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As the study of cooperative breeding systems expands, a number of key species form the examples that underpin our general understanding. The ostrich is increasingly becoming such a textbook species, on the basis of the results obtained in Brian Bertram's study of vigilance and egg discrimination in this extraordinary bird. Here Bertram presents new data on the ostrich communal nesting system, in which several females lay in one female's nest, with only one female and the male doing all the work. The Ostrich ...
As the study of cooperative breeding systems expands, a number of key species form the examples that underpin our general understanding. The ostrich is increasingly becoming such a textbook species, on the basis of the results obtained in Brian Bertram's study of vigilance and egg discrimination in this extraordinary bird. Here Bertram presents new data on the ostrich communal nesting system, in which several females lay in one female's nest, with only one female and the male doing all the work. The Ostrich Communal Nesting System unravels the basis of the cooperation observed, and explains how a system involving apparent altruism is maintained by natural selection. It is now possible as never before to explain and quantify the effects of the different choices these birds make and to integrate ecological and morphological factors such as predation and size. Based on three seasons of study in Tsavo West National Park in Kenya, this book depended on recognizing individual birds, detecting and monitoring well-concealed nests, determining motherhood of eggs from their surface appearance, and time-lapse photography of nests. Key findings were that females could switch rapidly between reproductive strategies, that a nesting female could recognize her own eggs and when necessary discriminate against those of other females, and that the whiteness of ostrich eggs is an adaptation that protects them against overheating but at the cost of greater vulnerability to predation.
Originally published in 1992.
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1.1 The Bird Itself
The ostrich is well known to be the world's largest living bird. An adult male stands 2.1–2.7m high (Cramp and Simmons, 1980). Females are generally a bit smaller. Information on measured weights of these birds in the wild is characteristically scarce; a small sample (n = 13) from Kenya averaged 111 kg, ranging from 86 to 145 kg (A.V. Milewski, pers. comm.).
Their size alone would render ostriches incapable of flight. Instead, they are adapted to a walking and running way of life. The very long bare legs carry the large bird economically (Fedak and Seeherman, 1979) and when necessary at up to 60–70 km/h (Brown et al., 1982), with strides of up to 8.5 m (Smit, 1963). The reduction in the number of toes from the normal five to only two in the ostrich is probably another adaptation for fast running. The large inner toe (originally the third) and occasionally the smaller outer toe (originally the fourth) carry a powerful nail.
Ostriches' wings are considerably reduced. The feathers are without barbs, which contributes to their loose, soft appearance. The primary feathers are developed as large plumes, used in display and sought after by human beings. Much of the body is devoid of feathers, particularly the long neck, the whole of the legs and patches on the underside of the body. The adult male's body plumage is jet black, with the exception of white plumage on its wings and tail. The female's feathers are of a fairly uniform earthy pale brown-grey colour.
The eye is very large (50 mm across: Hurxthal, 1979), is protected by long eyelashes, and with good visual acuity it provides excellent vision from its high vantage point.
Unlike most birds, the male ostrich has a penis, which is everted when the bird defecates and urinates.
The eggs vary in size, weight (1100–1900 g) and shape (from ellipsoidal to almost spherical). The shells are creamy white in colour and about 2 mm thick.
The species name Struthio camelus, given by Linnaeus in 1758, derives from the Greek and Latin name Struthocamelus, by which the ostrich was known. The 'camelus' is based on a supposed similarity to camels, probably in the strong fleshy feet, the prominent eye lashes, the large size and the desert habitat it can survive in.
The ostrich is the sole living species in the family Struthionidae. Half a dozen extinct species of Struthio have been described on the basis of fossil bone and eggshell fragments (Brodkorb, 1963). These species were mostly larger than the present-day species, and during the Pliocene and Pleistocene periods up to 5 million years ago, they occupied wide areas of China, India and Eastern Europe as well as Africa.
The higher classification of the ostrich has been considerably debated. It is generally considered (e.g. Cracraft, 1974, 1983) that the Rheas (Rhea and Pterocnemia) from South America are their closest relatives, also in the family Struthionidae. More distant are the Cassowaries (Casuarius) from New Guinea to Australia and the Emu (Dromaius) from Australia. All of these large, flightless birds, along with the New Zealand Kiwis (Apteryx) and some other extinct giant birds including the Moas (Dinornis) of New Zealand and the Elephant Birds (Aepyornis) of Madagascar, are classified together as the Ratites. Their relationship to other birds is not clearly understood, but they are generally considered to have descended from a common ancestor (Sibley and Ahlquist, 1981), which was capable of flying (Bruning, 1991).
1.2 Ostrich Subspecies
During the twentieth century, five subspecies of ostrich have been validly recognized (Brown et al., 1982). The Masai Ostrich, Struthio camelus massaicus Neumann, forms the subject of this book and so is dealt with first. As the name suggests, it is found in East Africa, from central Kenya to south-west Tanzania (see Fig. 1.1). The bare skin of the male's neck is pink, and becomes red during the breeding season; his thighs likewise, are bright pink in colour. The white tail feathers are almost invariably soiled and so usually brownish or reddish in colour. There is a small ring of white feathers as the black body feathers give way to bare skin about a third of the way up the neck.
Adjacent to the Masai Ostrich to the north-east is the Somali Ostrich, Struthio camelus molybdophanes Reichenow, from the horn of Africa. It inhabits north-east Kenya and south-eastern Ethiopia and Somalia. It is distinguishable from the Masai Ostrich particularly by the male's deep grey-blue neck and legs, bright red tarsal scales, and because he has no white neck ring. His tail feathers are usually unsoiled and therefore bright white. Also, Somali Ostriches have a bare crown patch, a grey rather than a brown iris, and they usually hold their beaks a little higher (Hurxthal, 1979).
The South African Ostrich, Struthio camelus australis Gurney, occurs south of the Zambezi and Cunene Rivers. It is another blue-necked race, also with red tarsal scales but with no bare patch on the crown. It is mainly this race, hybridized with others, which has been domesticated on ostrich farms (see Section 1.4).
The North African Ostrich, Struthio camelus camelus Linnaeus, is distributed across the southern Sahara and Sahel region from Mauritania to Ethiopia. This is the tallest race, and the most similar to the Masai Ostrich in having a bright pink neck and thighs; but the bill and tarsus are redder, particularly in the breeding season, and there is a bare crown patch.
A so-called Dwarf Ostrich, S. c. spatzi, from the Rio de Oro, described on the basis of a small zoo specimen and eggshell fragments, is no longer recognized as being a valid race (Cramp and Simmons, 1980). The Arabian Ostrich. S. c. syriacus Rothschild, which once inhabited Arabia and neighbouring countries, was wiped out between 1940 and 1970.
Wild ostrich populations everywhere are declining, their ranges are shrinking, and the survival of some subspecies is severely threatened. The North African Ostrich in particular which once occupied a vast range, has totally disappeared from the northern side of the Sahara (Egypt, Libya and Tunisia) within the past 150 years, and is now almost extinct in the western Sahara. Its numbers are no longer high anywhere, as the species is vulnerable to disturbance, delinquent shooting and human plundering of nests.
The Masai and Somali Ostrich races are holding their own in the good wildlife areas within their range; but as human numbers in their habitat rise, ostriches become increasingly vulnerable to disturbance and casual plundering of their nests. The South African Ostrich has been eliminated from most of its former large range, mainly due to conversion of its habitat for farming and ranching activities. It survives now in the more arid parts of its range, particularly in Namibia and Botswana.
The only subspecies which are nowadays contiguous are the Masai and Somali Ostriches in parts of Kenya where they overlap for a few dozen kilometres. No naturally occurring hybrids have ever been reported there, although a limited amount of interbreeding did take place in the Nairobi National Park following the release into the local Masai Ostrich population of a couple of male Somali Ostriches. There are differences in the courtship displays of the two races (pers. obs.; Hurxthal, 1979) and these may prevent hybridization except in artificial situations such as the above and on ostrich farms.
Despite the declining status of some races, the ostrich is not yet listed in the Appendices of the CITES (Control of International Trade in Endangered Species) Convention; this means that ostriches or their products taken from the wild can still legally be taken across most international borders. Domestic legislation in a number of countries provides a degree of protection against human activities. In Britain, for example, ostriches are classed in Group Al of the listing of the Department of the Environment, and import licences are required to move ostrich eggs into or out of the country.
1.3 Habitat and Feeding
Ostriches may be found in a variety of open habitat types (Brown et al., 1982). They avoid areas of thick bush or of heavy tree cover, but inhabit wooded grasslands and other more open country. Semi-arid, open, shortgrass plain is usually associated with the highest ostrich densities. Highly productive and very well vegetated areas usually support high populations of competing herbivores, and therefore high predator populations. Adult ostriches are vulnerable to predation particularly by lions (Panthera leo) if there is a great deal of cover. And ostrich nests are vulnerable to nest predators such as hyaenas (Crocuta and Hyaena) for about 2 months; a very high hyaena density in an area could result in almost all ostrich nests being discovered and destroyed before hatching takes place.
At the other extreme, ostriches are able to thrive in very poorly vegetated areas, and can be found in dry savannah to desert. They exhibit a number of adaptations for dealing with the harsh desert conditions – extreme temperatures, a lack of water and little food.
Ostriches are tolerant of high temperatures. They rarely seek shade, as most desert antelopes regularly do. Their feathers are excellent insulators, minimizing heat gain due to direct solar radiation, as well as reducing heat loss during cold desert nights. Temperature control is achieved by erecting and flattening the body feathers (Louw et al., 1969) and by panting at very high temperatures (Crawford and Schmidt-Nielsen, 1967).
The bulk of ostriches' water needs can usually be obtained from their food, achieved partly by their feeding early in the morning, particularly on hygroscopic plants with a high moisture content (Louw, 1972). If free water is available, the birds will make use of it, and under certain conditions they will trek long distances to obtain it (Sauer and Sauer, 1966a). Generally, their distribution is almost independent of water as they can withstand a considerable degree of dehydration (Cloudsley-Thompson and Mohamed, 1967). Their urine consists of uric acid carried in mucus, so minimizing water loss (Louw et al., 1969).
A desert environment has sparse food items, and ostriches are well adapted to harvesting them. The bird's excellent eyesight detects the food item at a distance, and the beak at the end of the long, light, mobile neck can gather the food delicately and economically.
Ostriches eat a variety of species and parts of plants, particularly dicotyledonous ones. The plants eaten include succulents, herbs, shrubs, grasses, creepers and bushes (Brown et al., 1982); leaves, flowers, small fruits, seeds and seed pods or heads are all selected and consumed. In captivity, ostriches are considered omnivorous, as they relish meat and will pick up almost any small bright object. In the wild, however, animal protein (in the form of lizards, locusts, termites, etc.) clearly forms only a minute proportion of their diet and they appear not to go out of their way to obtain it.
The food selected is evidently of relatively high quality, and ostriches thrive and grow quickly on it. They can lay down quantities of subcutaneous fat reserves against times of food shortage.
1.4 Relationship with Man
Ostriches have had a long and varied relationship with man. They feature in the folklore and carvings of the Kalahari Bushmen (Bleek and Lloyd, 1911), appear as paintings and carvings in caves in the Sahara dating from 5000–10000 years B.C., and were considered holy by the Assyrians (Smit, 1963). The Bible (Leviticus 11:13 and Deuteronomy 14:12) prohibited the eating of ostriches along with a whole range of other birds, apparently as being unclean. Elsewhere in the Bible (Job 39:17), referring to the ostrich, it is stated that 'God hath deprived her of wisdom, neither hath he imparted to her understanding'.
The belief in the bird's stupidity persists in the still current notion that ostriches bury their heads in the sand in face of danger. No-one has witnessed this at first hand. There are several ways in which the story could have originated: a feeding ostrich may have its head concealed among low vegetation for considerable periods; the small neck and head of a very distant ostrich can be invisible; and a nesting ostrich when approached by humans or other predators will often stretch its neck and head flat out along the ground where it is not easily noticed.
Ostrich eggs have long been prized. The Bushmen, and later European sailors, found them an invaluable food source which kept fresh for long periods. The Hottentots used complete empty shells as practical water vessels (De Mosenthal and Harting, 1879). As carved vases and cups, ostrich eggshells were being used in Mesopotamia 5000 years ago (Laufer, 1926), as well as in Ancient Egypt, Carthage and post-Renaissance Europe. The holy properties of the shells were used to help and protect Ethiopian Coptic churches (Brown et al., 1982) and buried Phoenicians. And shell fragments have been extensively made into necklace beads by Bushmen and others.
People have also made use of the birds themselves for many centuries. The Struthophagi tribe in Egypt used to hunt ostriches by stalking them disguised in an ostrich skin (De Mosenthal and Harting, 1879), as did the Kalahari Bushmen (Sauer, 1971). (The Australian Aborigines also hunted emus in this manner.) In Tutankhamen's tomb, there is a depiction of him hunting ostriches by bow and arrow from his chariot, apparently a privilege of the Pharoahs. The Arabs also used to hunt them on horseback. Guns have nowadays made ostrich hunting much easier.
Ostriches have been hunted for a variety of reasons, and most parts of a killed bird are put to use. The skin, being flexible but tough, has been used for making protective jackets worn in the Arab world. The mad Roman emperor Heliogabalus of the second century A.D. had 600 ostrich brains served at a banquet, but in general the meat has not been specially sought after. A large number of ostriches were shot in South Africa following the discovery of diamonds in the gizzard of one of them (Sauer, 1971). But their feathers have been the birds' main attraction.
Ostrich feather plumes have been used for decoration for many generations. They are common in the headdresses of African warriors. They adorned the fans of Assyrian Kings and of Popes, the horses of Tutankhamen's chariot, and the headdresses of Greek, Roman and Turkish generals. The ostrich plume frequently featured in the hieroglyphics of Ancient Egypt, where with its symmetrical appearance it was adopted as the symbol of justice and truth (Smit, 1963).
The European demand for ostrich feathers for personal adornment began in the fourteenth century; it grew steadily, and resulted in widespread persecution of the species. Without domestication, ostriches might well have been completely exterminated by now. For centuries, a few people had captured odd young birds as chicks, tamed and reared them, and kept them as pets, or for riding as in ancient Rome and Egypt (Smit, 1963). But the farming and breeding of them in captivity only commenced in about 1863 in South Africa.
The ostrich farming industry was phenomenally successful, fuelled by prices of £5 and more per pound of feathers. The development of husbandry techniques, and particularly of ostrich egg incubators, resulted in an increase of the captive population from 80 birds in 1865 to over 150000 within 20 years (Smit, 1963). Large fortunes were made. Many ostrich farms sprang up, mainly in the Little Karoo region centred on Oudtshoorn in the Cape Province of South Africa, but also elsewhere in South Africa, Kenya, Egypt, Australia, New Zealand, the USA and Argentina. At the peak around 1913, there must have been at least 1 million birds in captivity.
Excerpted from The Ostrich Communal Nesting System by Brian C. R. Bertram. Copyright © 1992 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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|4||The Breeding System||49|
|6||Strategies Adopted by Major Females||102|
|7||Major and Minor Female Strategies||124|
|9||Discussion: The Evolution and Maintenance of the Communal Nesting System||159|