A Tiger's Dinner

One of the basic concepts in ecology is that of the food chain; the idea that plants are eaten by herbivores are eaten by small carnivores are eaten by large carnivores. The reality is both more complex - because, for example, omnivores exist - and simpler, because, at least on land, many of the largest carnivores eat large herbivores, not smaller carnivores. Nonetheless, there's still an underlying truth, and it introduces us to concepts such as the apex predator.

An apex predator is essentially a carnivore that has no predators of its own, an animal that sits at the top of its local food chain. Many mammals fit this description, including wolves, big cats, bears, and killer whales. (The last of those, of course, being an example of a large carnivore that does mainly eat smaller carnivores). Outside the world of mammals, one could add eagles, crocodiles, and sharks, among others. Humans could count as another example, given that we obviously don't have regular predators, but this does depend on your exact definition, since we're clearly omnivorous and, in many parts of the world have a nearly or totally herbivorous diet.

A key feature of apex predators is that they are rare. Since an animal cannot convert its food into energy with 100% efficiency there is less energy available to support herbivores than there is to support plants, and less energy for carnivores than there is for herbivores. And so on up the chain. You can't have an ecosystem where everything's big and scary; predators have to eat something, and whatever it is has to be more numerous than they are. On top of that, the reality is that humans don't really like apex predators - at least, not wild ones living close to where they are.

The tiger (Panthera tigris) is a clear example of this. Tigers are among the most iconic mammalian apex predators, and for all that we admire their beauty and grace, most people probably wouldn't want one prowling the countryside near where they live. The fact that tigers happily eat our livestock given the opportunity and, on occasion, will attack and kill humans has led to conflict between them and ourselves, and it's a fight that, in the long run, the tigers aren't winning.

Tigers used to live from eastern Turkey to the Pacific coast, but no longer. They have vanished entirely from western and central Asia, as well as from Pakistan, Korea, Vietnam, Cambodia, and the islands of Java and Bali. Although estimates vary, it's thought that there are not much more than 3,000 adult tigers left alive in the world, about 80% of them in India. Unsurprisingly, they are officially listed as an endangered species, and most of those in India are found in legally protected areas. 

But still, around a third of them are not, and even the protected areas aren't always as safe as they are supposed to be.

Since so much of the conflict between tigers and humans is driven by the predators eating livestock, an understanding of the diet of tigers across different areas would clearly be beneficial, and many such studies exist. But how exactly to do this, given the difficulty of following the tigers about and watching them all day? As an illustration, we can look at one recently published study, surveying tigers in part of northern India between 2014 and 2020. 

This was a wide-ranging study, looking at tigers in an area called the Terai Arc Landscape which stretches for about 900 km (560 miles) along the foothills and lowlands (terai is Sanskrit for "lowlands") on the southern side of the Himalayas in northern India and southern Nepal. With a total area of about 15,000 km² (5,800 square miles), this encompasses eight wildlife sanctuaries and national parks on the Indian side of the border, where the study was conducted. It's a diverse area, ranging from high foothills to lowland forests, grassland, and swamps.

Despite its high human population density, it's an area rich in wildlife, with predators including leopards, hyenas, wild dogs, and (omnivorous) bears in addition to tigers. It is also home to rhinos and wild elephants as well as the endangered hispid hare, and birds such as the near-extinct Bengal florican (a type of bustard). Still, the tigers are probably the most famous wild residents, with the region being home to around 650 of the animals - which you'll note is somewhere around a fifth of the worldwide population.

To see what the tigers had been eating, the researchers followed animal trails at multiple different locations across the landscape, including both protected areas and unprotected forests. Rather than observing the tigers directly, they brought along wax bags and placed any large carnivore droppings they found inside them to take back to a laboratory. 

In total, over the six years of the study they collected 1,689 dung samples. However, one large carnivore dropping looks much like another, so the first thing they did with them was to test them for the presence of tiger mitochondrial DNA. That left them with 525 samples that definitely came from tigers, which they dried out and sieved to obtain undigested bits of hair that they could check under a microscope to try and identify the prey species. (Genetic analysis would likely have helped here, but seems to have been beyond the budget of the study and the researchers argue that tiger diet isn't varied enough for this to be an issue).

It's no great surprise that the study found that the great majority of what the tigers were eating were various large-bodied herbivores, with deer as the most common prey item. In the foothills, sambar were the favoured prey, a species of deer about the size of a red deer or elk, and thus almost as large as deer get. In the lowlands, where sambar are less common, they switched to the smaller chital deer but there's a suggestion that they didn't really consider these a filling meal, eating them only because they were so common. Instead, when they could, tigers in the lowlands were eating a much higher proportion of livestock than those in the foothills, apparently seeking out oxen and the like when their preferred larger prey were not available.

So how does this help us? For one thing, the pattern of locations in which the researchers found tiger droppings shows that they are by no means only restricted to the tiger reserves and other protected areas. They can and do wander across the landscape more widely, into places where they are neither protected from attacks, nor monitored to assess their population. Furthermore, it allows us to highlight areas where tigers are eating livestock, putting them at the greatest risk of retaliation by local farmers.

Significantly, tigers in lowland protected areas, where there shouldn't be many livestock were, in fact, eating just as many agricultural animals as those elsewhere. This may be due to local farming practices, which tend to allow animals to graze freely, perhaps even escaping and becoming feral. In other parts of the world, changes in animal husbandry and paying compensation to farmers can be effective as means to reduce conflict between humans and wild predators, and to reduce the risk of disease spreading from livestock into the wild.

Knowing where the tigers are most frequently eating domestic animals is a good place to start when it comes to focussing our conservation efforts.

[Photo by "Seemaleena", from Wikimedia Commons.]