The impala breeding season, known as the rut, is that time of year when impala rams are flooded with testosterone; that time of the year when all their attention is focused on mating with the ewes; that time of year when all other males are seen as fierce competition.
During the rut rams are so immersed in the behaviours associated with the rush of testosterone that they have little time to eat and keep themselves clean. Their energy is solely focused on developing their fighting ability enough to acquire a herd of females for themselves. Once the ram has succeeded at this (by driving out competing males, usually violently) his energy turns to herding his ewes and keeping them together (it’s surprisingly easy to lose a ewe) as well as keeping competitors away from his prized herd!
What this season brings is a cacophony of sounds made by the rams. When a ram chases a competitor away from his herd or is establishing dominance over a male within a bachelor herd, he will blurt sharp rasping calls (a similar sound to their alarm call) followed by a unique roar/grunting (which sounds as if it should be made by a predator!). Whilst producing these sounds he opens his mouth and projects his tongue, sometimes curling his tail upwards and exposing a flash of white hair.
I know, strange and interesting, right? It gets better.
The peak of the rut (when rams hold dominance for sometimes as little as eight hours before getting displaced by stronger rams) occurs in the last two weeks of April and into the first week of May. This high level of competition and rapid turnover of dominant males keeps up a high level of genetic diversity within the species. Most of the mating occurs over this period and it usually occurs at night. The ewes will be more subdued and less inclined to leave the area and get lost at night. This is because impalas instinctually know not to move very fast or very far in darkness as predators are moving around and hunting during these hours. It is a matter of survival to remain in that area (usually an open plain) after dark and keep watch, making the dominant male’s job much easier!
We have discussed why the mating mainly happens at night, so why has it been happening largely in the last two weeks of April and into the first week of May?
Here is a possible reason:
The first quarter of the waxing moon (waxing: when the moon gets larger in the sky) occurred around the 21st of April this year. The moon was bright on that evening and because it is in its waxing phase it grew larger and brighter until the 28th (the night of the full moon) after which it slowly reduced in size and brightness. This is the perfect time for impala to be mating because predators hunt more effectively when the moon is not illuminating the night. When mating, impalas are vulnerable because they do not utilize all their senses effectively; they are solely concerned with the act of mating. They are safer to mate in lighter conditions because predators do not hunt as effectively when the moon is bright for the simple reason that prey species are able to see the predator more easily. Therefore, impalas mate when the night is lighter because it is safer!
There is another reason why impalas mate over this time; with a gestation of 194-200 days their lambs will be born in early November, a couple of weeks after the first summer rains have descended. This means that the newborn impala lambs will have new shoots of fresh green grass to feed on in the first weeks and months of their lives. Over the course of time evolution has structured their lambing period this way.
If you have ever been to the bush in the South African Lowveld before, you will have witnessed the incredible abundance of this antelope species. Are you starting to understand why? There is a high level of evolutionary intelligence in the development of this species. The fact that they have an instinctual drive to mate during a period when the moon is fuller and brighter as well as structuring the birthing period to drop their lambs shortly after the rains have arrived shows why the impala are doing so well out here. These decisions are not conscious decisions; they are instinctive and are a product of many years of evolutionary success.