The laws of physics that govern collisions between the car and the moose
and that explain why such collisions have a high mortality rate
|by Fiona Clements|
There are some words that are inherently funny, and for some reason, most of these words are the names of animals. Moose. Weasel. Wombat. Goat. Its a scientific fact that any sentence that contains one of these words is automatically funny. Or at least... its a scientific fact in the Fionaverse.
Having a heightened sensitivity to these words gave me some awkward moments when I was working for a Norwegian company. At least once a year the monthly company newsletter would contain an obituary for a Norwegian colleague whose car had run into a moose, and Im sorry, Im sorry, but there was a sentence with the word moose in it and I always had to fight back a giggle. It would (probably) have been different if Id known the person, but it was a large company and one got very used to reading the names of unknown Scandinavians in that newsletter.
Still, I did feel guilty about those stifled giggles, and while I wasnt consciously looking for a way to atone, I was very quick to pick up any serious information relevant to such collisions that came my way. Over the years I stumbled across two main sources of information that are relevant to this article:
Prisoners of the Sun (BBC, 1992)
This was a superb three-part documentary looking at the different strategies that different animals have for managing their energy-budgets, where the main contrast is between the low-risk/low-yield strategy of the herbivore and the high-risk/high-yield strategy of the carnivore.
The strategy of the herbivore is low-risk because your food cant escape from you: every time you put effort into getting a mouthful of food, you will get it. But its low-yield because grass is thin on nutrients, so herbivores have to spend most of their day eating. The strategy of the carnivore, on the other hand, is high-risk because your food can escape, and you can burn up a vast amount of energy in the chase with no guarantee of a result. But its quality food when you get it and carnivores have a lot more leisure time than herbivores.
There are also different energy strategies for ensuring the survival of the next generation: from creating millions of offspring but investing no energy whatsoever into caring for them; to creating a small number of offspring and investing decades of care into each. That isnt directly relevant to this article, but it is part of balancing that energy budget and the program covered it very well.
On Being the Right Size by J.B.S Haldane
This was one of a series of articles that Haldane wrote for the Daily Worker in the 1920s and that I encountered in The Faber Book of Science (edited by John Carey).
This article deals with the physical laws that affect the possible range of size for different types of creature, and the way that different properties change at different rates as you increase or decrease the size, which means that you cannot scale up or down indefinitely.
So... Im going to be talking about size in relation to energy - in relation to the collision between the car and the moose.
The Size (and Shape) of the Moose
The moose is a large animal, and it has long thin legs which means that its centre of gravity is high - above the level of the bumper of most cars (certainly of most cars in Norway). In a collision, the car hits the moose below the centre of gravity, and so the moose doesnt fall to the road and disappear under the tyres - which wouldnt be healthy for the moose but would give the driver some chance of riding out the bump - but instead it flies over the bonnet and through the windshield, which is frequently fatal for both moose and humans.
Now, there are reasons why the moose is large and has long thin legs, and these both involve issues of energy-management for a herbivore living in a cold climate. The size and the shape of the legs can be considered separately, and Im going to deal with the size first.
Why is the Moose Large?
Even when an animal isnt doing anything, it loses heat from its skin to the air, and the amount of heat that it loses is proportional to the surface area of its body. This means that the amount that it has to eat in order to keep warm is proportional to its surface area and not, say, to its weight.
As the size of an animal increases, its weight increases more quickly than its surface area. For example, if you consider a cubic animal (not unknown in science-fiction stories) and imagine doubling its dimensions in each direction, then the surface area will increase by a factor of four (2x2), while the volume (the weight) will increase by a factor of eight (2x2x2). A human weighs about 5,000 times more than a mouse, but has a surface area that is only about 300 times that of a mouse. So the amount that a large animal has to eat in order to keep warm, considered in relation to its weight, is much smaller for a large animal than for a small animal: a mouse in a temperate climate has to eat an amount equal to about a third of its own weight every day just to stay warm, whereas the proportion for a human is about a 50th.
A third of the weight of a mouse might not sound like very much, but consider that a mouse has a proportionately small mouth so has to take many, many mouthfuls of food every day - which means that it spends most of its time eating. The larger the animal the less time it needs to spend every day eating enough to keep warm; for something the size of a human eating the same type of food as a mouse, this would be a 17th of the time required by the mouse, which gives a much better safety-margin in case food becomes hard to find. Being a small animal is hard work.
The figure of a third of its own weight applies to a mouse in a temperate climate. In a cold climate such as northern Norway, animals lose heat much more quickly and the amount of energy that a mouse obtains in each mouthful is less than the amount that it loses while searching for and eating that mouthful, which means that it simply cant survive in that climate.
In the case of a large animal in a cold climate, the balance is different. If the animal is large enough, it can survive, and the size of the moose shows what large enough means. In a very cold climate, its very large indeed.
Now, what Ive just said applies to animals living above ground, exposed directly to the cold air and particularly to the wind. If the animal can take refuge in a tunnel under the ground or under the snow, then it will reduce the rate of heat-loss dramatically and the minimum size will also be reduced - to the size of the vole, for example, or the lemming. But only small animals can burrow successfully, and the vole and the lemming also represent the maximum size for this lifestyle. So we have the vole below the surface and the moose above, and no herbivores in the size-range in between. Carnivores are different. Ill talk about carnivores at the end.
Why does the Moose have Long, Thin Legs?
The moose has to travel in search of grass, sometimes over long distances. There are many different methods of walking, or, rather, many different possible designs of body in order to walk, and the design which uses the smallest amount of energy is one which has long, thin bones and also long, thin muscles.
Long bones can take long strides, which uses less energy than a large number of short strides when covering a given distance. Thin bones are light, and need less energy to lift in each step. However, if they were too long or too thin, they wouldnt be able to support the weight of the moose, so there has to be a balance.
The muscles are thin because the amount of energy burned by a working muscle is in proportion to its size. To use less energy, muscles are needed that are large enough to lift the leg - and not an ounce more.
And that, more or less, is the leg of the moose, and of all herbivores that live above ground. I say more or less because there is probably no herbivore that has the most efficient leg possible. The design that is most efficient for walking at a slow, steady pace between pastures does carry some disadvantages: with such weak muscles, its not possible to run in order to escape from predators. The more pressure an animal is under from predators, the more energy it has to invest in some manner or another to ensure that the herd survives. This investment might be in legs that are stronger but less efficient, or it might be in raising more children to balance the losses to the predators. The balance is different for different animals.
There are predators that threaten the moose - the wolf, for example - but wherever the point of balance for the moose is located, it still places the centre of gravity of the moose above the level of the bumper of most cars.
What about Carnivores?
Ive been talking about herbivores because theyre the ones involved in road collisions of the type that get documented in company newsletters. Carnivores eat high-energy food and can afford to be smaller - as small as an arctic fox, for example - and the type of leg thats required for a successful chase is much shorter and thicker. Carnivores that live above ground are much more low-slung than their herbivore prey and tend to go under the wheels rather than over the bonnet, so they only get this tiny paragraph right at the end.