The Impala of Rifa

The Impala of Rifa, reproduced below, was written by Leslee Maasdorp, and published in Harare by Zimbabwe Hunters Association, 1st Edition, November 1998, 2nd Edition, February 2006. (ISBN 0-7974-3 1 06-3     ©Rifa Conservation Camp.)

“Not only are impala intrepid evolutionary survivors, but they are a phenomenal ecological success story as well, and have actually increased in numbers and broadened their range over the past century in concert with rising human population.” Michael Mooring, “The Living Fossil”, Africa – Environment & Wildlife, Vol 3. No, 5

Introduction to the Book

Schools, colleges and societies with an environmental bias, use Rifa Conservation Camp for education purposes. The facility is built on a channel bank very close to the Zambezi River.

Impala herds are seen every day. One of the animals is killed and provided to each school party for study by the Zimbabwe Hunters’ Association which funds and runs the camp.

The impala is the keystone animal for the course. Scholars measure and skin the carcass and dissect it under the guidance of field lecturers, then they eat most of the meat and place bones and scraps out for vulture and hyaena watches.

One of the reasons for writing this book is to provide more background information for the dissection work and for data collection on the condition of the antelope and its parasitic infestations.

Despite having the record of being the most studied antelope of Africa, impala have managed to keep some aspects of their lives hidden until recently. In the 1980s researchers were surprised to find some of impala’s front teeth provided a comb for grooming and in the 1990s even more surprised to discover that these teeth are wobbly, an effective design for trapping ticks. It is research findings such as these that students and adults find fascinating, yet have little chance of finding out this information for themselves. This is not only because the vast literature on impala is scattered in so many journals published all over the world, but that the few libraries that hold these papers in this country are not easily accessible to the public. Nor have students and adults the time, or even the inclination, to wade through the papers, most of which are long and of technical nature. Resumes of this recent work are to be found in this handbook as well as accounts of the nutrition, parasitology and ecology of the animal. Areas well dealt with by field guides, e.g. descriptive features and behaviour have been scantily covered here.

Those reading this handbook may gain a deeper insight into the adaptability and resilience of this species and may have their appetites whetted to extend their interest to other antelope. The reader may well come to appreciate and respect the impala as it strives so successfully to overcome the harsh conditions of the mid-Zambezi Valley.

LESLEE MAASDORP

Resources Manager, Rifa Conservation Camp near Chirundu under auspices of the Zimbabwe Hunters’ Association

Introduction to the Impala

 From diverse East African strata, fossil remains of impala have been found that are over three hundred million years old. In the following pages some of the reasons for the success of this antelope are examined.

Besides being so successful from a survival pointy of view, impala are also noted for their beauty and grace. They possess the best leaping skills of any antelope, being able to float through the air in a graceful arc which may take them over a distance of 10 to 12 metres.

Impala Taxonomy

Class : Mammalia –

Order : Axtiodactyla (even-toed ungulates)

Family : Bovidae (antelope, buffalo and cattle)

Sub-family : Aepycerotinae

Genus : Aepyceros (impala are the only living members of this genus)

Species : melampus

Sub-species : melampus

Impala Vital Statistics

Shoulder Height: Males 90 cm : Females 85 cm

Mass: Males 55 kg : Females 40 kg

Horns: Male – lyrate horns, females none

Maturity: Females 17 – 20 months

Gestation: 6 ½ – 7 months

Births: November – December

Lactation: +/- 5 months

Impala names

The impala was first described to European audiences by German zoologist Hinrich Lichtenstein in 1812. He gave it the scientific genus name Aepyceros (literally ‘high-horned’) from Ancient Greek aipus (‘high, steep’) + keras, (‘horn’). This refers to the tall lyre-shaped horns of the males. The species name melampus (literally ‘black-foot’) comes from melas (‘black’) + pous (‘foot’).

The first English name for this antelope, mentioned in 1802, was palla or pallah, from the Tswana word phala meaning ‘red antelope’. The name impala, (also spelled impalla or mpala) first occurs in 1875 taken directly from Zulu (and is also the Ndebele name). The Shona name is mhara. The Afrikaans name, rooibok ‘red buck’, is also sometimes used in English.

Distinctive features of the impala are:

Diagnostic or distinctive features that set the impala apart from other antelope are:

l. The lyre-shaped horns of the males, i.e. shaped like the ancient Greek musical instrument (The females are hornless);

2. Two pairs of mammae or teats (many other antelope have one pair);

3. Hind legs with metatarsal glands covered with a noticeable tuft of black hairs;

4. No false hooves are present. (False hooves are the toes on an animal which do not normally touch the ground. These toes are present on what one would normally consider an antelope’s ankle.)

Distribution

Two subspecies are recognised, the common impala (Aepycetos melampus melampus), and the black-faced impala (Aepycetos melampus petersi). The latter is confined to southwestern Angola and Kaokoland in northwestern Namibia. The common impala is widely distributed throughout the eastern savanna woodlands of Africa reaching from eastern Natal to Kenya.

Climate where impala are found

Where impala occur, the summer season is warm to hot and rains generally fall in these months.

In the study area at Rifa Camp (location ref. l6°00S 28°5 lE) and elsewhere in the mid-Zambezi Valley, the climate is described as being very hot. The mean annual temperature is 25°C, the highest recorded temperature being 54°C at Chirundu which is close to Rifa Camp. In some years in November, the Valley has had the distinction of being the hottest place on earth.

Rainfall is confined to the months of November to March, the average being about 600 mm.  With the high temperatures the effective rainfall is less. Furthermore, much of the rain often falls during violent storms and rapidly runs off the surface into rivers and down-stream to the ocean.

Cooler nights occur from April to August but from mid-August temperatures begin to rise, and the humidity drops. By the end of September desert-like conditions prevail with a very dry atmosphere and intense heat. It is these months before meaningful rain arrives in December that make it difficult for wildlife to survive. Nowhere in its range does the impala have to contend with a harsher climate than the mid-Zambezi Valley.

Habitat

Impala have a localised distribution preferring light woodlands or dambos (seasonally inundated grasslands) or channel grasslands. They are very common in the mid-Zambezi Valley where they are seldom found too far from water. They are not found in mountainous regions.

They are associated with Colophospermum mopane, (mopane) woodlands where in the rainy season pans are common. As the pans dry out, herds within several kilometres of the Zambezi tend to concentrate on the flood plains where some grass and good water supplies are available. Other herbivores follow suit, and the condition of alluvial plains deteriorates until the rains arrive. It is then that animals disperse once more, leaving the alluvial plains to recover.

Other woodlands, e.g., Acacia, Combretum and some teak woodlands also provide good habitats for impala. The trees and bush provide shade and shelter and browse while fairly open areas give the herds a refuge to sleep in, providing opportunity to flee from predators.

Impala appear to be more sensitive to human encroachment than some of the other antelope. As a result they have become extinct in areas where they once occurred.

Coat & Colouring

Does the hair colour and patterning contribute to survival?

The impala has a shiny red colouring to the hairs on the upper body separated by a clear line separating this from the flanks which are light brown, while the underparts are pure white.

This design not only helps in camouflaging the buck, it also seems to give a two dimensional appearance which confuses the predator.

It is interesting to note that each antelope species has hair with a different cross-section. That of impala is a cricket-bat shape – triangular with rounded corners.

Other ways in which colour and pattern are linked to survival

1. The white underbelly helps in reflecting away heat radiating from the ground.

2. Around the eyes there appears to be white make-up. These white eye stripes assist in reflecting light at night (starlight or moonlight) back into the eyes.  In the Namibian desert where reflection off the sand is dazzling, a different strategy is adopted. The black-faced impala have developed black eye make-up. Guides in Namibia tell tourists the impala are wearing sun-glasses.

3. At a distance and viewed from the side, the impala has a tawny appearance that hides its outline from lions, leopards and cheetahs—all of which hunt by sight. However, it is strikingly marked on the rear with a black-and-white “flag” that is most conspicuous when the animal is leaping or running. At a warning snort from a sentinel guarding the herd, each impala leaps, flashing the black stripes on its tail and hindquarters together with the white underparts of the tail, and black spots on its heels. From the point of view of a predator this “bomb-shelling” of stripes is confusing and agitating.

It is also thought to indicate the direction of escape to the others behind  and so keeping the herd together and promote re-assembling.

Impala grooming

When an impala shot at Rifa is placed on the slaughter block, students invariably comment on how shiny and smooth the coat of the animal is. How is this sleek appearance achieved?

On the lower jaw of the impala the second and third incisor and canine teeth have sharp prongs forming the teeth of a comb. The impala grooms its fur by using this toothy comb in an upward movement, drawing the teeth through the fur and trapping ticks in the comb. It seldom grooms by licking with the tongue. The comb teeth, amazingly enough, can move about 2 mm in their sockets making them more flexible and hence more efficient in raking out the ticks. (The looseness of comb teeth can be felt in fresh carcasses.)  Self-grooming and scratching the skin with the feet is often undertaken.

Mooring, Michael S. “Programmed Grooming after 30 Years of Study: A Review of Evidence and Future Prospects”, Animals, 2024, 14(9), 1266; https://doi.org/10.3390/ani14091266

However, a grooming antelope cannot reach its own head and neck areas. To rid these parts of ticks, two impala face towards each other and in turn “comb” each other’s fur. This type of behaviour is known as reciprocal or allo-grooming and can be carried out by both related and unrelated individuals. Partners receive about the same amount of grooming as they give.

Tick populations are highest in the wet season from January to April and lowest in the dry season, September to December in the Lowveld.  It has been noted that impala groom less when the tick load is lightest.

 Ox-peckers de-tick mammals. Does this activity reduce grooming?

Ox-peckers also remove ticks from the larger mammals, especially from the head and neck areas. There are many large mammals at Rifa that the red-billed ox-peckers Buphagus erythrorhynchus can detick, e. g., buffalo, kudu and hippo. Ox-pecker requirements are host animals, e.g., antelope and hippo

 Impala are sometimes seen with these birds on their backs. In the Matobo National Park, the yellow-billed ox-pecker Buphagus afiicanus plays a big role in ridding impala of ticks. As a result, it was no surprise to find that there is less allo-grooming in this park.

 Tick loads are lower in the mid-Zambezi Valley than in the Lowveld and Matobos and this may be why allo-grooming has so far seldom been noticed at Rifa. In primates grooming is thought to keep social bonds and defuse aggressive attacks, but with impala its purpose is to remove parasites, especially ticks. Impala is the longest surviving of all extant antelope species, so it can be concluded that grooming behaviour has stood the test of time. (Grooming is also dealt with in the tick section of parasites in another post).

Is there evidence that grooming reduces the tick load?

 If animals have less time for grooming, it would be reasonable to predict they would carry a higher tick load. Territorial male impala, busy with keeping their harem together and chasing off challengers, would have less grooming time and hence more ticks. Just such a situation was found when impala, including sire males, were culled in Omay in May 1992 and examined for ticks. A sampling technique, called partial or patch sampling was used involving collecting ticks from selected body areas and it showed that territorial males harboured six times more adult ticks than did the females. (In passing, it is interesting to note that being a territorial male has its drawbacks – its heavy tick load drains reserves by taking up blood, its territory holding activities reduce its feeding time yet use up much energy and at the same time make it more inattentive and hence more vulnerable to predators.)

What about the impala lambs? Do they have to learn from adults about grooming?

It seems not. Lambs are born with a built-in grooming behaviour pattern and soon develop the allo-grooming habit. This behaviour has no doubt evolved because lambs are born into a period when tick levels are increasing (November – December) and they must be able to combat high tick infections right away.

Nutrition

How impala and other herbivores cope with food high in indigestible cellulose.

Mammals do not produce enzymes that can break down cellulose. The ruminants and non-ruminants, however, harbour bacteria, protozoa and other microbes which do produce these enzymes. This is a symbiotic relationship and enables these herbivores to cope with the commonest carbohydrate source on earth, cellulose. See Post entitled Ruminants, including “Detailed account of ruminant digestion and absorption in the impala” by Graham Smith

Impala are on the whole diurnal feeders and favour early morning and late afternoon sessions of feeding. When the rumen is full, they rest and chew the cud either in a standing or a lying down position. The cud is slowly chewed by the massive cheek teeth.

 Impala dentition

      The above means:

• There are no incisors in the upper jaw and three pairs in the lower jaw.
• The incisors project more or less forward and are flattened.  The incisors work against a dental pad (a feature which is found in ruminants who are grazing or foraging vegetation).

• Canines are also absent in the maxillia (upper jaw), present in the mandible (lower jaw).
• Three premolars are present in each half jaw. Cheek teeth have the characteristic herbivore enamelling.

 The cheek teeth (premolars and molars) with their elaborate enamel patterning, together with strong jaw muscles, provide a powerful grinding machine reducing tough cellulose and lignin in the diet to finer pieces enabling digestion in the rumino-reticulum to proceed.

Much saliva is added during this sideways grinding action and when the finer food mass is swallowed, the saliva maintains a suitable pH for microbial activity. The main fermentation vat or rumeno-reticulum harbours some of the richest living systems on earth. Millions of bacteria break down the cellulose while protozoa, both ciliates and flagellates feed on the bacteria.

Impala (and other ruminants) can lose condition more quickly than non-ruminants

The impala (like other herbivores) needs only leafy or twiggy materials to supply all its nutrients. However, the processing of this food is time consuming in a ruminant. This major drawback may result in the impala losing weight if the quality of food drops (i.e., protein level falls below 6%). Thus, if the food contains much fibre, the processing from eating to defecation may take as long as four days and the animal begins to suffer from malnutrition. The ruminant system is very efficient except when food quality deteriorates. In the post-gastric processors or non-ruminants, e.g., elephant which is a bulk feeder, big quantities of low-quality browse and grass are eaten but this is processed in half the time taken by a ruminant. Zebra is also a bulk feeder and eats grass in all its forms, green or dry, stalks and flowers. It remains in good condition longer than does the impala under adverse conditions.

Plants that impala feed on

Impala are mixed feeders and one of the reasons for their success is their ability to switch with ease from grazing to browsing. In a Hwange National Park study it was found that 28 species of browse and 13 grass species were eaten by impala and that grass is invariably a component of what had been eaten. In the Zambezi Valley, on the other hand, the proportion of grass to woody leaf reaches, on average, a peak of 75% grass in January. This level declines to a minimum of 9% in June/July at the start of the dry season. Thus, the proportion of grass in the diet reflects the availability of grass in the habitat. In the dry season impala are forced to rely heavily on browse. The stomach contents usually contain leaves, twigs and sometime pods of Colophospermum mopane. New shoots and pods of this tree provide valuable food in November when food resources are low. The new leaves are higher in unpalatable tannins than are the shoots, but it is not known whether impala select shoots preferentially or not. Other browse includes leaves and pods of Faidherbia albida, leaves and flowers of Combretum species, leaves and fruits of Xanthocercis zambesiaca (nyala tree), Boscia species, Fleuggea virosa (snowberry) and young leaves of Lonchocarpus capassa (raintree) and flowers of Kigelia africana (sausage tree).  Trichilia emetica (Natal mahogany) is not browsed on by elephant unless it is close to dying of starvation, but it is eaten by impala, kudu and eland around the end of August as other food becomes scarce. These antelope trim the lower branches back to about 1 to 2 metres, forming a noticeable browse line. The impala must contribute to this pruning early on before the leaves get out of reach.

Feeding Behaviour

A Feeding Strategy for the Whole Group

Linked to impala’s gregarious habit are a number of feeding patterns including one thought to involve an assessment of potential feeding sites. It is fairly obvious that if impala fed as individuals (rather than in competition with a large group) they would obtain more food. However, they feed in groups. Recently it has been suggested that they take group size into account when choosing feeding sites.

Impala, as feeders, appear to be frequency-independent predators; they seem to look out for high profit ‘prey types’ (often shrubs and trees). This suggests they (i.e., experienced members of the group or one leader) may be taking into account the group size when selecting shrubs in a particular area. It is thought that some individuals scan the shrubs that appear in the field at bite-height, the field appearing to them as a patchwork of different bite densities. After the assessment of the feeding site in relation to numbers in the group, the herd is led into the chosen site by the leader(s), a site that will give the highest yield of food for the given size of group.

Apparently, there are trade-offs between the bonuses accruing from the reduction in predation risks and the costs accruing from the increase of feeding competition. The strategies adopted by the group seem to be sub-optimal in foraging (the cost of feeding together as a group) but optimal for the survival and reproductive success of the individual impala.

How impala fit into the ecosystem

Impala require a habitat of woodland, mopane and miombo and can live marginally on open grassland. Selected sleeping areas are rather bare surrounded by trees and shrubs. Other environmental requirements are a food supply throughout the year and a reliable water supply. Impala make adjustments as the food resource dwindles in the hot dry months by

1. Spending more time feeding,
2. Moving to floodplains where some new grass is found,
3. Increasing their range. (In one study it was found that impala increased their range from 40 kms in April to 150 kms in November)

Impala’s role in preventing bush encroachment

By exploiting their resources, impala (together with other herbivores) have an effect on the vegetation and water supplies. Impala are often found in big herds and tend to downgrade the habitat, especially the shrub and ground cover strata, pushing back the succession a step. It may be that this is a strategy to achieve a preferred habitat as there follows an invasion of herbs and shrubs after degradation and leguminous forbs form an important part of the diet of impala. In other words, large impala populations can have a significant effect on the vegetation if their feeding patterns suppress bush establishment.

Evidence for this comes from a northern Tanzanian study in Lake Manyara National Park where bush establishment occurred in episodes that coincided with drastic reduction of impala populations in anthrax epidemics (in 1997 in the south and 1984 in the north of the park). The East African study points to the establishment of Acacia seedlings as being a rare event, usually suppressed by high grazing pressures especially of impala.

Reduction of impala and other ungulates by epidemics create narrow windows for seedling establishment, explaining even-aged stands of Acacia. This study challenges other earlier claims by researchers that elephant are implicated in the establishment of Acacia trees. Elephant gradually kill off old Acacia trees and then move off to other areas and without shade and browsing pressures the Acacia seedlings establish themselves again.

The rinderpest pandemic in the Zambezi Valley of the 1980s resulted in the establishment of Acacia trees which today can be seen as three even-aged stands in the park of Vachellia tortilis. At Rifa and Mana Pools the even-aged stands of Faidherbia albida (formerly Acacia albida) may well have become established when impala numbers were very low after rinderpest wiped out up to 90% of the ungulate population in the 1890s.  

However, at Rifa and Mana it is more probable that the even-aged mature Faidherbia albida stands became established in isolation on sandy islands sheltered from browsing pressures of impala and elephant. By the time they were big enough to withstand these pressures, the islands had become incorporated into the mainland. The 15 year old trees on Bwarambwa Island opposite Rifa Camp have become established in this way and may eventually become pan of the mainland as the channel silts up.

 Faidherbia albida trees, Cynodon dactylon grass impala, macrotermes termites and leopard and elephants are among the commonest species in their groups present on the floodplains along the Zambezi and channel in the camp to Tiger Point alluvial plain and play important roles in the ecology of the area.

Faidherbia albida have high protein levels in branches, pods and seeds (highest level). Elephant eat all parts of the tree but do not chew the seeds which pass undamaged through the gut and this process may facilitate germination of seedlings. At Rifa, the impala crunches up the seed for food and eats Faidherbia albida seedlings and it is thought that only when impala populations are very low can albida seedlings become established. Such a window may have occurred 100 years ago.

Ecological separation and competition

Several medium to large mammalian herbivores share the habitats in the Rifa area, e.g. warthog, impala, bushbuck, waterbuck, buffalo and hippopotamus. Despite living together they do achieve ecological separation to a large degree by, for instance, occupying different habitats, selecting different food types, making use of different feeding levels, and seasonal use of habitats. It is only in the hot dry months that several of these herbivores compete directly for the channel grass. The waterbuck, impala and hippo all concentrate on the aerial parts of the grass while the warthog dig up the rhizomes. It appears that impala and hippo out-compete the waterbuck for this scarce resource and the latter retreat to swampy or island refuges along the Zambezi where sedges, reeds and some grasses sustain them. Hippo also share these areas.

Another means of ecological separation is achieved through different feeding techniques. Ruminants browse by plucking off leaves and twigs, twisting the food round the tongue and cutting it off against the lower incisors. Grass is dealt with in the same way; however, this method does not permit close cropping of the grass. Lips are used to nip off short soft grass and forbs. Graminivorous (grass-eating) non-ruminants (e.g. zebra) can crop grass short with both sets of incisors while hippo use heavy muscular lips to mow the grass short.

Impala are bite selectors because with their narrow jaws and mouth, they are able to nibble off small soft twigs and leaves and the most nutritious parts of grass and are able to pick up fruits. They also move on all the time while they are feeding. They can afford the time to feed in this way as they select high quality food. By contrast, zebra and wildebeest are site selectors. Having chosen a good grassy area, they stand in one place stay feeding on what is in front of them until much of the grass is eaten.

The dry savannas, of which the mopane woodlands of the mid-Zambezi is an example, have a rich plant species diversity including woody components used as browse. Many of the trees, shrubs and forbs may defend themselves against browsers by production of thorns, spines and unpalatable toxic compounds, e.g., tannins. In the moister savannas there is a decrease in diversity of plants and herd mammals.

Dominating all these mammal communities are the ‘bulk-roughage’ feeders – the large herbivores that eat high fibre foods, e.g., grass and browse. These include elephant, buffalo, hippo, wildebeest, zebra and, of course, domestic cattle. Nowadays cattle dominate the African savanna and they, together with their owners, Homo sapiens, have changed the face of vast areas of the savannas.

“Tannins and Digestibility for Kudu”

Although woody plants produce secondary plant metabolites such as tannins which are useful as defenses against herbivores, their major role in evolution has been to protect plants against fungal attack. 40% of the dry weight of heartwood and bark consists of these compounds and without them woody plants would be killed off by saprophytic fungi. Leaves of trees and shrubs have much lower amounts of tannins (0,5 – 10% dry weight).

Tannins are valuable as defenses against herbivores and the role they play in kudu nutrition has been investigated. Kudu is a browser that eats little grass. The tannins in the browsed plants combine with dietary protein to form and indigestible complex. Kudu can die from malnutrition when:

(a) their selection of browse is limited due to fencing

(b) they consume plants with a high tannin content over a long period, e.g., during times of drought.

Impala browse to some extent and can be affected by tannins. There are digestive adaptations that assist browsers in coping with tannin, e.g., the salivary component gives better ability to ingest tannins. The parotid salivary gland is three times bigger in browsers than grazers. There is much controversy surrounding the role of tannins in browsers’ nutrition.

Ref; Rosenthal G. and Jansen D. (ed.) Palatability of Wood Plants to Browsing Herbivores: Their interaction with secondary metabolites. Academic Press. 1979

The indigenous herbivores in savanna habitats have very much higher biomasses per km² compared to domesticated herbivores. The mopane and riverine habitats at Rifa, like other dry savannas, support a bigger biomass of large animals than anywhere else on earth. A distribution table of the large wild herbivore species is included in the appendix.

Water – A habitat requirement

It seems that water is required for impala throughout the year in the Valley. In some parts of Africa impala appear to exist for a period in the dry season without free water. In the dry season and droughts impala conserve water by remaining quietly in shade during the heat of the day, feeding early in the morning on channel grasses when dew may be on the leaves and urinating

less frequently. In a Tanzanian study, impala were seen to lick the dew off the leaves in an area where grass was plentiful but surface water on the whole non-existent.

Diminishing water supplies of the floodplains of the mid-Zambezi Valley

Before the damming of the Zambezi River at Kariba, seasonal flooding filled up the channel systems along the river which even at the end of the dry season still had strings of pools providing drinking for game. In the years of big floods, huge marshes developed within the vast floodplains whose effects were felt for several years. Around Rifa Camp and encompassing the tributary, the Chipandaure River, marshes several kilometres in extent developed with huge reed, sedge and grass beds. Water supplies were plentiful.

Once Kariba Dam was built and filled in 1959 a seasonal flooding, when the gates were opened, provided the channel systems with water but the flood gates were shut early in 1981, and the channel systems slowly dried out with less and less water flooding back into the inlets. These inlets had provided relatively safe drinking spots for game. The steep banks of the Zambezi are hazardous for antelopes – only elephant and lion drink regularly at the river.

In the last two decades a consequence of the drying out of the floodplains is less grass and inlet water being available to game and populations of buffalo, impala and warthog have decreased. Buffalo have migrated to the attractive grazing and water resources of the escarpment hills where better rains have improved this habitat. Even with occasional above-normal hydro-electric power production leading to extra water flowing into the Zambezi, little water flows into inlets.

This is because the Zambezi has downgraded its bed, leaving the inlets high and dry. To counter this, it was hoped to release enough water from the lake at times when it is most needed by game and suitable for birds breeding in the river. This would revitalise the Valley floodplains, enabling them to attract and support large herds of game, including impala. However, on average, the levels of water in Kariba are declining and the water released for hydro-electrical generation is increasingly spasmodic.

Social Structure & Breeding

The Gregarious Habit

When animals live together in groups or colonies, they are said to be gregarious. It is thought that antelope evolved in forests and thickets, leading solitary lives or associating in small family units, the way bushbuck and duiker do today. It is when some antelope species emerged into more open habitats in response to improved food resources that they became gregarious to counter their greater exposure to predators. The gregarious state makes possible protective behaviour strategies to be developed, e.g., communal warning systems and laager behaviour, demonstrated by a buffalo herd when lions attack. Bulls launch attacks from the outer circle defending cows and calves inside the circle.

Herd structure

A herd of impala can number well over a hundred and this group has a social organisation which is connected to breeding, protection and caring for the young and dispersion. This group or herd is subdivided into

(1) breeding herds made up of adult females, calves, yearlings and often some rams except in the breeding season,

(2) bachelor herds made up of old and young males, and

(3) (in the rut) the breeding males become territorial, leaving the bachelors for a couple of months.

Below is a map showing home ranges of the female herds, the male territories in the rutting season and the ranges of the bachelor herds often located in areas where food quality is poorer than in the ranges and territories.

In the mid-Zambezi Valley breeding groups number from 30 to 120 impala and each occupies a discrete home range. Though groups may assemble when green grass is plentiful, they very seldom associate with each other. Spacing within the group occurs when males establish their territories towards the start of the rut.

Breeding – The rut and territorial male behaviour

The breeding rams are in their prime between 5,5 and 7,5 years of age, showing they may be successful in matings over three or more ruts. Most top breeders, however, hold their status for only one to two ruts.

The males reach peak condition as the rut begins. With good quality grass to feed on during the rains, they put on weight and lay down fat reserves. Males practise head pushing and horn clashing throughout the year but as mid-March approaches this activity increases. The breeding cycle is initiated by shortening day lengths in autumn, triggering the release of higher hormone levels. Towards the middle of March some of the males show behaviour such as horn clashing contests, roaring and snorting, which initiates the rut. This behaviour stimulates the females to come into breeding condition.

Breeding males begin to establish their territories.  They mark territories by forehead and facial rubbing on grasses and twigs. Dung heaps called middens are produced by breeding males. No females contribute to them. On approaching the midden the ram smells the mass before stretching to urinate and then squatting to defecate. This dung collection is produced by several males, maybe any males passing by contribute to it. It seems to act like a newspaper passing on information to the males.

A harem of adult females and juveniles are herded by the ram into his territory.

The males begin to strenuously defend their areas by chasing, snorting, strutting, head bobbing and fighting, followed by roaring. These dominance fights are energy consuming and sometimes lead to serious injuries. Much energy is consumed by the rams establishing a territory, holding it against competitors and in keeping the harem together. Males begin to lose weight and condition and some become exhausted before the end of the rut and retire, giving way to a rival. The males that breed have very low fat deposits at the end of the rut.

The peak of the rut occurs between April and May. Its centre, as measured by roaring frequency, was found to vary across a 20 day interval over five years at Sengwe. This timing was probably influenced by the lunar cycle. Most matings fell approximately between full moons in the April/May period. (Matings seldom occur at full moon.)

The success of the breeding strategy

This synchronised system is successful when judged by the high breeding rate of the species. Females become pregnant from about 18 months of age and may produce a lamb every year for the rest of their lives. In Sengwe it was found that they produce seven to nine offspring in a lifetime. During a harvesting operation in Omay it was found that 85% of 400 females were pregnant.

Foetus development and parturition

Despite a widespread belief that impala can delay the birth of a lamb if conditions are adverse, there is no supporting evidence for this. This question of delayed birth is being investigated by the Veterinary Department of the University of Zimbabwe.

Growth and survival of the young

New-born lambs lie up in the thickets for a few days and then join nursery units within breeding herds.  Weaning takes place 4 to 5 months later and the lambs are still able to grow rapidly for another two months before food quality drops to its lowest level.

In a Sengwa study it was found that mothers keep their daughters with them longer than their sons who at 7 to 12 months of age join the bachelor herds and often end up in unsuitable habitats for further growth. The daughters will learn from their mothers where food and water holes are to be found in the dry season and droughts, giving them a better survival rate than their brothers. Males are expendable in the population. In one study the percentage of one year old males dropped from 48,7% to 35,4% altered in a month from though the sex ratio was probably 1:1 at birth.

Population Dynamics

Deterioration of the range is the main factor in controlling impala numbers.

Predators and predator attacks account for some animal deaths. By far the biggest factor controlling population numbers, however, is the deterioration of the impala’s range mainly through drought but to some extent through over-utilisation. Poor nutrition leads to a drop in weight and condition thus lowering the animal’s resistance to disease. With range deterioration breeding rates are also affected and there is a reduction in the number of male foetuses conceived which on average exceeds the number of female foetuses.

In drought years a birth rate as low as 14 per 100 breeding females has been recorded. More resorptions (a process in which a substance, such as tissue, is lost by being destroyed and then absorbed by the body) or abortions occur and lower birth weights of calves result in lower survival rates as does reduced milk supplies from the mothers. Surviving young would be smaller and slower to mature reducing the number of sexually mature yearlings.

Predation

Not only do predators help in reducing impala numbers but they are also an important factor in maintaining a healthy population – the lame, young, old and sick members are quickly removed from the herd. Impala represent fast food for an array of carnivores and other predators including lion, leopard, cheetah, hyaena, painted/hunting dogs and crocodile.  Lambs will be taken by them and by jackals, civets, eagles and pythons.

Lions are not fast enough to catch many adult impala except the inattentive, nor are hyaena who attack the weak and young. Leopard depend heavily on impala in their diet using stalking and ambush techniques to catch them. The leopard dispatches them through a throat bite but if the prey emits a cry, the leopard may lose its kill to a lion or hyena. The leopard cannot afford to leave the carcass on the ground for long and risk having it stolen by stronger competitors. After plucking the fur off the abdomen, the carcass is opened up, stomach and intestines removed and liver and, perhaps, haunch eaten by the leopard.

 If undisturbed, the leopard may then cache the impala up a tree.  Drag marks with leopard spoor and claw scratches up trees are often seen around Rifa.

A little over a century ago hunting dogs were in large enough numbers to be a significant factor in controlling impala numbers (though still not as important as poor nutrition). Following a determined all-out slaughter of the dogs through implementation of government and agricultural policies, painted dogs were but exterminated by hunters and farmers. Protection afforded to a few remaining packs in the Hurungwe Safari Area has allowed dog numbers to improve and a pack has made an impact on the impala and bushbuck numbers around Rifa. From 1994-1997 dogs in groups of between 4 and 22 hunted on the floodplain in front of camp.

When dogs were denning down around July 1996, a hunter protecting Springvale scholars noted that three impala were killed during one day, the first early morning, another during the day and the third late afternoon. The group, a lactating female and some adult males, always returned to the same area of the channel upstream of camp, possibly to the den site. Hunting by the dogs is sporadic at Rifa except when young pups are being fed at a den. Impala herds may be fragmented by this hunting which in turn could halt excessive habitat degradation.

Anti-predation characteristics and strategies

Impala have excellent eyesight and having two focal points on the eye for close-distance and long-distance vision also assists this sense. When a predator is spotted impala stretch their necks and stare fixedly at it for a while before running off. Their hearing is very acute. They rely on operating as a large vigilant group and on communal warning systems. They use speed and ‘bomb-shelling’ (individuals jumping in all directions, confusing the predator) to escape predation. (see reference to “bomb-shelling” above in “Coat and Colouring”).

Though the impala is judged to be a poor swimmer it will jump into water to escape predators. (Two chased into the Zambezi by hunting dogs in front of Rifa were eased back to land by a fisherman in a boat. They emerged exhausted.)

Conculsion

The above highlights some of the reasons that contribute to impala being the longest surviving antelope in Africa. Its physical attributes such as long legs and the camouflage colouring of the coat, good sight and hearing enable it to escape predators very successfully. lmpala’s efficient way of ridding itself of ticks by combing them out with wobbly teeth free it from heavy tick loads.

As a ruminant, it benefits from its symbiont-aided digestion of cellulose and other insoluble polysaccharides. (The disadvantage here is that it needs time and space for this type of digestion to be efficient.) One of the impala’s strengths is its ability to switch from grazing to browsing very readily and to use a wide variety of plants for food. Impala have evolved strategies for surviving hot dry months and droughts though the latter take their toll.

These animals have the ability to remain healthy despite brittle climatic conditions and infections from disease-causing organisms. Its gregarious habit has led to communal warning systems and fairly involved breeding strategies which are very successful – mature females produce a lamb almost every year. From an ecological point of view impala is regarded as having a high biotic potential with a big impact on other species.

Their numbers increase rapidly under favourable conditions but when a shortage of food occurs, degradation of the land takes place. Impala is an antelope with localised distribution and when they occur in large numbers, a drought, for instance, would cause a crash in numbers. This would be followed by recruitment or regeneration of plants lost from the habitat by over-grazing. The impala numbers would then increase and so the cycle would be repeated.

Acknowledgements

Many people have assisted me and taken an interest in the writing of this book and I am grateful to all of them. In particular I would like to thank both Veterinarian Research Departments, that of the University of Zimbabwe and of the Department of Lands and Agriculture. Professor G. Vassilev’s help from the latter institute was much appreciated, as was Dr. G. Smith’s provision of notes on digestion and Harriet Davies’ work on Traffic S. & E. Africa’s data on meat production. I also appreciated Mrs. F. Willmot’s kindness in reading and commenting on the text and Mrs. P. Evans (of Zimbabwe Hunters’ Association) expertise and patience in typing the text and helping on the production side. Thanks are due to Roger Bone for a sketch, Dick Pitman for a photograph and Margie Grobbelaar for the fine sketch of a leopard trying to catch an impala. I thank Dr Geoff Stiles for putting in the graphics so well despite working with such a mixture of good and indifferent material. Finally, I would like to record that I learned to appreciate the admirable qualities of the impala on the many long walks that Hugh, my husband, took me on at Rifa.

L. MAASDORP

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