If you visit the savannahs of southern Africa, there is one animal you can be very confident in seeing and seeing a lot of. An antelope that within its range is the most numerous and successful. None other than the Impala. Although common in most wild areas, being an impala is not easy, they have almost everything out to get them. From the moment they enter this world there is a constant expectation that they will end up as food for someone else, and very often sooner rather than later, and tourists seldom show much interest in impala unless they are being eaten by something else. Although often overlooked these beautiful animals are arguably the perfect antelope. Their success can be attributed to a large collection of superb physiological and behavioural adaptations. From a mixed diet (they are browsers and grazers) to super senses, they have a wide variety of survival strategies.
Have you ever stopped to admire the humble impala?
Before we get into the nitty-gritty it may be beneficial to share a brief overview of the species. Impala are found throughout the savannahs of eastern and southern Africa with their range extending from southern Kenya through to northern South Africa (a small population of “Black-faced” impala exist in the western savannah of Angola and northern Namibia). Impala are gregarious and sedentary. Females live in discrete herds with their young within a traditional home range (minimal average group size is 6-20, ranging as high as +100). Males typically leave their natal herd by age four and live in bachelor herds and later become seasonally territorial, while outside of the breeding period males and females may be seen in mixed herds. They are regarded as edge (eco-tone) species, preferring light woodland with little undergrowth and grassland of low to medium height, and are considered predominantly grazers while grasses are green, and a browser of foliage, shoots, seed pods and forbs at other times. This mixed feeding gives impala an unusually varied, reliable and predictable food supply, enabling them to lead a sedentary existence and reach recorded population densities of up to 240 per square kilometre! This success, however, comes at a cost. Because they are often the most numerous prey species (as well as the perfect size for the multitude of large African carnivores) they fall prey to predators more often than most. With the likes of leopard, cheetah, wild dog, lion, African rock python and even martial eagle (to name a few) all preying on the poor impala. In fact, wild dogs, cheetahs and leopards all actually focus on impala where they occur but note that behaviour differs slightly between populations found in differing habitats and climatic conditions.
A young male impala soon finds himself cast away from his natal herd.
From here on out we will be only referring to populations found in the southern part of the range where seasonality results in a distinct short breeding peak. Although there is so much to explore when talking about impala, I'd like to focus primarily on the species' reproductive strategy and on what is going on in to Okavango population at the moment… The annual rut. For impala, as with most other herbivores in southern Africa, the best time to give birth is just before or during the first rains of the wet season (November-December). This gives mothers increased food availability and thus the ability to produce enough milk for their young. It also means that later these same young have ample grazing as they mature and develop. A survival strategy that promotes birthing on mass at the peak time of the year has obvious benefits both for food provisioning as well as predation avoidance. With so many predators out to get them impala have a lot to worry about. Even more so when they are at their most vulnerable… With all the young being born during a short period of time the strategy acts to “flood the market”. There are literally too many young for all the predators to hunt and thus it is guaranteed that at least some will survive. Despite this, it is believed that up to half of the young are eaten by predators.
Leopards are one of the major predators of the poor impala.
It goes with out saying that if the Impala strategy is to birth on mass then the females need to fall pregnant at around the same time. This time is termed “the rut”. Every year for a few months (March - May) shortening day length as well the lunar cycle stimulate hormonal changes in both male and female impala. Males begin gearing up for the rut as early as March, when shorter days stimulate gonadal growth and hormone production, leading to increased aggressiveness and territorial behaviour. Territorial competition takes many forms and initially does not involve much physical conflict. Posturing, chasing, threatening and vocal displays begin to separate the prime suiters (typically 5.5 - 7.5 years old) from the sub-prime bachelors. Once the prime males have been identified each begins to establish a territory. For a human observer, the stand-out feature of this period is the increased vocalisation involved. Males use a wide variety of loud bellows, grunts, roars and snorts as part of their territorial behaviour. Even a seasoned bush person may mistake these sounds for something more like large predators like lions or leopards fighting.
Something quite extraordinary happens to male impalas during the breeding season. They take to attacking sage bushes and making the most bizarre sounds!
Observations in some areas indicate that the three-week peak rut among southern impala is influenced by the lunar cycle with mating usually occurring at night under a bright or full moon. During this period territorial vigour is at its zenith and males work franticly, investing up to one-quarter of their time rounding up and attending to females that enter their grounds. Interestingly the preferred areas for territory establishment remain pretty constant through a male's prime years and attachment to a particular place is characteristic of males in their prime. Territories are maintained through an aggressive attitude towards other males that ramps up as the time for female readiness approaches. One might wonder why set up a territory in the first place especially considering the age-old idea that “females follow resources and males follow females”. Well in impala, as well as many other species, males play the system by establishing territories that hold the resources and let the females come to them.
During the rutting season, impala rams are so distracted by one another that they forget to keep their wits about them!
While female impalas are on a male's grounds, the male will mill around the group testing for receptiveness in the form of urine testing. Having located a female in oestrus, an excited male immediately begins an energetic courtship. This involves a variety of flirtatious behaviours where the seemingly reluctant female is relentlessly pursued by the male. There is evidence that, despite the prime-male's best efforts, a large proportion of young may actually be fathered by sub-prime males who opportunistically rush in while the territory holder is otherwise engaged and attempts to mate with whichever females he may find that are receptive before he is the subject of the territorial males attention.
In response or to prevent this, territorial defence may involve fierce physical fighting that can result in injury and broken horns. Death is not unheard of but is uncommon. The actively territorial males probably lose 25 per cent of their body mass during rutting as they do not find much time to feed and groom. With such huge costs, territory tenure can be short-lived as the males condition deteriorates with the average tenor in some areas being as short as 8 days! That being said, the reproductive rewards for these prime males can be extraordinary. A prime territorial male may father young with up to thirty females. One study in the Sengwe Research Area in Zimbabwe showed that only 12 of 69 males that were present in the range of 67 adult females during one rut were seen to do any mating. The same four prime males accounted for 78% and 66% of observed matings during two successive ruts.
Once all of the females that were receptive have become pregnant and the rutting period ends, impala life returns to some form of normality. Territorial males still tend to stay within a given area but refrain from serious territorial behaviour and are often even seen associating with female and bachelor herds as they roam within his home range. All of the territorial vigour subsides, the bush returns to its peaceful nature and, with any luck, six and a half months later, with the beginning of the rains, a flood of new young impala are born.
Blog written by Greg Hartman.