Science Sunday: The Diversity of Locomotion
As promised, here is the post about locomotion in the animal kingdom. For the purpose of not making this into a never-ending wall of text, I’m limiting the concept of locomotion to movement along the ground.
Now, moving along the ground might seem a bit boring. I mean, we do it all the time, no fuss. However, moving along the ground in an efficient way is quite a sophisticated task. The environment is never static, and there are always obstacles in the way. Animals have dealt with these obstacles in different ways, and have evolved a number of ways to move from place to place.
One of the main ways to move around is by using appendages, or legs as I’ll call them from now on. This is the most common form of locomotion on land, as it is used by vertebrates, spiders and insects, among others.
Animals that move around on two legs are called bipedal. There aren’t many animals that are exclusively bipedal. The mammals that are bipedal usually move around by hopping, like kangaroos and wallabies. Humans are actually an exception, as we move by putting one leg in front of the other, in an alternating walk. It’s not without reason that so few animals rely on only two legs. Among other things, shifting from four to two legs puts lots of strain on the spinal column, which is why so many humans struggle with lower back pain. It will also make you really prone to losing your balance and falling. Some animals are bipedal for short periods of time, like lizards that run on their hind legs, or bears that stand up. The only big group of animals that exclusively are bipedal are birds; when they move along the ground, they usually do so by hopping. Of course, most birds fly in addition to walk, and quite a lot more energy on that form of movement.
Most vertebrates are quadrupedal, meaning that they move on four legs, including mammals, amphibians and reptiles. Since there are many animals that are quadrupedal, there are also many different quadrupedal gaits. The so-called pacing gait connects two species as different as the cat and the camel. They both walk by moving the legs on one side first, and then the legs on the other side. However, most other mammals walk by moving the hind legs and forelegs diagonally opposite to each other at the same time. The gait of an animal may also change as the animal moves with a different speed. Horses, for example, have four different gaits which – in increasing speed – are the walk, trot, canter and gallop. Frogs principally move by jumping, which is why their hind legs are usually much larger and stronger than their forelegs.
Of course, there are many animals that move with more than two or four legs. Insects, like flies and scarabs, are hexapedal, as they use six legs to get about on the ground. Spiders have eight legs, making them octopedal and separating them from insects. Segmented arthropods like centipedes and millipedes can – as the names suggest – have up to several hundreds of legs. The distinction between centipedes and millipedes is that centipedes have one pair of legs per body segment, while millipedes have two pairs per segment. In other words, the possible number of limbs is quite high, if not infinite.
The other main way of movement is – you guessed it – without using legs. Limbless locomotion is used by both vertebrates and invertebrates, and one thing that they all have in common is that they are cold-blooded. As these animals have no legs – or at least no usable legs, they move by creating a forward force with their bodies.
There are about 3,400 species of snakes, all of which have been quite successful without any extra limbs. So, losing some baggage has obviously worked out for the snakes, form an evolutionary perspective. Different species also implies that snakes move differently, and indeed they do. The most common form of movement is called lateral undulation, where the snake’s body moves alternately to the right and left, creating a wave motion. The forward motion is made as the snake pushes against different irregularities on the ground. Snakes living in desert usually don’t have many rocks or twigs to push against, making lateral undulation impossible. Instead, these snakes will sidewind. When sidewinding, all the parts of the body that are oriented in the same direction touches the ground, while the rest of the body is lifted up. This will make the snake move diagonally across a surface, in a sort of rolling motion. The forward force is created as only non-moving parts of the snake are touching the ground, therefore hindering slipping. It should also be mentioned that there are legless lizards which share many of the characteristics of snakes. These include the wrongly named glass snakes, a group of lizards which have simply lost their legs at some point in evolution. They can be distinguished from snakes quite easily, as they have eyelids and external ear openings.
Slugs move quite differently from snakes, as they are invertebrates. To make life easier for themselves, slugs create a layer of mucus on their underside, reducing the friction as they move. A large part of the underside of a slug is called the foot, which is equipped with the muscles that the slug needs to move. Slugs move by contracting the muscles in the foot in a rhythmic way. As the foot has many small bumps that can help the slugs adhere to a surface, slugs can even move up vertical walls. Not bad for a spineless little creature.
Remember, these are just a few examples of movement among species today. I haven’t mentioned any extinct species, and there are many details which have been glossed over. However, locomotion is a fascinating subject, and does not happen purely along the ground. Maybe we’ll take this series to the skies next week.