clingiest infants had biological mothers who tended to rebuff their foster children. In other words, the pushy-baby genes and the tough-mom genes were bundled up as a package. As mothers become tougher, the genes that favor pushy babies get favored. Maestripieri has taken a snapshot of a struggle between parents and children that has lasted for millions of years.

All of this doesn't help me feel more awake this morning, but at least it helps to remind me that Charlotte and Veronica aren't in this tag-team match out of personal spite. It's just evolution, Dad.