| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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feature The Mark of Cain Sibling Rivalry to the Death
My two sons fight, and so do blue-footed booby brothers. Fortunately, my sons haven’t killed each other. Booby family quarrels, however, regularly result in the death of a sibling. And baby boobies aren’t the only ones to start fatal fights. They’re just one example of about two-dozen bird species and a smattering of other animals that routinely practice some form of siblicide, the death of a youngster at the hands, or rather paws, beaks, or teeth, of a brother or sister. You don’t have to be an evolutionary scholar to know that “survival of the fittest” doesn’t mean that animals simply do away with whomever is nearest in proximity, usually a brother or sister if there’s more than one offspring in a brood. On the contrary, we instinctively feel that the truism “blood is thicker than water” is right, and rooted somewhere deep in our DNA. If we were in a capsized boat, we’d probably reach out to save that sister who drives us crazy before a stranger or even a close friend. Hugh Drummond of the National Autonomous University of Mexico in Mexico City has been watching blue-footed booby siblicide behavior for over 20 years. Blue-footed boobies are large, gull-like birds that belong to the Sulidae family, which includes nearly a dozen species of gannets and boobies, and are closely related to pelicans and cormorants. Blue-foots occupy arid islands in the Gulf of California and along the Pacific coast from Baja California to northern Peru, with large populations on the Galápagos Islands. They breed year-round and lay eggs in nests on the ground in dense colonies along the shoreline, making them easy to observe for scientists. Drummond’s fieldwork has been on Isla Isabel, about 45 miles off the coast of Nayarit, Mexico. When Drummond describes booby sibling interactions, he makes my fractious sons look like a couple of choir boys. “The dominant chick will bludgeon the subordinate’s head onto the sand and sometimes only relaxes when it lies completely still,” Drummond says. During a five-minute period, a youngster may shriek loudly while delivering up to 30 pecks to its little brother or sister’s head and neck. The picked-on sibling, almost always the youngest, is either pushed from the nest and finished off by an unrelated neighbor or dies from starvation. It’s easy to see how siblicidal behavior benefits the winner: either one of my sons would, at times, happily admit to wanting to get rid of the other to have solo access to parental attention and future college funds. Of course, we don’t permit this to occur, and for good evolutionary reasons: my sons are both equally related to me, and there is no advantage in favoring one over the other. But from either son’s point of view, it is to his immediate advantage to obtain as many resources as possible from our family. If parents want to maximize the survival of all their offspring while each child looks out for his or her own interests, we might expect siblings to act selfishly and parents to protect the vulnerable. We might see parents, for instance, having conflicts with aggressive offspring, or giving some kind of preferential treatment, such as extra feeding, to subordinate ones. But, according to Drummond, blue-footed booby parents don’t intervene in sibling spats—even fights to the death. When not providing for the family, they may be standing about, seemingly disinterested in the Cain and Abel drama occurring at their feet. In fact, mom actually sets up a family hierarchy that favors older chicks by laying two or three eggs in five-day intervals. Each chick hatches about four days apart, creating a considerable size difference between nestlings. Not surprisingly, the older chicks almost always grow larger than the younger ones, putting them in a position to dominate the nest. Furthermore, parents feed the dominant chick more often than subordinates. “Parents and senior chicks cooperate as if their fitness interests were congruent,” says Drummond. In 1964, W.D. Hamilton expanded Darwin’s “survival of the fittest” theory to take into account the survival of other related individuals, and termed this concept “inclusive fitness.” According to this idea, close relatives, which are likely to share many of the same genes, are valued more highly than distantly related individuals. So it’s to any chick’s advantage to have its siblings survive—unless its own survival becomes threatened. Taking this into account, one might expect to see a difference in sibling interactions depending on environmental factors: good times might bring peace to the family, but when food is insufficient for both chicks to thrive, siblings might fight. Drummond has found that blue-footed booby siblings seem to be fine-tuned to get along when sufficient resources enable bigger families to thrive, and to prune the family through siblicide only when the going gets tough. Pecking rates increase during food shortages: a 20 percent weight loss in the senior chick seems to be the critical threshold when aggression becomes intense and younger chicks are likely to die of starvation or expulsion. In contrast, blue-foot parents able to bring in abundant food are more likely to have broods in which two, or in rare cases, even three chicks survive. Although kittiwakes, ospreys, and black guillemots have also been known to vary their aggression according to food ingestion, not all birds show such “facultative” siblicide. Other booby species routinely practice siblicide, even in times of abundant food. Researchers term this phenomenon “obligate” siblicide. Dave Anderson of Wake Forest University studies Nazca boobies, a species recently classified as distinct from masked boobies. Nazcas often lay two eggs, which usually hatch some five days apart. Within a day or two of emerging, the younger sib is inevitably nudged from the nest by the older chick and eventually dies. Unlike blue-foots, which Anderson describes as using a “market economy approach,” Nazcas raise only one chick no matter how many groceries are on hand. Then why lay two eggs to start with? Anderson says that’s relatively easy to explain: “Their hatching success is lousy,” with a failure rate of up to 40 percent. When Anderson artificially supplemented females’ food supplies, more than 90 percent laid two-egg clutches, compared to only 70 percent under natural conditions. The system serves as an insurance policy: if the first egg doesn’t hatch or the older sibling is sickly or taken by a predator, the second chick can save the breeding season from total failure. An analogous phenomenon is seen in primates, including humans. Women occasionally ripen more than one ovum per menstrual cycle, resulting in twins. It’s unlikely that having twins was an evolutionary advantage in premodern times: twins tend to be born smaller and at an earlier gestational age than singletons, and have lower survival rates. Sonograms of women in the first trimester show that twins are actually conceived two to four times more often than are born. In most cases, however, the smaller fetus disappears during pregnancy, apparently resorbed by the mother. “By producing more than one ovum, parents increase the probability that at least one will produce a neonate,” says Anderson. “The insurance value of extra ova...is functionally identical to that of insurance eggs laid by some birds that typically raise only one chick to fledging.” But why, Anderson wondered, don’t Nazca boobies try to raise both offspring in good times, like humans usually do? (Not all human cultures try to raise both twins: one survey of 60 societies discovered that 14 practiced infanticide of one or both twins, a phenomenon found on five continents.) Drummond theorizes that obligate siblicide in birds develops when, under normal circumstances, resources are adequate for the survival of only one chick. Nestlings may require large amounts of food over periods of many weeks, so that in typical Nazca booby habitat, short-term abundance may not serve as an accurate indicator of food availability over the long-term. But Anderson has found that the explanation for Nazca behavior may be more complex. To see what would happen if he prevented siblicide from occurring, Anderson occassionally removed and fed subordinate chicks to allow them to gain weight before placing them back into the nest. He found that 30 percent of these two-chick families were successfully raised. Although growth rates were slowed, neither chick was underweight by the time of fledging. But the mothers ultimately paid a price. About 15 percent fewer mothers who had raised two-chick families returned to breed the following year. Apparently the strain of feeding two young, even under ample food conditions, proved fatal to some mothers. Anderson also found that offspring of two-chick families ended up producing fewer offspring themselves than their peers who were raised as singletons. Siblicide has been most extensively studied in birds, but other animals also engage in the practice. Piglets are born with weapons in their mouths designed exclusively to battle on home turf. Eight so-called “wolf” or “needle” teeth, are used to shove aside siblings and establish an “udder order.” The nipples nearest to the sow’s head deliver the most milk, and piglets who gain control over these teats clearly have a growth advantage. This is how pig farmers came to use the phrase “sucking the hind teat” for getting the short end of a stick. Less aggressive piglets turn into runts and may end up starving to death, particularly if there are more piglets than teats. Farmers routinely clip off teeth shortly after birth to avoid harm to the other piglets, as well as to the sow, who can get cut in the melee. Without interference, wolf teeth fall out within two to three weeks, and later give way to tusks in adult males. Another mammal with family problems is the spotted hyena of Africa. These stocky, 150-pound animals are incredible hunters: a lone hyena can bring down a 550-pound wildebeest, or, as part of a group, a Cape buffalo three times that size. Spotted hyenas usually give birth to twin cubs, which are born with fully erupted front teeth, well-developed motor skills, and extremely high levels of androgens, or male hormones, regardless of their sex. Unlike most mammals (and even other hyena species), which are relatively helpless at birth, the super-androgenized spotted hyena newborns start out with the tools, skills, and personalities of teenage psychopaths. “Within minutes of birth, a newborn attempts to fasten its teeth into the neck and shoulders of its sibling and shake it violently,” according to UC Berkeley’s Laurence Frank. “Fighting is most intense and prolonged on the day of birth, and dominance is all settled within a few days.” The dominant cub clearly has many advantages: it will get more milk from the mother, grow faster, and eventually attain a larger size and extremely aggressive personality, enabling it to compete more effectively among other hyenas for food. But having high levels of testosterone on board in utero exacts a cost to the species beyond the poor survival of subordinate twins, who often die from their wounds or malnutrition. While the genitals of male and female mammals start out morphologically similar, female spotted hyenas develop a large clitoris that looks very much like a penis yet contains the birth canal, along with empty scrotal sacs. This is probably a result of high androgen levels, although the Berkeley group, led by Stephen Glickman, has yet to work out the precise physiology. The resulting birth canal is so narrow and tortuous that mortality rates for both mother and baby are very high for first deliveries. After a first birth, enough stretching and tearing has occurred to make subsequent births easier. Frank is convinced that the advantages conferred by the male hormones in terms of dominance in feeding situations have driven this evolutionary oddity. The high costs of mortality of both cubs and mothers during delivery are unfortunate by-products. Battling newborns, in all likelihood, are another negative consequence of high androgen levels. Sand tiger shark siblings don’t wait until they are out of the womb to start attacking one another. Shark scientists still speak of the day Stewart Springer, a distinguished fish biologist of the 1940s and ‘50s, hauled up a shark on the deck of his research vessel. Seeing that she was pregnant, Springer reached inside the uterus to investigate. To his surprise, a plucky little fetus gave him a sharp bite on the finger. Sand tiger sharks have one of the most unusual reproductive strategies known among vertebrates. Shark mothers initially produce a number of fertilized eggs and package them in capsules before sending them down an oviduct into two separate uteruses. (Although a female sand tiger shark has just one large ovary, she has a uterus on each side of her body.) Some of the eggs, but not all, develop into embryos—each capsule may contain as many as three—which grow by feasting on the other eggs in the capsule. As the embryonic sharks mature, they grow special teeth to chew their way out of the capsules. They then turn to eating other egg capsules as well as each other. By the time the feeding frenzy is over, only one embryo survives in each uterus. But the story doesn’t end there. The mother’s ovary at this point grows enormous, and she continues to send down unfertilized eggs to junior as a sort of room service. Up to 1,700 capsules containing 17,000 ova may be consumed during a single pregnancy. By the time the sharks are born, they are experienced predators, ready to take on ocean prey. While most of us have some concept of what a functional family is like, few of us (fortunately, perhaps) bear in mind the disparate evolutionary paths taken by animal families of other species. Sometimes I watch my sons expend so much energy fighting over something so trivial that I stop trying to intervene, concluding that the tendency towards fraternal conflicts must be deeply ingrained. But don’t throw in the towel for sibling relations yet. When the going gets rough, desert-dwelling spadefoot toads do resort to cannibalism, but actually try to avoid munching their brothers and sisters in the process, according to David Pfennig of the University of North Carolina who studies kin recognition signals in spadefoots. When their often short-lived ponds start to run dangerously dry, some spadefoot tadppoles change into “cannibalistic morphs,” growing an enormous, flat head with a huge jaw and a sort of hawk’s bill that enables them to consume other tadpoles. While siblicide is a part of the ensuing feast, Pfennig has found that cannibalistic morphs are actually less likely to swallow their brothers and sisters, especially when they’re not too hungry. They apparently perform a “taste-test,” probably based on an ability to discriminate odors between immediate relatives and others. Then they preferentially spit out kin. Now there’s an example of looking out for family. Nora Steiner Mealy is a nature and medical writer living in Davis. |
