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The Gnu's World

Serengeti Wildebeest Ecology and Life History

Richard D. Estes (Author)

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ISBN: 9780520958197
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This is the first scholarly book on the antelope that dominates the savanna ecosystems of eastern and southern Africa. It presents a synthesis of research conducted over a span of fifty years, mainly on the wildebeest in the Ngorongoro and Serengeti ecosystems, where eighty percent of the world’s wildebeest population lives. Wildebeest and other grazing mammals drive the ecology and evolution of the savanna ecosystem. Richard D. Estes describes this process and also details the wildebeest’s life history, focusing on its social organization and unique reproductive system, which are adapted to the animal’s epic annual migrations. He also examines conservation issues that affect wildebeest, including range-wide population declines.
Introduction: The Author’s Fifty-Year History of Wildebeest Research

1. Africa: The Real Home Where Antelopes Roam
2. African Savannas: Understanding the Tropical Climate, Vegetation, and the Gnu’s Ecological Niche
3. Introducing the Wildebeest’s Tribe: Similarities and Differences among the Four Genera and Seven Species
4. The Four Wildebeest Subspecies and the Status of Migratory Populations
5. Increase and Protection of the Serengeti Wildebeest Population
6. Serengeti Grasslands and the Wildebeest Migration
7. Social Organization: Comparison of Migratory and Resident Populations
8. Male and Female Life Histories
9. Cooperation and Competition among Twenty-Seven Ungulates That Coexist with the Wildebeest
10. The Amazing Migration and Rut of the Serengeti Wildebeest
11. The Calving Season: Birth and Survival on Calving Grounds and in Small Herds
12. Serengeti Shall Not Die? Africa’s Most Iconic World Heritage Site under Siege

Bibliography
Index
Richard D. Estes is a behavioral ecologist and chairman emeritus of the Antelope Specialist Group of the International Union for Conservation of Nature (IUCN). He is a research associate of the Smithsonian Conservation Biology Institute and an associate of the Harvard Museum of Natural History. His books include the successful Behavior Guide to African Mammals (UC Press) and The Safari Companion. Estes chose the Serengeti white-bearded wildebeest as the subject of his doctoral dissertation while living in Ngorongoro Crater from 1963–1965. He continues to study antelope and associated mammals in the Ngorongoro and Serengeti ecosystems and is considered the world’s authority on wildebeest behavior.
"A masterful explanation of Richard Estes truly long-term studies of wildebeest . . . Both informative and entertaining."—James M. Vose Ecology 96, no. 1
"Rich in detail and conservation-related issues."—Staff Conservation Biology

2015 Outstanding Academic Title, Choice

Chapter 1

Africa

The Real Home Where Antelopes Roam

The diversity and abundance of antelopes sets Africa apart from all the other continents. Africa has 72 to 75 different species, and Eurasia has 12; the other continents have none. The American pronghorn, adapting to similar plains habitats, looks a lot like an antelope but actually is so different that it is placed in a family of its own. Then what, exactly, is an antelope?

Good question. Antelope is the common name for all members of the family Bovidae other than cattle, sheep, or goats, plus a few tribes with no domesticated species (mountain goat and chamois, muskox and takin) (fig. 1.1). Bovids stand apart from all other ruminants-notably deer, the other major ruminant family-because males of all species and females of some have horns consisting of a bony core covered by a sheath of horn. Horn is made of keratin, the same stuff as our fingernails. So the name "antelope" was derived through a process of elimination and has no taxonomic meaning. It most likely comes from the Latin name of the Indian blackbuck, Antilope cervicapra, making this the only antelope deserving the name, taxonomically speaking.

Taxonomists have divided the Bovidae into different subfamilies and tribes whose genera and species share a common ancestry. But the tribes are as different from one another as cattle are from sheep and goats. This is true even of tribes placed in the same subfamily. Look at cattle, tribe Bovini, and spiral-horned antelopes, tribe Tragelaphini: they all belong to the subfamily Bovinae. The tribes in the subfamily Antilopinae are much more diverse. The latest classifications, based largely on DNA analyses, assign all the bovids to just these two subfamilies.1

In my view, lumping them this way obscures the affinities between sister tribes. Now goats and sheep are included with all the antelopes in the subfamily Antilopinae, which used to include only the gazelles (Antilopini) and the dwarf antelopes (Neotragini: dik-dik, klipspringer, etc.). Previously, goats, sheep, and goat-antelopes were cleanly separated in the subfamily Caprinae. Consequently, I'm sticking to the traditional arrangement followed in my two behavior guides2,3when considering the similarities and differences between the hartebeest-wildebeest tribe and other tribes of African antelopes.

The Bovidae are the latest family of ungulates to appear in the fossil record. The first known representative, Eotragus, dates from the Miocene, 20 million years ago (ma) (fig. 1.2). The adaptive radiation of the family tracked the global change from tropical to cooler climates and the replacement of tropical woodlands and swamps in Eurasia, North America, and Africa by grasslands. Most of the tribes present today evolved within the first few million years. By then, bovids were already adapted for particular biomes: gazelles and oryxes for desert and subdesert; duikers for rain forest; wildebeest/hartebeest for savannas, and so on. Goats and sheep occupied niches in the mountain ranges of the North Temperate Zone; only the Barbary sheep and two ibexes made it into sub-Saharan Africa.4

Why Bovids Rule

There are four main reasons.

a) Being the latest herbivores to arrive on the scene

The bovid radiation reached its peak within the last several million years in the Pliocene and Pleistocene (5-2 ma; fig. 1.2). Most of the genera are still with us, though greatly reduced in numbers and geographic range.

b) Advantages of the ruminant digestive system

The difference is plain to see in any pasture occupied by horses or donkeys together with cattle, sheep, or goats. The cowpats and pellets dropped by the ruminants are fine-grained, whereas the road apples of the equids contain many bits of undigested fodder. In all nonruminant herbivores, the digestion of plants occurs in the large intestine and in an appended intestinal pouch, the cecum, of which our appendix is a useless relic. Nonruminants, such as pigs, rhinos, and elephants, are hindgut fermenters, because digestion of plants occurs only after being processed in the stomach. In contrast, ruminants are foregut fermenters, as much of the nutrient processing occurs in the rumen and interconnecting stomachs (pouches) prior to entering the stomach and also afterward in the cecum, which many ruminants still retain (fig. 1.3).5

What I still find amazing is that vegetation eaters of whatever family or order cannot themselves digest cellulose, which forms plant cell walls, but depend on bacteria and other microorganisms in the digestive tract to perform this service. A more striking example of codependence, or mutualism, can hardly be imagined. What the ruminants did was to greatly expand the accommodation afforded these organisms in the form of the rumen, a large anteroom that is in essence a fermentation vat. Fermentation is the metabolic process of converting carbohydrates to alcohols and carbon dioxide, or organic acids under anaerobic conditions.

Some knowledge of how ruminant digestion works is important for understanding the advantages ruminants have over hindgut fermenters. The description I wrote in the introduction to the ruminants in The Behavior Guide to African Mammals (4) will suffice for present purposes.2

The rumination process is both mechanically and biochemically complex and still not fully understood. First the animal feeds until the rumen is comfortably full by gripping foliage or grass between its lower incisors and upper dentary pad, plucking, and then swallowing after briefly chewing each mouthful. Then it settles down to chew the cud, either lying or standing, grinding each bolus with rhythmic side-to-side jaw movements. To make this sideways action possible, ruminants had to lose their upper front teeth; they pluck (grazers) or snip (browsers) plants with their six lower incisors against a tough dentary pad in place of the upper incisors.

The cud consists of the coarsest plant particles, which float on top of the semiliquid rumen contents and are regurgitated a mouthful at a time through contractions of the rumen and its annex, the reticulum (the "honeycomb tripe" relished by some gourmets). As the ruminant grinds each mouthful at a steady rate, on the same or alternate sides of the mouth, enlarged salivary glands secrete a buffered solution that helps to maintain the rumen pH preferred by the microorganisms in the rumen. Chewing the cud promotes the full extraction of nutrients by increasing the surface area that is exposed to bacterial action. Some nutrients are absorbed through the rumen wall, which is lined with tongue- or finger-shaped papillae; these both vastly increase the absorptive area and provide crannies in which bacteria and protozoans multiply.

Rhythmic contractions of the rumen and reticulum keep stirring the "vat," sorting food particles according to size and specific gravity. The smallest particles sink to the bottom and from there are pumped through the reticulum into the omasum, also known as the "book organ" or "psalter" because of the leaflike plates that line it. Here the semiliquid ingesta are filtered once more before being pumped into the abomasum, the true stomach. Afterward, during passage through the intestines, the residue undergoes some final cellulose digestion in the cecum.5

To enable the stomach compartments to move freely, ruminants have to lie on the brisket or stand while ruminating. They rarely lie on their sides for more than a few minutes at a time. Anesthetized ruminants left lying on their sides are likely to ingest rumen contents and suffocate. The need to maintain a certain position and to keep chewing the cud may explain why ruminants do not sleep soundly as do nonruminants. This complex digestive system seemingly precludes normal sleep. Yet rumination goes together with a relaxed state (the "contented cow" syndrome), and the brain waves of ruminating animals resemble those associated with sleep in nonruminants. And until newborn ruminants begin ruminating, they sleep comfortably lying on their sides.

In addition to the more complete utilization of plant fiber in ruminant digestion, the constantly reproducing and dying rumen microorganisms that do the work provide the host with energy in the form of volatile fatty acids that they excrete as metabolic wastes, and the organisms themselves become a major source of protein as they pass through the digestive tract mixed with the rumen contents.

Ruminants possess the further important advantage of being able to recycle urea, thereby retaining and recycling inorganic nitrogen that the ruminant bacteria use to reproduce and to synthesize more protein. From this bacterial protein ruminants acquire the essential amino acids that nonruminants have to gain from their plant food. As an added bonus, recycling urea cuts down on urine excretion, helping to conserve water and contributing to the water-independence of desert-adapted ruminants.

There is one major drawback to rumination: the thorough digestion of cellulose takes time, and the more fibrous the food, the longer it takes-up to four days from ingestion to excretion. When protein content falls below 6 percent, ruminants cannot process their food fast enough to maintain their weight and condition. Hindgut fermenters consume and partially digest large quantities of low-quality forage in half the time; they can thereby manage to obtain adequate sustenance from vegetation too tough and fibrous for ruminants to process. Thus a horse can extract only two-thirds as much protein from a given quantity of herbage as a cow, but by processing twice as much in a given time, its assimilation of protein will exceed the cow's by one and a third times.

c) Habitat diversity and the ability of antelopes to specialize

Africa has by far the largest tropical landmass. It is the only continent that spans both tropics and also extends into the temperate zones to 37°N and 35°S (see map 2.1). Diversification of life zones, or biomes, began in the middle Miocene, providing new opportunities for the antelopes to exploit. The extraordinary versatility of the ruminant digestive system helps to explain how the ruminants could become so diverse, filling a greater variety of ecological niches than any other group of herbivores. The ability to structure the digestive system precisely for a given diet has enabled ruminants to partition African ecosystems into much narrower feeding niches than can nonruminants, which require a greater variety and amount of vegetation to meet their nutritional needs. For instance, the horse (Equidae) and pig (Suidae) families occupy much broader niches and have relatively few species. The bovid design for a particular ecological niche combines size, conformation, feeding apparatus (especially width of muzzle and incisor row), development and complexity of the digestive tract, dispersion pattern, and reproductive system to produce the best fit for a specific niche.

Climatic and geomorphic changes (e.g., volcanoes, rifting, plate tectonics) kept creating more habitat diversity during the Pliocene and Pleistocene, inserting new niche spaces, which antelopes proceeded to occupy.

Speciation within Africa was promoted by expansion and contraction of the Equatorial Rain Forest during wet (pluvial) and dry (interpluvial) periods of the Ice Age. During pluvial periods the rain forest stretched from coast to coast, barring interchange between northern and southern savanna and arid biomes but facilitating the dispersal of forest forms. In succeeding dry periods (Ice Ages), the rain forest was reduced to a number of isolated islands, and a drought corridor extending through eastern Africa connected the savanna and arid biomes. This explains the presence of some of the same antelopes in the Somali and South West Arid Zones, separated by the whole Miombo Woodland Zone, for instance, the oryx, hartebeest, topi/tsessebe, wildebeest, dik-dik, and steenbok.

Exchanges of fauna and flora also occurred during the Ice Ages when lowered sea level created land bridges connecting Eurasia and Africa. Thus, a major faunal revolution occurred in the early Pliocene, after many Eurasian fauna crossed into Africa. Over 85 percent of the Pliocene genera were new, including nine genera of bovids, all but five unknown outside Africa. As late as the early Pleistocene, new genera continued to appear in the fossil record, due both to Asian immigrants (eight more bovid genera) and in situ evolution. The duikers, dwarf antelopes, and reedbuck tribes are the only bovids that evolved in Africa and never reached Asia.

The Sahara Desert has imposed a formidable barrier to intercontinental movement since the second half of the Pleistocene, as reflected by a much higher proportion of endemic African mammals. Eurasian species could still disperse to North Africa, but only desert-adapted forms could penetrate the Sahara. Most of the Eurasian tropical savanna fauna became extinct during the Ice Age, leaving Africa as the final refuge of the Plio-Pleistocene fauna. In the variety and abundance of large mammals-with antelopes foremost-Africa represents the Golden Age of Mammals.6

The areas of sub-Saharan Africa with the greatest biomass and diversity of herbivores clearly show their tropical origins. Optimum conditions occur where mean annual temperature ranges between 19° and 22°C and mean annual rainfall ranges between 750 and 1,000 millimeters (mm). These areas are associated with semiarid to moist (mesic) savanna ecosystems.7 By far the greatest known herbivore biomass of any terrestrial ecosystem has been reached in the African grasslands.8,9,10,11 Even without the elephant and the hippo, the wild ungulate biomass of the best African savanna far exceeds that of recent mammals of the Great Plains, the South America savanna, the tundra, and the Eurasian steppe.12 In an area with diverse habitats, over a dozen (up to seventeen) species may be found within a few square kilometers (sq. km) (e.g., Serengeti, Kafue, Hwange and Chobe National Parks).

Specializing for an Ecological Niche

Every kind of grazer prefers new green grass. That's only natural, as the fiber content is low and the protein content can be as high as 30 percent or more. A pasture covered with fresh green grass is every grazer's dream come true. But no grazer bites off more than it can chew, and how big a bite it can take depends on the width of its muzzle and incisor row (dental arcade). The wildebeest, with its wide muzzle and incisor row, should take bigger bites than a hartebeest or topi. The size of the bite also depends on the height and density of the grass sward. For animals the size of a topi or wildebeest, the sward has to be at least a couple of centimeters high to reward the feeding effort.

We're talking costs/benefits here. (This concept, borrowed from economics, is now pervasive in ecologists' thinking.) The first plains game to move onto a postburn flush are small selective gleaners such as Thomson's gazelle or oribi and zebra, which with their full set of choppers and flexible lips can feed as closely as gazelles. For cattle or buffalo, the stand has to be a good 10 centimeters (cm) (4 in.) high to feed efficiently. In long grass they can take the biggest bites, because with every bite their prehensile tongue sweeps out and bundles the grass. This tongue action is hard-wired in the whole tribe (Bovini) and at least some members of the sister tribe of spiral-horned antelope (Tragelaphini), most of which are browsers.

Grazers are distinguished from browsers by having high-crowned (hypsodont) cheek teeth and proportionally large stomachs. Their row of incisor teeth is also more inclined from the vertical, designed to pluck grass pressed against the dentary pad with a slight chin nod. The nearly vertical incisor row of browsers can cleanly snip foliage. Grazers need high-crowned molars because the silica content of many grasses increases the molar wear of chewing the cud. Silica(SiO2) deposited inlarge quantities in the shoot system reinforces the cell walls, reducing palatability by increasing the abrasiveness of the leaves and making extraction of starch and proteins more difficult. Thus grasses defend themselves against herbivory by increasing the costs of consumption.13,14

Browsers are classified as concentrate selectors because their diet consists of foliage and forbs with low fiber and high protein content, making dicots more digestible with less effort than monocots. An incisor row with the two central teeth enlarged also distinguishes and enables browsers and mixed feeders to feed more selectively than grazers, whose incisors are similar in width and allow bigger but less selective bites.15 (Fig. 1.4.)

Size is also a determining factor in the diet of grazers. Small size goes together with rapid metabolism. Larger animals have a lower metabolic rate. Because it takes more time to ferment and break down grass, most pure grazers are of medium to large size. The smallest grazing antelope is the 14 kilogram (kg) (30 lb.) oribi. So it is no accident that the bulk feeders are medium to large, as it can take several days to extract the nutrients from fibrous grasses.

The number of bites a minute is another measure of feeding efficiency. The ecological concept of optimal foraging applies here. In theory, natural selection should lead to the most efficient feeding behavior a species can achieve. Thus a grazing animal should feed at the fastest possible rate in the pasture of its choice. This rate is species-specific, dependent on size, feeding apparatus, and selectivity of grass stage. The quickest eaters take up to thirty thousand bites in a day.16

According to Ledger,17

Tropical wild animals appear to be physiologically incapable of depositing large amounts of fat, in the carcass, thorax, or viscera, even when ample food is available. Conversely, there is so far no evidence that their body composition is materially affected when food is less plentiful. The evidence suggests that game animals may be physiologically geared to a near-maintenance level of production and that once these requirements have been met, their appetite is satisfied and they cease to feed. From a meat production point of view this represents a degree of inefficiency because such animals are then incapable of making use of an abundance of food. From a soil conservation standpoint such a limitation may be an advantage as it means that the vegetative cover is then easier to maintain. (138)

Putting all this together, we see that adaptation for a particular ecological niche involves a wide range of different factors. But when all is said and done, size and digestive system stand out as the major determinants of niche separation.18,19

d) The dominance of the grazers

Three tribes of grazing ruminants dominate Africa's savannas.

Reduncini (reedbuck, kob, waterbuck) occupy valley and floodplain grasslands within a short distance of water.

Hippotragini include the desert-adapted addax and oryxes, as well as the water-dependent sable and roan, which inhabit the well-watered savanna woodlands.

Alcelaphini exploit savanna ecosystems with extended wet and dry seasons. They disperse into dry savanna during the rainy part of the year and concentrate on green pastures near water in the dry season.

The Bovini are the fourth tribe that falls into this category, but the Cape buffalo (Syncerus caffer) is the only native African bovine. Bovini evolved in Eurasia. The Sahara was an impenetrable barrier to these water-dependent grazers. Ironically, domestic cattle now far outnumber all Africa's wild bovids but only because of human intervention.

The members of these tribes include the most advanced ruminants. They have the most developed digestive systems, capable of subsisting on dry hay with as little as 6 percent protein content. They are of medium to large size, built to travel long distances in search of green pastures. They prefer open over wooded habitats and flee rather than hide from predators. They include the most gregarious African bovids with the most conspicuous coloration and markings.20 A few species (wildebeest, topi, blesbok, kob, lechwe, buffalo) form large aggregations and dominate whole ecosystems. These are the keystone species.

Bovid ancestors were small, solitary dwellers in forests and other closed habitats where they relied on concealment to avoid predators. Duikers like the blue duiker and red duiker and dwarf antelopes like the dik-dik and steenbok are most like the early bovids.

To leave cover and exploit the abundant grasslands required a whole suite of profound changes:

Size and conformation

From hiding to avoid predation to reliance on flight in the open

From browsing to grazing

From solitary and pair-forming social systems to herding and polygyny

From cryptic to conspicuous coloration

Development of visual displays as a major avenue of communication

The Division between Bovids of Closed and Open Habitats

Many of the morphological and behavioral adaptations to closed and open habitats are opposite. Thus antelopes of closed habitats that hide to avoid predation have hindquarters that are more developed than forequarters and cryptic or disruptive coloration and markings (duikers, dik-dik, bushbuck, kudu). This conformation enables quick starts, sharp turns, and high bounds when an animal is flushed from its hiding place. Antelopes of open habitats have long, evenly developed limbs (gazelles, oryx) or higher, more developed forequarters (hartebeest tribe) adapted for long-distance travel and the speed and endurance necessary to outrun predators.

In an analysis of the correlations between conformation (phenotype), socioecology, and mating system of African bovids,21 I sorted the bovids into two distinct ecotypes, one adapted for life in closed habitats and the other adapted for life in the open. It appears that this dichotomy applies to the whole family.2,20

Group I, comprising bovids that live in closed habitats, have cryptic and disruptive coloration, and possess the conformation just described, is associated with a concealment strategy. The duikers and dwarf antelopes are solitary or live in monogamous pairs and have minimal sexual dimorphism and approximately equal adult sex ratios. Two reedbucks fall into this category but are of medium size (up to 80 kg [175 lb.]) with well-developed horns in males only. Most members of the bushbuck/kudu tribe (Tragelaphini) also belong in Group I but are larger, polygynous, and very sexually dimorphic, with skewed adult sex ratios.

In Group II, comprising bovids that live in open habitats, antipredator strategies depend on the ability to outrun predators in open flight, or on refuges (cliffs for goats, swamps for lechwes and sitatunga), or on large size and/or group defense (buffalo, other Bovini). All the member species are gregarious, have polygynous mating systems with adult sex ratios favoring females, and have at least some degree of sexual dimorphism (males larger, with more developed horns). The most conspicuous and distinctive coloration is seen in the most gregarious species that live in the most open habitats. Hippotragini, Alcelaphini, and most Reduncini, the gazelle tribe, goats and sheep, and bovines (buffalo, etc.) belong in this category.

How so? When there is no need to hide, selection for visual communication causes conspicuous coloration to evolve.21,20 The need to be as conspicuous as possible to conspecifics and different from other associated bovids can account for the color schemes that distinguish bovids of open habitats: dark or light coats that contrast with the natural background, black and white markings that contrast with coat color, and reverse countershading (darker below, lighter above, as in the wildebeest). Furthermore, coloration is only one of a suite of visual recognition characters, including size, conformation, locomotion, horns, and other appendages, so redundant that associated species can be recognized by silhouette alone. Such redundancy of recognition characters is in itself evidence of the premium placed on development of a distinctive appearance that promotes communication and mating with one's own species while selecting against attraction to the traits of other species.22,23 The contrasting coloration and conformation of topi and hartebeest subspecies are illustrated in chapter 3 as cases in point.

That virtually all bovids that live away from cover are social while virtually all solitary species depend on concealment suggests that gregariousness is an essential adaptation for life in the open. One might go further and postulate that herd formation was a prerequisite to abandoning concealment. Antelopes band together in the open for mutual security: the herd takes the place of cover for the individual. Take away their cover, and even Group I antelopes like reedbuck and oribi join in temporary herds. In short, the mutual security offered by a group may be the foundation of sociability.

Leaving aside sick or injured animals, the only individuals of gregarious species seen alone are mothers keeping watch over concealed young calves and males guarding territories. The willingness of males to stay alone places them at greater risk but is a prerequisite for reproduction in territorial mating systems.

To become sociable it was necessary for females to forgo territoriality. In fact, male territoriality is the rule in all but nine African antelopes, all of them in the bushbuck-kudu tribe (Tragelaphini). This tribe is allied with the buffalo-cattle tribe in the subfamily Bovinae, whose lineage has been separate from the Antilopinae subfamily (as currently classified) since the Miocene.24 Instead of territoriality, a male dominance hierarchy, based on prolonged physical development, determines mating success.

Consequences of the Male Arms Race

Competition between males to monopolize breeding opportunities with females in herds selects for development of characteristics such as size, weapons, and ornamentation that make for dominance. The larger the herds of females, the more intense the male competition. It is the classical peer competition Darwin described.25 When the reproductive unit is a mated pair, competition is minimal, and so are physical differences between the sexes. In fact, female duikers are slightly larger than males, and both sexes have horns. In solitary and monogamous systems, grown offspring of both sexes perforce leave home. Facing equal risks, mortality rates are similar and the adult sex ratio remains more or less equal. In herd-forming species, the intolerance of breeding males toward perceived rivals forces male offspring to leave their natal herd and home range, whereas females can remain at home. Consequently, males suffer higher mortality and the adult sex ratio is skewed toward females, supporting the polygynous (multifemale) mating system.

Among the male secondary sex characters that evolve through peer competition, development of large and elaborate horns makes for the most obvious gender difference, especially in species with hornless females. In the impala, for instance, color and markings are the same in both sexes. But in the Reduncini, in which females are also hornless, not only the horns but also male coloration and markings of the most gregarious species (white-eared kob, lechwe) enhance gender differences (sexual dimorphism). In species with horned females, male horns are typically far more developed than in females, as seen in the gazelles, sheep, and goats (fig. 1.5).

Color dimorphism is pronounced in the sable antelope, alone of all the Hippotragini: mature males are jet black with white markings, while females stay brown. In order to stand out in the crowd to maximum effect, males need to keep growing for up to three years longer than females. Sable cows conceive at two years, while males finally mature at five years. But the most extreme maturation difference is seen in the nonterritorial, male-dominance mating system. Thus eland cows also conceive in their third year, whereas bulls mature at eight but have to keep growing in order to compete with their seniors, who slowly but surely continue to add bulk.

How to Explain Minimal Sexual Dimorphism in Wildebeest and Oryx Tribes

Except for the sable, antelopes in the Hippotragini and Alcelaphini display surprisingly little sexual dimorphism (SD). As they too have territorial, polygynous mating systems, males would be expected to respond to peer competition by clearly advertising their sex. Yet the similarity of males and female can be close enough that sexing individuals is challenging for human observers (fig. 1.6).

The notion that females might mimic male secondary characters arose from the difficulty I had sexing passing wildebeest: cows had a faux penile tuft that looked a lot like the male's real one. I wondered, was this annoying little addition designed to deceive other wildebeest as it deceived me? Years later, an analysis of African antelope social organization21showed that minimal sexual dimorphism in gregarious species went together with the habit of forming mixed herds, particularly in species with migratory and nomadic habits. This correlation suggested that minimal SD could enable males to mingle with minimal aggression. It seemed that some force countered selection of conspicuous male secondary characters, leading to reduced SD and intolerance between males. It was this male intolerance that led to the expulsion of adolescent males from female society, as I had observed countless times.

Peer competition is by definition between equals. But competition between different age classes is by nature unequal. Aggression by an adult against a younger male is a form of despotic competition.27 This form was not considered in Darwin's theory of sexual selection and has rarely been considered as the antithesis of peer competition since. Yet it can counteract peer competition by suppressing or postponing the development of male secondary characters.2,27,28 In his seminal treatise on sexual selection, Darwin actually pointed out an example of despotic competition that he could nott explain: the horns of yearling elands were more developed than the horns of yearling male kudus, whose horns are among the largest of any bovid.

The plausible explanation is that male kudus cannot afford to advertise their sex by developing this most obvious male secondary character before they are grown up enough to survive eviction from the maternal herd. Both sexes of the eland bear horns. Females and young males look enough alike that males do not stand out until their third year or even later.

The same holds true of other bovids. As horns of both sexes are equally developed in the young of horned females, males are tolerated until they develop beyond female limits. Being thus disguised, they can begin horn development sooner than species with hornless females-in response to peer competition. That would explain why horn development begins earlier in all bovids with horned females, typically within the first two months, compared to six months or so in offspring of hornless females.26

This train of evidence led to the hypothesis that females evolved horns to buffer male offspring from despotic competition by mimicking this iconic male secondary character. How else could they disguise this permanent appendage? Fossil remains show that females of the earliest bovids were hornless and that horns evolved in females of the different tribes independently.29 Females in thirty-one of the seventy-two African antelopes remain hornless.

The conventional view of female horns is that they evolved as weapons of self-defense and offspring defense against predators.30,31 Certainly females use them as weapons but so seldom and ineffectually against predators that their advantage over hornless females is hard to see or quantify.26,32 Prolonging the stay of male offspring, on the other hand, should be a strong source of both natural and sexual selection. It enables sons to stay in the maternal herd until the benefits of leaving outweigh the costs of staying while at the same time getting a head start (!) on horn development. In this way mothers also promote their sons' reproductive success, which is potentially much greater than that of daughters, who produce only one offspring a year.

If the horn mimicry hypothesis sounds far-fetched, consider the fact that male mimicry is not limited to horns. Females also replicate beards, manes, dewlaps, and coloration to lesser or greater degree. And don't forget the wildebeest cow's penile tuft (see chapter 4).* But horn mimicry stands out as most essential because males cannot compete without them, and their size is important in reproductive competition.

*Apparently the Western white-bearded race is the only subspecies that has developed the penile tuft. See chapter 4.

I find it fascinating that male sexual competition in the bovids can have opposite effects depending on whether it is between peers or between older and younger males. It is despotic competition that checks and slows the development of horns and other male secondary characters that arouse male intolerance. As long as males and females look and act alike, despotic competition is minimal. In species with pronounced SD, females stop mimicking males at the stage where the cost-benefits favor their joining all-male herds wherein they can openly engage in peer competition.

The minimal SD exemplified by wildebeest and oryx is the outcome of females tracking male secondary characters to the adult stage. As already noted, reduced SD correlates with social groups that include adults of both sexes. In the desert-dwelling oryx, the chances of evicted males finding a bachelor herd are so slight that natural selection favored remaining permanently in the company of females. The resemblance of the sexes is so close that the alpha male treats both sexes like females.

How far females track male characters depends on the species' social and ecological circumstances. Horn mimicry extends no further than early adolescence in the Thomson's gazelle and in various sheep and goats, in which females have short, thin spikes for horns, all the way to maturity in the Alcelaphini and Hippotragini, whose horns, though never as robust as in males, are of similar shape and size.

For an in-depth review of the male mimicry hypothesis, see Estes.26 In a later paper,20 I propose that the evolution of conspicuous coloration and markings in bovids with gregarious, polygynous social systems was promoted by continuing female mimicry. That could happen because of peer competition to develop distinctively male secondary characters. If females proceed to track these characters, then males have to keep trying to be different. When females copy these features to the adult stage, you end up with a gaudy animal like the gemsbok, the most conspicuous and least dimorphic antelope.

References

1. Gentry 1990.

2. Estes 1991a.

3. Estes 1999b.

4. Kingdon 1982.

5. Hofmann 1973.

6. Huxley 1965.

7. Thackeray 1995.

8. Bourlière and Verschuren 1960.

9. Talbot and Talbot 1963.

10. Lamprey 1964.

11. Field and Laws 1970.

12. Bourlière 1961.

13. Kaufman et al. 1958.

14. Hunt et al. 2008.

15. Janis and Ehrhardt 1988.

16. Murray and Illius 1996.

17. Ledger 1964a.

18. Bell 1970.

19. Jarman 1974.

20. Estes 2000.

21. Estes 1974.

22. Dobzhansky 1937.

23. Mayr 1963.

24. Kingdon 1997.

25. Darwin 1871.

26. Estes 1991b.

27. Schein 1975.

28. Wilson 1975.

29. Janis 1982.

30. Packer 1983.

31. Stankowich and Caro 2009.

32. Walther 1966.

33. Brooks 1961.

34. Estes 1967a.

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