Sunday 23 October 2011

The Mother Elk's Dilemma

A male elk
Giving birth and looking after newborn young are particularly dangerous times for the large hoofed herbivores. That isn't to say that it isn't difficult for other mammals, too, or, indeed, other animals in general, but the larger herbivores are especially vulnerable. Smaller animals can hide their young in burrows, or other secluded dens, until they come of age, while the larger carnivores don't generally have to fear being eaten. Large herbivores do not have such a luxury.

However, hiding from predators while the young are still small and vulnerable is not the only concern that mothers have to face. Giving birth is costly in terms of resources, and after that, the young must be fed milk until it is large enough to search for food on its own. The mother, therefore, needs to keep herself well fed, and needs a good supply of food to keep both herself and her infant healthy. While this is obviously true at any time of the year, during the birthing season it becomes particularly important for the mother to find an area that is both free of predators, and has a ready source of available food.

That can be a problem if the areas that have the best food are also the ones that have the most predators.

Which is a common situation; if nothing else, predators do tend to congregate where their prey prefer to live. So how does a mother resolve this problem? Clearly, she will have to accept some sort of trade off, depending on whether she is more worried about her child starving to death or being eaten. Exactly how she will do that depends on the exact environment and the nature of any challenges that the animal will face.

The way that animals approach this dilemma has been studied in a range of different animals, including antelope, moose, reindeer, and goats, among others. Spencer Rearden and colleagues, of Oregon State University, have recently extended this to study elk living in the Blue Mountains of their state. Before I look at that, though, I should explain that there is considerable confusion as to what exactly an "elk" is.

Firstly, there's the common name. In Europe, the word "elk" refers to the animal Americans know as a moose, and that is, in fact, the original meaning of the word in English ("moose" comes from the Abenaki language). In an attempt to clear this up, the name "wapiti" has been suggested for the American "elk", although it has never really caught on with the general public. Anyway, whatever you call it, that's the animal I'm talking about here.

But, unfortunately, the confusion doesn't end with the common name - it is also unclear what the scientific name for the animal should be. That's because scientists can't agree on whether or not the elk is a species. Most of the usual sources, such as the IUCN, consider the elk to be merely the American form of the animal Europeans know as the red deer (Cervus elaphus). So, when Europeans say "elk", we usually mean "moose", and when Americans say "elk" they mean a red deer.

Or do they? The elk (or wapiti) has long been thought to be a separate species, and it's only been relatively recently that it has been downgraded to being another type of red deer. There has been considerable argument over whether or not that was the right thing to do. From what I have seen, it very probably wasn't, and the elk likely is distinct enough to be considered a species in its own right. So, I am going to say: the elk (Cervus canadensis) is a large, forest-dwelling deer, native to the northwestern USA and neighbouring parts of Canada, as well as to parts of the Far East.

Red Deer    Elk     Sika Deer
   |         |          |
   |         |          |       Thorold's
   |         |          |         Deer
   -----------          |          |
        |               |          |        Sambar,
        |               |          |         etc.
        -----------------          |          ^
                |                  |          |     Pere David's
                |                  |          |      Deer, etc.
                --------------------          |          ^
                          |                   |          |
                          |                   |          |
                          ---------------------          |
                                    |                    |
                                    |                    |

The picture may be more complicated even than this. Everyone agrees that the red deer and sika deer are species, and, as I've indicated above, I think there are good grounds for supposing that the elk/wapiti is one as well. It is, however, entirely possible that each of these three actually represents a group of very similar-looking species - for example, the elk of eastern Asia may not be the same as those from America.

The elk of the Blue Mountains live in forests dominated by ponderosa pine, especially around forest edges and clearings. Here, the density of the trees is crucial. Where many trees crowd together, the forest is cast into shadow, which may make it easier to hide small and relatively immobile young. The dappling effect of sunlight coming down through the overhead branches may also be useful in acting as a form of camouflage, in combination with the spotted coats of baby elk. But that same deep shade also means that there is less undergrowth, and it is that undergrowth that supplies the deer's food, as well as potentially hiding predators. So, to get enough food to produce milk for your young, you also have to run the risk of them being eaten.

Elk in this part of the world give birth round about May, so the researchers captured and radio-tagged female elk in March, in the hope of seeing where they would later give birth. They also used a special transmitter that is able to detect when and where an animal gives birth (it is expelled from the body when this happens, and the sudden change in temperature sets it off). Together with the use of GPS, that makes it possible for the researchers to visit the site once the mother and fawn have left, and will no longer be disturbed by their presence - and then you can look and see what sort of location she chose.

When they looked at the areas around those where the mother gave birth, they found that tree cover was lower than average for the surrounding forest, and that the terrain was also slightly flatter. The former in particular would mean greater visibility, perhaps making it easier to spot stalking predators, but it also means that predators were more likely to be there in the first place, suggesting that the availability of plentiful forage was more important than the risk of attack.

However, the deer were choosing their birth sites more subtly than that. Because, when the researchers looked on a much smaller scale at the actual birth sites, those were generally among dense stands of trees or heavy vegetation. In other words, mothers were looking for well-concealed patches in the midst of more open ground - and not, for example, equally secluded dens in denser forest. The tactic seems to work, at least in the short term. For the first four or five days of their life, young elk fawns stay hidden in vegetation or among logs, with the mother patrolling and feeding in the land about them. Yet few of the local predators were eating fawns of that age; if they were looking for them at all, they clearly weren't finding them.

So, for the first five days or so, the fawn is, apparently, quite safe. But that can't last, because sooner or later it begins to get up and follow its mother about, re-joining the herd. Instead of simply hiding, if it needs to escape from a predator after the first few days, the fawn has to run. Once that happened, the balance began to shift. We already knew that most fawns just don't make it to adulthood, as is the case with many wild animals. To find out why that was, the researchers fitted some of the fawns with 'mortality collars' - expandable collars that send out a signal if they don't any sense any movement over a four hour period (if the animal survives, they are designed to eventually fall off anyway, so that they can be retrieved without the need for re-capture).

The primary predators in the area turn out to be cougars and, as expected, they were more successful in open terrain with good visibility. Evidently cover was not a big concern for them; it was more important that they be able to quickly identify elk herds with young and that the latter had little chance to flee deeper into the woods. Of course, we can't tell, just from this study, how often the cougars tried to go after animals in less suitable terrain and failed, but we can see that forest edges, in particular, were good hunting grounds.

So the elk fawns over five days old had a much tougher time of it, although presumably the mere fact of being in a herd, with several vigilant adults, offered at least some protection. But, for the first few days, they are - relatively speaking - safe and cozy, protected not just by the vegetation immediately around them, but by the clearer terrain beyond that where there may be danger, but there is also food for the mother, and maybe a better chance to see what's coming.

[Picture from Wikimedia Commons. Cladogram adapted from DeMiguel et al, 2008]

No comments:

Post a Comment