Question #14: What is that thing sticking off your body?

This will be the first in a series of several posts dedicated to identifying the science terms surrounding arms and legs.

First we need to start with the most general scientific term; appendage. This is more or less anything sticking off the head or torso of an animal. Appendages include: arms, legs, tails, antennae, tentacles, wings and fins among others. Most animals have some form of appendage.

A term that comes up in science discussion is the term Paired Appendages. This means we have paired items; two arms and two legs. We as humans are part of a group called Tetrapods which means we have two pairs of paired appendages plus some other traits that distinguish us from something like a grasshopper which also has paired appendages. We share this, Tetrapods, with mammals, dinosaurs, reptiles, amphibians, birds and snakes.... Wait. Snakes? Yes. Snakes are descendants of lizard like animals that had paired appendages. If you look at pythons you can still see their pelvis on x-rays and see the external extension of these called the spurs.

Python Skeleton showing the pelvic bones

Illustration of where the pelvic bones are in a python

Image of a python showing the exposed spurs or hind legs.

Pythons have vestigial legs. Vestigial is a term applied to a body part that still exists but does not seem to serve a purpose. Usually vestigial structures are much smaller than the original structure and can not perform the function of the original structure. Vestigial limbs are quite common in the animal world. In addition to snakes, some amphibians and birds also have vestigial appendages.

Dwarf Burrowing Skinks is a type of lizard that has vestigial legs. This animal moves more like a snake but you can still see the tiny legs in all of the species.

Some birds have vestigial wings; Kiwis and Cassowaries do. Their wings are barely short stubs that are not used, to the knowledge of science, for any purpose that other birds use their wings for.

Kiwi Wing

What about Ostriches and Penguins? While Ostriches and Penguins do not use their wings for aerial flight they are not considered vestigial because they have a purpose. In ostriches the wings are used for threat and breeding displays and also to shade chicks and eggs from the hot sun. Penguins still perform the motions of flight with their wings but instead use them to swim. In both cases these wings still perform necessary biological functions that other birds use their wings for. Other birds use their wings to shelter chicks from the weather and many water birds such as cormorants use their wings to swim.