Probably the best way to really get a grip on births in an animal population is to study it for several generations, seeing what changes and when, and what factors are particularly important for infant survival. For the majority of small mammal species, a generation isn't a very long time, so it's practical to do that - although the sheer number of such species inevitably means that the great majority have never been studied. For larger, longer lived, animals, its a different matter, and the task can be far more difficult.
A new study of this kind has, however, just been published on leopards (Panthera pardus). The authors comment that such long-term studies are "particularly rare" for large, carnivorous mammals, although, despite what I just said above, I can't help wondering "compared with what?" Not anteaters, I should think, nor marine mammals, nor, for that matter, anything much that's neither a mammal nor a bird. (If this blog were about reptiles, it would be a lot harder to find material to fill it than it is).
Be that as it may, such studies certainly aren't common. Indeed, while there have, of course, been short-term studies on the same subject, this does seem to be the first of its kind on leopards - as opposed to lions, tigers, pumas, or cheetahs. The study took place in the Sabi Sand game reserve, a set of privately owned properties lying next to the Kruger National Park in South Africa. The researchers collected records compiled by the gamekeepers, going back to 1979, which enabled them to reconstruct the life histories of a several female leopards living in the area.
Leopards, like many mammals, exhibit a behaviour pattern called "female philopatry". This means that daughters tend to hang around close to their mothers, even after they've grown up. Each animal will have its own 'home range', through which it wanders in search for food, but mothers often give up part of their home range to their daughters, while other daughters take over territory from deceased relatives. Males, on the other hand, wander away from their parents, travelling long distances in search of new land - thus ensuring they don't end up mating with their own sisters and nieces.
So long as the population density is stable, this should work perfectly well in the medium to long term, and means that it's possible to follow individual females from birth to old age. Even so, the researchers had to examine 3,500 photos taken over the years, to confirm that they were looking at exactly the same animals each time.
One of the first findings was the confirmation that most leopard births occur during the wet season. This isn't really surprising, since mothers need to drink a lot in order to provide milk to their cubs, and also want there to be as much food around as possible. That's especially important, since they can't travel too far looking for it, when they have cubs to look after. Since the wet season is the time when plant growth is at its best, it's also the time when herbivores choose to breed, which means more young ones for mother leopards to hunt.
Still, not all the litters were born at this time of year, and its interesting to note that those that weren't didn't do any worse than those that were. That may partly be because there's less competition from other hungry mothers, but it could also be because the game reserve has a number of artificial water sources. They're there for the benefit of impalas, keeping their numbers steady throughout the year. But anything that's good for impalas is also good for animals that eat them, so mother leopards giving birth in the dry season aren't at the disadvantage that their reproductive biology tells them they should be.
The most precocious female leopards in the study got pregnant at three years of age, but most waited until the age of four. This is interesting, because they showed all the signs of coming into heat aged just two. If they're sexually mature at that age, why do they wait for so much longer before having cubs?
There are a number of possible reasons for this. For one, it takes time for a young leopard to establish her home range, finding a suitable patch of land of her own in which to live. Without one, the odds of being able to raise a litter of cubs is probably small to zero, so it really does make sense to wait at least that long. Another reason may be to avoid mating with your own father. As-yet-unpublished data apparently shows that, in the Sabi Sand, males hang around for about four years on average, before leaving for pastures new. Waiting for four years before breeding does not, therefore, mean that your dad has definitely left the area, but it does make it a lot more likely.
Some evidence for this comes from the fact that an earlier, shorter, study in the Phinda game reserve showed that the leopards there tended to first give birth at around three years. The population density there is quite a bit lower than at the Sabi Sands, making it easier to establish a home range, and possibly also meaning that males head off sooner in search of new females.
Pregnancy in leopards lasts about three months, and usually results in the birth of two cubs, although singletons are common, and as many as 10% of births result in triplets. Given that leopards mate several times over the course of their lives, and that their population is fairly stable, it's obvious that not all of these cubs survive to adulthood.
In fact, the number that do seems to be higher than we might think - almost half (47%) seem to manage it. Of course, we do have to bear in mind that it's only possible to count the cubs we have actually seen; if some die before they're even seen by game wardens, then they're obviously not going to be counted in the figures. Allowing for this, the authors estimate that only about 37% of cubs are really reaching adulthood. Still, we can determine how long the cubs we do know about live, and, perhaps more importantly, what happens to those that don't.
As we might expect, the most dangerous time for cubs is when they're youngest. Like any cat, very young leopards are helpless. They're initially blind, and, even once they can see, they stay in the den, and aren't mobile enough to, for example, climb trees to safety, until they're about two months old. Indeed, it may be that not much more than half of cubs survive their first three months of life, although, once they do, their odds of survival start getting quite good.
By far the most common cause of the death of young leopard cubs turns out to be... other leopards. Specifically, about 40% of all deaths are due to male leopards catching and murdering the cubs. That is, from a human perspective, a shocking figure, and exactly why the males do it isn't entirely clear. The best bet is that, because it takes so long for a female to rear a set of cubs, if a male can kill them all, she'll come into heat again much sooner, giving him a better chance to mate with her.
That, of course, only makes sense if the cubs aren't his. And, indeed, the most prolific cub-killers are new arrivals, confident that they can't possibly have fathered any cubs they come across, and desperate to mate as soon (and as often) as possible. When they do attack, such males tend to slaughter the entire litter, and the females seem all but powerless to stop them. Yet they do have one tactic up their metaphorical sleeves that may help to mitigate the danger: have sex with as many males as possible.
That only works if you're already between litters, but it does mean that any males that do hang around can't be sure that any given cub isn't theirs. A trick that lions use is to become less fertile when new males are in the area; it may give them more time to evaluate the newcomers, and, perhaps more importantly, allows for things to settle down before they have any more cubs. It's possible leopards do the same.
The second most common cause of cub death, incidentally, turns out to be lion attacks, with hyenas in third place. Here, it's most likely that they're just hungry, and that a defenceless leopard cub is as good a target as a tasty impala. Other causes are relatively rare; less than 10% are killed by other animals (mainly snakes), and a few die from accidental causes, loss of their mother, or, in just one recorded case, illness. Only one cub was deliberately abandoned by its mother, and that was adopted by its grandmother, and subsequently survived to adulthood without a problem.
Normally, we measure how well a mother mammal does by it's "lifetime reproductive success": how many of its offspring survive to have children of their own. This study only looked at how many survived to adulthood (the males, remember, left the area), which is presumably a slightly higher number. However, it does show that the most important factor for leopards in the Sabi Sands is how often their cubs are attacked, rather than, say, how much food there is. Although we should, of course, note that that may not be true in more marginal habitats.
You can't do much about the attacks from lions and hyenas, and it is worth noting that the population of leopards over the thirty years of the study did remain stable; all the deaths were balanced with new births, and that's just part of life out in the wilds. But it does have a lesson for conservation, and that's that you don't want to kill too many young male leopards. That will lead to a more rapid turnover of new males entering the territory of females abandoned by a male who would otherwise have hung around for a bit longer. And new males coming into an area is, as we've seen, a bad thing.
And that's the sort of thing that, on a game reserve, its potentially possible to control. In these sorts of places, leopards are going to be hunted for sport and trophies; that's what the reserve is there for. But if you can restrict hunters to the older males, who would have left or died soon anyway, there may be more leopards around for the future.
[Picture by "FrontierEnivro", from Wikimedia Commons]
Great stuff. I was Leopard-watching in Sabi Sands last year. One of the females we saw was believed to have Cubs, but we didn't see them. (We did see two leopards in the process of making more leopards though...)
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