Sunday, 21 October 2018
The Best Time for Breeding
It may also not matter too much if the animal lives somewhere where the seasons don't change too much, although, even in the tropics, where there's no winter, there's usually a rainy and a dry season. A great many mammals, however, do have a specific breeding season, timed to be one gestation-length away from the best time to give birth. If pregnancy lasts six months, and the best time to give birth is in the spring, when fresh plants are sprouting, you'd better do your mating in the autumn. If pregnancy lasts twelve months, however, you want to mate in the spring. And so on.
The longest gestation periods for mammals are those of elephants. This might surprise you, because whales are often quite a bit larger than elephants. But, no, for whatever reason, even a blue whale is only pregnant for around 12 months, and this, or a little over it, is fairly typical for the larger whales. Even sperm whales, which take the longest (so far as we know - it's hard to tell with some of the more obscure whale species), give birth after no more than 18 months. Elephants take nearly two years.
Which means that, if they really do want to give birth at a particular time of year, they'd have to mate in the same season, only two years prior. On the other hand, they live in the tropics, and not in the temperate regions of the world, where seasons are more marked, so do they bother? And is there a difference between African and Asian elephants - which live in regions with a notably different climate?
I should, at this point, note that African elephants (Loxodonta spp.) are commonly regarded these days as constituting two separate species. Although the evidence for this seems pretty strong, it's not absolutely accepted yet, and a great many studies on elephant breeding would have been conducted before it was even hinted at. So, for the purposes of this post, I'll lump them together, just as I obviously will for the three subspecies of Asian elephant (Elephas maximus).
One thing we can say is that elephants, of both types, can breed and give birth at any time of year. There is more than enough evidence, both from the wild, and from animals kept in zoos, to demonstrate that this is the case. However, this doesn't necessarily mean that they aren't more likely to give birth at some times of year than others, especially in the wild where they don't have a steady year-round supply of nutritious food to keep them in peak condition.
In the case of Asian elephants, in most parts of the world, there seems to be evidence of a seasonal peak in births. In Sri Lanka, for example, mating has been said to occur not long after the start of the rainy season, although there is no such peak in the Western Ghats of India, where the monsoon is said to be less marked.
For African elephants, the situation is more complex, likely due to the wide variation in climate across their range, which includes everything from jungle to near-desert, with much seasonal savannah in between. In some places, especially where there is more than one rainy season in a given year, there may be multiple peaks in birth rates. In others, where is only one such season per year, conception does seem more likely after heavy rain, and, indeed, less likely in years when the rains "fail" (relatively speaking).
So, what causes this? How, in short, do the elephants know when the best time to mate is? The finding that mating is less likely when the rains fail would suggest that, for African elephants at least, how physically fit the elephants (most notably, perhaps, the female) is must have something to do with it. If they're well fed, then they're more likely to mate when they are in heat, in the hope that the same happens again once it's time to give birth.
Similarly, we know that Asian elephants in Myanmar are at their peak fitness when the monsoons begin, which is also typically when they start mating. But, while general body condition does seem to be the most important factor close to the equator, in most places, there's clearly more to it than that.
One way that we can tell this is that there's at least some evidence of seasonal peaks in elephant births from zoos, where the animals are (hopefully) in good condition year-round. There's also something of an advantage to it, in that just because the weather does one thing in a particular year, it doesn't necessarily mean that it will do the same again two years later, once it's actually critical.
Fortunately, one can tell the time of year by what the day length is doing. That won't work close to the equator, where modern elephants first evolved, but it does become more significant as you move further north or south. And, while they may be associated with tropical climes, both kinds of elephant are found in warm temperate zones outside the tropics - Asian elephants in Nepal and northern India, and African elephants in the far south-east of their continent.
Here, there is evidence, from both kinds of living elephant that they tend to breed at times of the year when the days are getting longer. This, in fact, is typical of a great many mammal species in temperate regions, and, indeed, other vertebrates, too. In the absence of wrist-watches, smartphones, and calendars, animals can tell this because the hormone melatonin is secreted primarily when it's dark, and high levels of melatonin can either increase or decrease the desire to mate, depending on whichever is preferable.
Understanding this can be significant, since elephants kept in zoos can suffer from high infant mortality and fertility problems. Much of this is surely due to stress, since, even in the best maintained zoos in the world, elephants cannot roam as far as they would in the wild, or have as broad a diet. Not to mention having to put up with human visitors. Increasing contact between male and female elephants, and increasing female body condition, as the days lengthen might help to provide a more natural environment for them, and help to conserve both species as comfortably as possible.
The Asian elephant, after all, is an endangered species...
[Photo by Jayanand Govindara, from Wikimedia Commons.]