Pick from the Past
MY OSTRICH RESEARCH in Kenya began some sixteen years ago, very much as a family affair. I had made a trip into the bush outside Nairobi and with official permission had gathered six ostrich eggs, intending to hatch and raise the young in captivity. Nancy, my wife and co-worker, who was recovering from surgery, agreed to brood the grapefruit-sized eggs in her bed while I constructed an incubator.
Three weeks later, six healthy young hatched in the incubator. The precocial chicks, each about the size of a barnyard hen, proceeded to imprint on Nancy, the first living being they saw after hatching. They followed her everywhere, emitting tremulous, mournful cries when left alone. This scenario seemed appropriate, since my research interest lay in ostrich breeding strategies.
Nancy and I soon learned that we were far from the first humans to raise ostriches or even to incubate the eggs ourselves. These birds, the largest on earth, have had long and intimate associations with humans. Prehistoric rock artists of the Sahara frequently depicted ostriches, and the ancient Egyptians and many other African peoples raised the huge birds in captivity, as did the Romans. In the late nineteenth century, ostriches were domesticated, and some ostrich farms are still in operation today, although tourism and the sale of skins have largely replaced the flourishing plume trade.
Ostriches and their relatives, most of which are also large and flightless, are ratites. The ostrichs closest relative is the South American rhea, which weighs only a third of the ostrichs 220 pounds but prefers similar habitats and exhibits some of the same behavior. Well adapted to the semiarid conditions of savannas, as well as to areas of higher rainfall where vegetation is often thick and lush during the rainy season, ostriches live throughout Africa, although their range is dwindling.
In spite of its long association with humans, the detailed behavior and ecology of the ostrich in the wild were virtually unknown in terms of modern zoology when I began my study at the University of Nairobi. I was especially concerned with the breeding behavior of the Masai ostrich, a subspecies found only in Kenya and Tanzania. In Nairobi National Park I found an ideal situation for my observations. Here many ostriches lived in their natural habitat, yet had grown accustomed to motor vehicles, so that I could approach them closely.
Only after several seasons of fieldwork could I outline the behavior, particularly breeding behavior, of the ostrich during a typical year in Nairobi Park. In May or June, adult males begin to display their breeding color, a pink flush on the skin of the neck and legs. About half of these males establish a territory of roughly one square mile, which they aggressively defend against other mature males. Adult females, more numerous than the males, do not defend territories, but each remains on a much larger breeding home range of about five square miles, which embraces five to seven male territories.
During July, individual hens begin to seek out and mate with territorial males. Soon they begin to lay eggs at nest sites already chosen and prepared by the males. Only about one in three hens finds a permanent mate; such females are called major hens, and their pair bond with a particular cock may last for many years. The remaining, minor hens form no pair bond. They mate with many different territorial males during a season but must lay their eggs in the nests of established pairs. (Minor hens and nonbreeding males may form temporary groups, which are unstable, with individuals joining or leaving at will.) Each males nest receives eggs from about ten minor hens. Thus, the resultant communal clutches are large, containing forty eggs on average. Only major hens incubate eggs, their own and those of the minor hens. The cock incubates more often than the hen, sitting at night and for part of the day. After the chicks hatch, the pair escort them away from the nest, leading them to edible plants and protecting them from predators.
Egg hatching takes place during October and November and coincides with Kenyas short rainy period, when a flush of tender young plants emerges. After the synchronized hatch, escorted broods of chicks from different nests eventually meet. The dominant parents aggressively round up other chicks to join their own, and by February all the young form one great nursery, or crehe, of thirty to one hundred chicks, escorted by only two or three of the original parents. Chicks grow rapidly and by July are left on their own, much reduced in number by predation by lions, jackals, and other carnivores. Their parents then start the breeding cycle for the next year.
This breeding pattern proved similar to other ostrich populations in East Africa. Nairobi Parks ostriches, however, numbered about 170 individuals in a forty-four-square mile area. This is a high density, and the many hens produce unusually large clutches.
Long before I had enough information to outline the ostrich breeding system, Mike Norton-Griffiths, a fellow ornithologist, challenged me to explain how such a breeding system could evolve or be adaptive. He argued that one hen obviously did not lay forty eggs, yet only one hen cared for them. Why should she sit on the ground in broad daylight, week after week, with lions about, incubating another ostrichs eggs? Mike called this the paradox of apparent altruism. But this was only half the problem, since an ostrich cock also risks his life on anothers eggs, sitting at one spot every night for six weeks of incubation. Early in the study, I found yet another example of apparent altruism. I sighted a flock of 107 two-month-old chicks, carefully led and guarded by one cock and one hen. I soon realized that these chicks must have come from more than a dozen nests, spread out over many miles of savanna, each with its own cock and hen. The single pair escorting the 107 chicks could not have been the parents of all. Yet these escorts commonly defended large broods and even attacked threatening predators.
Thus, parent ostriches expend time and energy, and risk being preyed upon themselves, to protect and care for many eggs and chicks, only a few of which are their own. Similar examples of apparent altruism are found in many animal species. In almost all cases studied in detail, however, the individuals receiving the benefits of an altruistic act are kin, that is, they are genetically related to the altruistic individual. This phenomenon, known as kin selection, makes sense in evolutionary terms, because the self-sacrificing individual insures the survival of the genes it shares with close relatives.
An ostrichs apparent altruism cannot easily be explained by kin selection. Because their communal nests contain the eggs of many hens, and because of their somewhat indiscriminate mating system, ostriches can never be sure which egg is related to which hen. We are left with a challenge to current evolutionary theory: What survival value does this behavior hold for the individual ostriches involved? How did such behavior evolve?
The explanation was likely to be found only through a detailed investigation of the breeding system. The apparent altruism seemed to be related to communal breeding or to environmental factors or perhaps to one of the other unusual characteristics of the ostrich, such as its extraordinarily large size.
Aspects of communal breeding were easily observed. One afternoon soon after I had begun my study, Nancy spotted a hen behaving in a peculiar way. Driving to a better vantage point, we discovered our first nest and on it a laying hen. As we watched, another hen appeared, and within twenty minutes, two others slowly walked up to the nest. The three were awaiting their turn to deposit eggs. We knew that the hens laying the next day would be different individuals, because an ostrich needs two days to form each egg. Eventually we found that thirteen hens had laid, placing a total of forty-two eggs is that one nest. This was about average for Nairobi Park. We were at a loss to explain why so many hens laid in each nest, and why, when a hen can cover only about twenty-one eggs, so many extra eggs, which could not be incubated, were placed these. Then an unexpected event made clear the hazards of the environment is which ostriches raise their young.
Tony Archer, a knowledgeable naturalist who arrived on the scene just after the occurrence, reported that an ostrich had killed two lions. A Masai man had witnessed the whole dramatic episode, and Tony had questioned him carefully. There is front of them were two dead lions, three-quarters grown, and a dead adult ostrich hen. The Masai said he was watching as ostrich pair with their newly hatched chicks feeding peacefully. Suddenly a lioness appeared with three of her own young. The cock immediately led the chicks to safety while the hen advanced on the predators. A hen with newly hatched chicks is often savage. The young lions began to close in and within a minute or two there was a brief but violent struggle. When the animals parted, two young lions had fallen, one with a broken back and the other with a blow to the head caused by the hens rapid, powerful downward kicks. Seconds later the lioness pounced on the hen and killed it with a few bites.
Ostriches naturally use their large size and powerful legs to defend their young. They are the only flightless birds of Africa, living in one of the most dangerous habitats on earth. The list of their larger predators is formidable. Jackals, hunting dogs, cheetahs, leopards, lions, and of coarse, humans all regularly prey on ostrich adults, young, or eggs. Smaller mammals such as servals also pose a threat. In addition, about ten species of birds of prey can kill a newly hatched ostrich chick, and one, the Egyptian vulture, can break open the thick eggshell. The other large ratitesthe rhea, cassowary, and ewehave only two or three major predators. Furthermore, the ostrich is the only ratite in which both parents escort and protect the young; for the others, the male alone is apparently enough of a defense.
The ostrichs reproductive potential, relatively high for a large animal, may be related to heavy predation. If no young died, the population could increase by five times in one year. But this could only happen is a predator-free environment. During one year of my study, 152 chicks left fifteen nests in Nairobi Park. A year later only 16 young had survived. Ninety percent had died, most killed by predators, and a similar rate of predation is common every year.
Predation has yet another influence on ostrich breeding: it does not affect both sexes equally. Another case dramatized this point. Judith Radnai, a good friend of mine studying lion behavior is Nairobi Park, kept me informed of lion predation on ostriches. One day she announced that one of her lions had killed one of my ostriches at a nest. I went to the spot she described and found eggs scattered about, although only a few were broken. The ostrich had been devoured, except for bones and feathers. Everywhere black-and-white feathers contrasted starkly with the brown-and-green landscape.
Conspicuous plumage may have been the reason the cock had been discovered and killed by lions. Gaudy feathers may be of value in courtship, but they are a liability for a bird sitting on a ground nest for six weeks. The following year I saw another cock dead at his nest, also killed by lions. More males than hens are killed, perhaps because the females brown-and-gray plumage blends in much better with the landscape.
The result of each unequal mortality is a biased sex ratio, with many more hens than cocks remaining in the population. But there are also other reasons for this bias. Hens mature more quickly, so the start of a breeding season for any age group finds more females than males ready to mate. Also, cocks most hold territories before hens will mate with them. To establish a territory, a young cock may have to pit his strength against older, experienced cocks. A younger male is usually chased away and even kicked for his trouble. He may have to try for years before he is successful. Thus, the need to hold territories further deters males from breeding.
As soon as a sexually mature, territorial male starts to prepare a nest, predators make an appearance. Egyptian vultures, jackals, hyenas, and lions discover and destroy many nests before the clutch is complete. If a nest is annihilated, the hens may then lay in another nest, while the cocks may not be able to start another nest until the nest year. So ostrich populations, like those of most animals, begin with equal numbers of males and females at birth, but by the time the young are ready to breed, there may be only one cock for every three or four laying hens.
In addition to sex ratio, the weight of the ostrich is significant. Large size is obviously an advantage against predators, but size also has a special meaning concerning eggs. Many small birds lay eggs that weigh about 10 percent of the weight of the adult. The ostrich lays an egg only about 1.5 percent of its body weight. Large birds in general lay much smaller eggs in relation to their body size than do small birds. Large eggs have disproportionately great volume. Their formation and growth time is prohibitively long. Most large birds have evolved relatively small eggs in response to this constraint. Thus, although it weighs as much as two dozen chicken eggs, the ostrich egg is smaller than that of most other birds relative to body weight. Even the combined weight of all the eggs of one female ostrich is small taking into consideration her body weight.
The relatively small egg provides a key to understanding communal laying. A major hen usually lays only about seven eggs but can cover about twenty-one. This means that when she finishes laying all her eggs and is ready to incubate, there is room for more. The minor hens, without a nest or permanent mate, put their eggs beside those of an established pair; some of these will be incubated. The small egg and the biased sex ratio are two factors that help us understand communal breeding and how so many hens manage to lay in one nest.
In September of one field season, I watched the cock I had named Interloper scrape the ground with his great claws and show the spot to Pointyhead, his major hen of many years. A week later, eight eggs were laid on the site during one afternoon, although only one was Pointyheads. During the next afternoon, nine different hens laid nine more eggs. I found that Pointyhead and sixteen other hens were laying in that one nest, and by the time they had finished, two weeks later, the communal clutch had grown to sixty-eight eggs, considerably larger than the average forty-egg clutch.
Typically, a major hen like Pointyhead initiates laying. She deposits one egg every two days for about two weeks, thus laying a total of about seven eggs. Within several days of her first egg, a few minor hens find the nest and also start laying. Other minor hens, not finding the nest until quite late, perhaps contribute only one or two eggs. In Nairobi Park, an average of ten hens lay eggs in each nest. When ready, the major hen rearranges the clutch. She places about twenty-one eggs in the center and moves all the rest about a yard away from the nest. She and the cock then start incubating the central eggs. The other eggs will perish. This behavior raises the question of how the major hen can recognize her own eggs. If she cannot identify them, the many eggs of the minor hens could confuse her and prevent her from incubating and hatching her own.
Zoologist Bryan Bertram became interested in what he called the behavior of ostrich eggs. After my study, he went on to investigate this phenomenon. In Tsavo West Park, in Kenya, he tried to find exactly which eggs the major hen moved out of her nest and which she kept in. This proved difficult to determine, but by marking eggs and identifying hens at many nests, he concluded that a major hen can distinguish her own eggs from the many others in a nest. She can thus keep them in the center of the nest to be incubated when she displaces the others. But since the minor hens have a chance to get some of their own eggs incubated, they also benefit. And the cock may benefit even more. If he fertilizes some minor hens, as well as his major hen, many more of the chicks that hatch will be ones he has fertilized.
Even though the cock and hen share the incubation task, he sitting at night and she during the day, the six weeks of incubation is a difficult time. Young, inexperienced pairs, in particular, seem to find sitting for six weeks a tedious job. Sometimes the cock or the hen is late coming back to the nest to take his or her turn. If the nest is left unguarded, even for minutes, predators may home in, stealing or breaking a few eggs. Almost every nest suffers losses.
This constant preying on eggs is what ultimately makes communal laying pay off for the major hen and her cock. If they had only their own eggs, with no minor hens laying, any egg taken by a jackal or other predator would be one of their own. By accepting minor hens eggs, the pair protect their own; if a predator takes an egg, it is less likely to be one of theirs. This behavior increases survival by the principle of dilution. A similar principle applies to some other species, for example, the huge calving herds of wildebeest and barren ground caribou. By associating her calf with hundreds or thousands of others of the same age, a cow reduces the chance that it will be killed by predators. This rule explains why ostriches, both in communal nesting and in crêching, appear to be altruistic. In reality, the parents are protecting their own eggs and chicks from predation by caring for the eggs and chicks of others. In the long run, the ostriches that practice communal breeding increase their numbers more rapidly than the few that breed alone. Thus throughout their range, most ostriches breed in great loose colonies that make communal nesting and crêching possible. Sound reproductive strategy underlies the sight of a wary adult pair shepherding dozens of chicks across African savannas.
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