Why so many of the bush's herd animals give birth at the same time

Every year, with the first summer rains barely soaked into the ground, the Klaserie produces a crop of babies so sudden and so plentiful that it looks for all the world as though someone has sent round a memo. Impala lambs appear almost overnight, wildebeest calves wobble to their feet beside their mothers, zebra foals start trotting about in miniature striped pyjamas, and within a few short weeks the reserve resembles an enormous open-air maternity ward. It's one of the most heart-melting spectacles of the safari year, and it also happens to be one of the cleverest survival strategies in the natural world.

Impala lead the charge, as they do in most things involving sheer numbers in the Greater Kruger. Their story starts months earlier in the cooler days of late autumn, when the rams are fizzing with testosterone, staking out territories and roaring at anything that'll listen during the noisy, chaotic few weeks of the rut, which usually peaks around May. Mating across the herds happens within a tight window, and because an impala's pregnancy lasts a fairly fixed 194 to 200 days, roughly six and a half months, the ewes all come due at more or less the same moment, which in this part of the world falls about now, just as the bush turns green.

Bush lore has long held that impala ewes can hold off giving birth until the first rains arrive, and it's a lovely story that guides have told around campfires for generations. The truth is a touch more prosaic and every bit as impressive, because the timing is set at conception and the ewes rely on thousands of generations of finely tuned instinct to deliver their lambs when the odds of survival are at their highest. 

Research shows that somewhere between 80 and 90 per cent of pregnant ewes give birth within a window of just three to five weeks. A study in Botswana's Chobe National Park found 90 per cent of lambs arriving within a fortnight in mid-November, and the researchers discovered that the more sharply defined the rainy season, the tighter the birth window becomes.

Safety in sheer numbers

The first reason for all this coordination comes down to wonderfully simple arithmetic. A predator can only eat so much, and when hundreds of vulnerable newborns arrive within days of each other, the lions, leopards, hyenas, painted wolves (African wild dogs), jackals, pythons and even the odd opportunistic baboon are faced with far more youngsters than they could ever hope to catch. 

Scientists call it predator swamping, and while it sounds rather brutal, the result is that a far greater proportion of the year's young survive their most dangerous early days than if births were spread thinly across the calendar, where every newborn would stand out as a lonely and conspicuous target.

The predators certainly aren't complaining, and the summer glut gives young cats and inexperienced hunters a rare chance to sharpen their skills, which is part of what makes game viewing at this time of year so compelling. Life and death play out side by side in the green season with a frankness that can be hard to watch and impossible to look away from.

Perfectly timed for the summer feast

The second reason is food. Producing milk is enormously demanding, and an impala ewe nursing her lamb needs the very best grazing the bush can provide. By timing births to coincide with the flush of fresh grass that follows the first rains, mothers are able to keep up with the demands of a hungry youngster, and the lambs themselves move on to tender new shoots packed with nutrients as they grow. 

A healthy, well-fed lamb grows faster, runs sooner and stands a far better chance of making it through its first year, which matters a great deal when only around half of impala lambs reach their first birthday.

Hide and seek or follow the leader

Different species handle those vulnerable first days in their own ways. Impala are what biologists call hiders, and when her time comes the ewe slips away from the herd to give birth in a quiet patch of cover, eats the afterbirth to remove any scent that might draw a predator, and leaves her lamb lying low and perfectly still for a day or two while she feeds nearby, returning regularly to nurse it. 

A newborn impala weighs between 4kg and 6kg and can stand within 15 to 30 minutes, and once it's strong enough the pair rejoin the herd, where the lambs gather in nursery groups and spend a great deal of their time leaping about for no discernible reason other than the sheer joy of having legs.

Blue wildebeest take the opposite approach. Their calves are followers, born out in the open and expected to be up and about astonishingly quickly, often standing within five or six minutes of birth and keeping pace with the herd within a day. 

In the Greater Kruger, wildebeest mate between April and June and calve in November and December after a pregnancy of around eight and a half months, which brings their youngsters into the world at almost exactly the same time as the impala lambs. Zebra are a little more relaxed about the calendar and foal throughout the year, though the great majority of foals in the Kruger region arrive between October and March, and they too are on their feet within about 10 minutes.

Ditto waterbuck, who also breed throughout the year with a peak in births during the summer months.

A bush full of beginnings

For anyone on safari in the Klaserie through November and December, the result is a landscape bursting with new life, and it changes the character of every game drive. Herds that spent the winter grazing quietly are suddenly full of tiny, spindly newcomers practising their bounds, mothers are on high alert, predators are on the prowl and the guides have their work cut out keeping track of it all. 

It's a season of wonder, of drama and of more photographs of impossibly endearing baby animals than any phone has the storage for, all thanks to an ancient piece of natural timing that the herds of the Greater Kruger have spent millennia perfecting.