Sunday, 4 September 2011

The Scariness of Tigers

Animals are naturally wary around signs of predators that might want to eat them. Even if they don't necessarily run away at first sight, they are at least likely to spend more time keeping watch and less time, for example, eating. Many animals may have a range of possible predators, and so have to be on the lookout for a range of different signs that one might be in the area. On the other hand, it makes no sense to be frightened of anything unexpected, so its unsurprising that they watch out for sounds, smells, and so on that are particularly associated with animals that regularly attack them.

Sometimes this behaviour may be learned, and at other times it can be ingrained deep in the genes, depending not least on what animal we're talking about. But what happens when the predator is not around any more? That completely removes the possibility of learning that the predator is dangerous, but it may also, over a number of generations, disappear from the genetic record as well - there isn't much point wasting energy looking out for signs that you'll never see.

Different studies have come up with radically different answers as to how long it takes to lose fear of a long-gone predator. This is hardly surprising, considering the diversity of different animals studied, and the likely differences in their exact circumstances. For example, there's a difference between an animal having no commonly encountered natural predators at all - for instance, if it's safely domesticated - and having just one local predator vanish while others remain. In the former case, almost any effort expended on looking for predators is wasted, and we might expect the fear of such animals to disappear relatively quickly.

A study in 2004 showed this effect in Tammar wallabies (Macropus eugenii). A population transplanted to New Zealand, where there is nothing large enough to eat them, lost essentially all fear of their former predators after, at most, 130 years of isolation. But what's true of wallabies isn't necessarily true of all other animals.

Père David's deer (Elaphurus davidianus) once lived in swampy environments across central to northeastern China. By the time French naturalist Père David introduced the animal to western science in 1865, wild populations were already dwindling - the animals he encountered were in the Emperor's private hunting reserve. In 1895, catastrophic floods destroyed much of the hunting reserve, killing all but a handful of the deer, which were, by now, critically endangered in the wild. A little over ten years later, during the Boxer Rebellion, troops occupying the reserve ate all of the survivors, and the last wild specimen was reportedly shot in 1939.

Which would have been the end of the species had the Chinese Emperor not sent a few animals as gifts to European rulers. Only a tiny number remained, all of them in parks and zoos, but a recovery program at Woburn Abbey in England ensured their survival, and some were even sent back to China in the 1980s. Even so, Père David's deer is entirely extinct in the wild, and has been for over seventy years. Every animal alive today is in a zoo, park, or similar refuge, and any plans to return them to the wild have so far stalled.

Pere David's      Eld's       Red Deer,
    Deer          Deer          etc.
     |             |             ^
     |             |             |      Fallow Deer,
     |             |             |          etc.
     ---------------             |           ^
            |                    |           |         Muntjacs
            |                    |           |            ^
            ----------------------           |            |
                      |                      |            |
                      |                      |            |
                      ------------------------            |
                                 |                        |
                                 |                        |

All of which makes them ideal as a test for how broadly applicable the multi-predator hypothesis really is. Chunwang Li and colleagues of the Chinese Academy of Sciences recently put the hypothesis to the test, seeing how the semi-domesticated deer would respond to different kinds of predator. The deer they tested lived in nature reserves in Beijing and Dafeng, and, like all living members of their species, are descended from animals that have lived in the relative safety of captivity since at least the nineteenth century - and presumably much longer, unless the then Chinese Emperor was restocking his supplies from the wild, which seems unlikely. Indeed, the authors estimate that these deer have not lived in the wild for the last 1,200 years, although, frankly, I'm not at all clear how they worked that out.

They used two types of test to see how the deer responded to potential predators; playing sound recordings of hostile animals, and showing them large cut-out pictures. For the sounds, they used crows, which shouldn't be scary at all, barking dogs, that the deer should be used to and unafraid of, white noise, to make sure they weren't just spooked by any sound, and the calls of a wolf, a lion, and a tiger. Tigers and wolves are both animals that are likely to have preyed on Père David's deer when they lived in the wild, so the question there is, are the deer more afraid of them than of the apparently harmless crows and dogs. Lions, on the other hand, don't live in China, so this particular deer species should never have encountered them at all, even in the distant past.

All of the natural sounds seemed to startle the deer a little, although they did completely ignore the white noise. Yet it was clearly the sound of the tiger that had them worried the most. They didn't actually run away - after all, they couldn't see anything to run away from - but they did stop eating, look about, stare in the direction of the noise, and even make alarm calls. In the case of the stags, it was clear that the tiger was the only animal they were really worried about, but the hinds were decidedly more nervous, and were almost as concerned by the dogs and lions. Oddly, wolves just didn't seem to worry them at all.

The cut-out pictures included bears and leopards, in addition to the tigers, lions, dogs, and wolves. Again, leopards and bears would be animals their ancestors would have been familiar with, although bears don't tend to attack deer so much as tigers or wolves, which might make them less frightening. As it turned out, the deer didn't really respond much to the pictures at all, perhaps because they weren't moving or making noises. In fact, if anything, they were more worried by the big piece of blank plywood the researchers used as a control. Random animals are perhaps something they're more used to seeing than mysteriously appearing bits of wood!

However, while they may not have shown any other signs of concern, they did stare at the picture of the tiger almost ten times as often as they looked at any of the other cut-outs. Something clearly told that this was something to at least keep a close eye on, although, so long as it didn't do anything, it wasn't really frightening.

So what we have here is a clear indication that, hundreds of years after they or their ancestors would ever have actually met a tiger, Père David's deer still find them frightening. Something deep in their DNA tells them that these are animals to be worried about, a menace from their distant past.

It's interesting that wolves, or the images of leopards, didn't concern them. The researchers speculate that wolves might have been less likely to hunt in the swampy habitats where the deer used to live, although its hard to believe that they didn't eat them from time to time. But perhaps they didn't do so often enough to remain in the genetic memory over hundreds of years - or maybe that memory disappeared in the random luck of the genetic lottery. At one point there were only eighteen Père David's deer alive anywhere on the planet, so its not as if they have a wide genetic diversity to draw on in their background; one random mutation could have been all it took to wipe out a trait in the population.

The hinds were more nervous than the stags, no doubt because they're smaller and sometimes have to care for vulnerable young. They were almost as worried by the roar of a lion as by the growl of a tiger, yet they clearly didn't know what a lion actually was, because the picture didn't concern them in the slightest. Of course, while its quite possible to tell them apart, there is a certain degree of similarity between the sounds that the two animals make (judge for yourself). Perhaps its just better to be cautious, in case that unfamiliar noise is really an odd-sounding tiger.
So, while there are animal that apparently lose their fear of predators after little more than a hundred years of breeding in safety, for some that fear is just too ingrained to go away.

[Picture from Wikimedia Commons. Cladogram adapted from Gilbert et al, 2006]

No comments:

Post a Comment