Excerpted from Animal Weapons: The Evolution of Battle by Douglas J. Emlen. Out now from Henry Holt and Company LLC.
Jacanas are bizarre birds, especially when it comes to their weapons. The female I’m watching has especially big spurs, one on each elbow, and she creeps about with delicate steps on very long legs. Her slender toes splay to a span of more than 5 inches per foot, and her gait brings to mind a carnival stilt walker. This female, “red-over-blue-and-white right” (named for the colored ring bands on her right leg), is making the rounds of her territory, checking the nests of each of her four mates. Her territory is difficult to get to, and we have to watch at a distance from a canoe. Jacanas defend areas of floating vegetation on wide tropical rivers. Panamanians call them “Jesus” birds because they look like they walk on the water and, in a sense, they do. They tiptoe across their floating mats, dispersing weight with each step over delicate toes, balancing atop the bobbing rosettes of water lettuce and hyacinth.
This morning, red-over-blue-and-white right is fighting again, as she so often does. An unmarked female had darted in from the adjacent shoreline, hiding in the hyacinth leaves behind one of the males, but our territorial female spots her immediately and closes in. Now, face to face, the two birds size each other up. Crouching low, elbow spurs flared out to the sides, each sidesteps the other in a slow circle. Then red-over-blue-and-white right pounces, leaping into the air and striking the intruder feet first on her way down, slashing out with her spurs. Everything whirls into a blur as both birds leap at each other over and over, crashing together and jabbing as they flail onto the mat of floating lettuce, pop to their feet, and leap once more. And then suddenly the fight is over; the intruder flies away, and the thick air rings with raucous ka-ka-ka-kas as our focal female proclaims victory to birds nearby.
Hundreds of vagrant females forage along the nearby shores of the river. These individuals have failed to secure an island territory of their own, and they challenge the resident females incessantly, pressing and probing, searching for weakness. For vagrants these battles are “do or die,” since failure to secure a territory is an evolutionary full stop, a dead end. Unless they can find a way to displace one of the owners, their chances of breeding are nil.
Female jacanas are fighters, towering over the males. They are stronger than males, vastly more aggressive, and they have the larger weapons. Sharp yellow spurs jut forward like daggers from each elbow. Bigger females fare better in fierce battles, and as a rule only dominant, top condition females manage to hold a territory for long enough to breed.
Males also fight for territories on the floating mats, though their battles are less vicious and are independent of the wars waged by the females. Males fight with rival males, and successful individuals defend patches of floating greenery sufficient to raise a clutch of chicks. These territories pack into the floating islands like tiles of a mosaic, with the female territories superimposed on top. Some females may be able to shove rivals away from only a single male territory, but the biggest and best females own enough island to house three or four males.
My female is engaged in yet another fight—her fourth of the morning. The male shepherds his brand-new chicks as they wobble from plant to plant, all feeding on little insects squirming at the water surface around the lettuce.
The next male over, his territory also nestled within the holdings of red-over-blue-and-white right, still has eggs, and he sits on his hidden nest sheltering his clutch. A third male has nearly grown chicks, and the last male is between broods, ready to begin the process again. Our female, when she’s not battling to hold her spot, moves freely among her males, mating with them from time to time. When one of her males is ready, she’ll lay a clutch of four eggs into his nest. But then she will abandon the eggs to the male and move on, placing her next clutch a few weeks later into the nest of a different male. Male jacanas spend several months tending to young each time they reproduce, preparing nests, incubating eggs, and shepherding the chicks as they grow to independence. Females show up to provide eggs when needed, but otherwise they leave the tending of young to the males.
Jacanas seem backward. Females are more aggressive than males, they are larger than males, they fight more viciously and frequently than males, and they have larger weapons. In the animal kingdom, usually it’s the other way around. In flies, beetles, mastodons, crabs, and elks, males are armed, not females. Jacanas excepted, in every species with weapons confined to a single sex, males have those weapons. Why should just one sex have weapons? And why is it (almost) always the males?
Females of all animal species produce larger reproductive cells (called “gametes”) than males. Eggs are bigger than sperm, and this difference in material investment is far more substantial than most of us appreciate. Humans are rather ordinary in this respect, but we’re a good place to start. The female egg is the largest cell in the human body. It measures almost a fifth of a millimeter across—about the size of a period (.) on this page—and it’s just visible to the naked eye. Sperm are the smallest cells in the body, and 100,000 could fit into the volume of a single egg.
In many animals differences in egg and sperm size are much more profound. A zebra finch mom fits nicely in the palm of your hand. She’s roughly 4 inches beak to tail, but she lays an egg that is more than a ½ inch across. By weight, a finch egg is 7.5 percent of the weight of her body. That’s equivalent to a human female producing an egg weighing 11 pounds. Kiwis have the most gargantuan of all gametes, relative to their body size: brown kiwi moms lay eggs that are a fifth of their body weight. Our human mom would need to produce a 30-pound egg to compare—the size of an 18-inch watermelon.
Asymmetry in gamete size has consequences that ripple through the biology of animal species. For one thing, females can’t produce as many gametes as males do. With the same amount of resources, males produce trillions of sperm. And these numbers stack up fast, since each male produces similarly copious quantities of sperm. A human female produces roughly 400 viable eggs over the course of her lifetime. A male, on the other hand, cranks out 100 million sperm every day, easily 4 trillion over his lifetime. Scale that up to a population of 1,000 people, and there are a quadrillion (that’s 15 zeros) more sperm than there are eggs. Scale it to the current human population and there are a septillion (24 zeros) more sperm than eggs. And humans aren’t even an extreme example. The simple fact is that in virtually every animal species there are nowhere near enough eggs to go around. The result is competition.
The size of female gametes is important for another reason. Large, nutrient-rich eggs are expensive, and they take time to produce. Depending on the species, females may take days or even weeks to recover from producing one batch of eggs before they are ready to lay another. Males, on the other hand, tend to need only a few minutes. Thanks to their gametes, females generally take longer to “turn around” between breeding events than do males.
Females also stand to lose more than males if a breeding attempt fails. Although each sex invests nutrients, energy, and time in producing gametes, the amounts they invest are different. Females spend more than males each time they reproduce and, because of this, the cost of abandoning a brood is much steeper for females than it is for males. As a result, whenever additional offspring care is required, it’s generally the females who provide it.
Females invest in offspring in all sorts of interesting ways beyond simply producing eggs. Cockroach moms hold fertilized eggs inside their bodies until they are ready to hatch, feeding and protecting them in an insect equivalent of a mammalian pregnancy. Scorpions wear bundles of babies on their backs for weeks after they hatch. Dung beetle moms excavate tunnels into the ground and provision their babies with balls of buried dung; a few species even lock themselves inside a crypt for a year so they can guard the young as they grow.
Preparing nests, pregnancy, guarding eggs, and feeding and protecting young all take time. These forms of maternal care can increase the latency between reproductive events still further, compounding the rift in relative investment between the sexes. Males can invest in offspring, too, as they clearly do in jacanas and humans, but it’s surprisingly rare in the animal world. In most animal species, males provide little more than sperm, and this means they recycle an awful lot faster than females.
Turnaround times are tremendously important for explaining animal weapons because whenever they differ between males and females the result is always competition. If you walk into a population of just about any animal species, and you count how many individuals of each sex are physiologically capable of breeding right now, you’ll find that all the males are able and willing, but many of the reproductive-age females are not. Some of the females are physiologically unavailable—out of commission, so to speak—in between broods. Female zebras in the midst of pregnancy cannot conceive new foals. Cow elks cannot start new pregnancies while they are nursing existing young. Females that are locked into a current breeding event are “out of the pool” since they are not available for conceiving new young. If all of the males are able to breed, but only a fraction of the females are, then there will not be enough females to go around.
Enter nature’s most pervasive and potent form of competition, what Charles Darwin called “sexual selection.” Individuals of one sex compete for access to the other. In principle, sexual selection can work both ways, with either males or females competing. In reality, except for rare cases such as the jacana, it almost always involves males competing for access to females. Female jacanas still produce the larger gametes (eggs are bigger than sperm), and it takes them a few weeks to recover between clutches. But this is where female investment stops. Male jacanas spend up to three months tending to eggs and chicks, and as a result their turnaround time is longer than the females’. Females recycle roughly three times faster than males (24 days compared with 78 days), translating into almost three times as many reproductively ready females as males on floating territories. At any point in time, roughly half of the males in a population are tied down with existing eggs or chicks, leaving just a few of them free to start new broods while most of the females are yolked up and ready to go. They could lay eggs immediately if only they had access to a territory-holding, reproductively ready male. In jacanas, there simply are not enough receptive males to go around, and females battle it out for the chance to breed. Sexual selection, rather than selection from predators or prey, drove the evolution of blazing yellow spurs in jacana females.
llustration by David J. Tuss. Excerpted from Animal Weapons: The Evolution of Battle by Douglas J. Emlen, published November 2014 by Henry Holt and Company LLC. Copyright © 2014 by Douglas J. Emlen. All rights reserved.