Drought and the Mother Rhino

You may be surprised to discover that the white rhinoceros (Ceratotherium simum) is not internationally listed as an endangered species. This is because it is reasonably widespread across southern Africa, poaching of the species has been in decline since 2014, largely due to effective enforcement methods. While it did almost go extinct in the late 19th century, well over 10,000 of the animals are thought to be alive today, with populations in some areas still rising in recent decades. In fact, it meets all the usual criteria for a species of "least concern", one that we wouldn't normally consider even close to being threatened.

This, of course, hides a fair bit of complexity.

Firstly, while it may be the largest rhino species, it isn't the only one. And the others... well, they are the ones that have the real problem, with one species being considered "threatened" and the other three being virtually extinct. Secondly, while the white rhino may meet the standard criteria for being considered of "least concern", it isn't listed as such. This is because it wouldn't take much for the current anti-poaching methods to fail - it's more vulnerable than mere population size and distribution would suggest.

Perhaps more significantly, I'm also talking about the species as a whole. There are two subspecies of white rhino, southern (C. s. simum) and northern (C. s. cottoni) and the northern one is not in a good place. Formerly native to DR Congo and South Sudan, none have been seen in the wild since 2006 and, even in such remote places, something the size of a rhino shouldn't be hard to spot in aerial surveys. While we can't be absolutely certain that there aren't any hiding out there, it looks unlikely, so that, as of 2010 the only ones we know for certain existed were six animals in a zoo in Czechia.

That's not much of a basis for a sustainable population. Moreover, since 2010, four of them have died, and the remaining two are both female. So long as they are alive, the northern white rhino is not extinct, but it might as well be.

So, sure the southern white rhino is not endangered, but we don't have to look far to see that the situation for rhinos in general is precarious and that conservation of the one (sub)species that's doing well isn't something we should ignore. Indeed, looking at the rhinos that are numerous enough to easily study may help us with those in a more perilous situation.

Precise details of where rhino populations can be found, and how large they are, tend to be kept confidential, so as not to give information to potential poachers. But those in some of the national parks are closely monitored, to see how they are doing. We can hopefully use this data to see what the causes of death are in rhinos, and whether there is any particular pattern to those deaths. Understanding such factors may not only help the white rhino but also the black rhino, which lives in similar areas and genuinely is an endangered species - its population having fallen by 85% since the 1970s.

Rhinos are grazing animals, feeding primarily on grass. They live in savannah country, tropical or subtropical climates defined by a distinct dry season during which very little rain falls. This, unsurprisingly, affects the availability of fresh grass. During the wet season (roughly, the local summer) grass growth is lush, and there is plenty to feed on. When the dry season comes, the grass dies back, and plants in general shift their nutrient reserves below ground so that leaves are not only harder to find but not as nutritious. 

Rhinos, like other grazing animals, cope with this by switching from ranging widely across the grasslands to restricting themselves to smaller patches where grass growth remains relatively lush. For the rhinos, this is often woodland where the forest canopy helps the air remain relatively moist, and Guinea grass tends to thrive. When that runs short towards the end of the dry season, they switch again to tussocks of red grass, which is less nutritious, but more resilient. These habits, however, are not a perfect solution, and many rhinos go hungry, surviving off their fat reserves as they wait for the rains to come again. 

Clearly, in the absence of poaching, habitat loss and all the other problems that humans have brought them, this is something that the rhinos can survive without any difficulty in the longer term. But that isn't the world we live in today, and calves may have a harder time of it than their parents. Although white rhinos can be born throughout the year, they are more likely to be born during, or shortly before, the dry season. For the first two months, however, they are feeding exclusively off their mother's milk, and they are largely finished with weaning by four months. 

At this point, however, they may weigh no more than 200 kg (440 lb)... which may sound like a lot, but implies considerably smaller fat reserves than a fully-grown animal weighing maybe ten times that much. So we'd expect mortality to be more strongly affected by annual weather patterns among calves than adults, on top of the fact that the harsh reality of nature means that death rates among infants are always high anyway.

Wardens in Ithala Game Reserve in South Africa have been watching white rhinos for many years, recording where they are seen and identifying calf carcasses - estimating the date and age at death from previous sightings with their mothers. An analysis of twenty years of this information, recorded between 1999 and 2019 has recently been published.

During the study period, 23 female rhinos gave birth to a total of 78 calves. Of these, 19 died of natural causes before reaching adulthood - almost a quarter. (The authors' definition of "natural" is, however, a rather narrow one - a further five died from lightning strikes, which apparently don't count. On the other hand, the reserve doesn't have any lions, so those aren't an issue). Only four of the 19 died outside of the rainy season, and three of those died during the first month of the wet season, so this seems to be the main determining factor. 

Furthermore, so far as they could tell, no calf died within four months of birth - that is, before they had finished weaning. So, assuming no neonatal deaths the researchers couldn't detect, the mothers are clearly doing a good job. Most of the deaths did, however, occur between the ages of four and eight months, when the calf experienced its first dry season. Only three died at an older age, all during the exceptionally prolonged dry season of 2002.

The likelihood of any given calf surviving, however, was affected by the amount of woodland available where they lived, but not in a straightforward way. Typically, woodland was a good thing, apparently allowing the mother and calf to graze on Guinea grass beneath the tree canopy in the early part of the dry season. But, if the dry season was not merely drier than usual, but also longer, the presence of readily available woodland actually made things worse. This may be because areas with a lot of woodland have less space left over for the tussocky "bunch grassland" where red grass grows, and it's this that rhinos rely on as their last-ditch food resource when things get bad.

Female rhinos are not territorial, select their home range based on the availability of food and potential mates. One might think therefore, that they would tend to select areas with plenty of bunch grassland, as security against a long dry season. This is because, if the dry season is short or not especially intense, while woodland may give them a benefit, their calves aren't likely to suffer much in the first place. So getting that good mix would be, on balance, the better bet.

But they really didn't seem to, apparently selecting their home based on some other criterion than how much woodland it possessed. And those that did pick relatively wooded areas suffered as a consequence in those years when a drought hit. So why did they?

The authors of the study dismiss the possibility that the female rhinos had little choice, given the low population density and large amounts of suitable land available in this part of the world. Nor do they think that they were simply staying close to where they themselves happened to have been born - because apparently, they don't. Another possibility, however, is that some picked areas with woodland because, when they reached adulthood and left home, the weather wasn't too dry, and these would have been the best places at that time, even if they weren't later on.

At least as plausible, however, is that it's all down to sex. Male rhinos may be less concerned with terrain suitable for raising calves, and the females may just go where the males are. There's no point in picking a home that's great for raising calves if it means you can't get pregnant.

Whatever the reason, the fact that so many white rhino mothers pick a bad place to raise calves and then apparently don't move is a concern because of climate change. In some parts of the world, summers are becoming wetter even while they get hotter, but in southern Africa, the prediction is that dry seasons will get longer and drier. Which is bad news not just for white - and black - rhinos, but also for other large savannah-dwelling herbivores, such as elephants and wildebeest.

'Safe' is sometimes relative.

[Photo by David Keats, from Wikimedia Commons.]

Synapsida will be taking a break next week, but will be back 4th August.