Last Word is 麻豆传媒鈥檚 long-running series in which readers give scientific answers to each other鈥檚 questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Why do many animals, such as mammals and birds, have white underbellies?
Herman D鈥橦ondt
Advertisement
Sydney, Australia
Being lighter at the bottom than at the top is called 鈥渃ountershading鈥�, and the main reason for it is camouflage. It is regularly found in animals like fish, which are often viewed from below by predators. However, though light usually comes from the sun above, it also works if the animal is viewed from the side. Since the top of the animal gets more light than the bottom, countershading makes overall tone more even and the animal blends in. In such cases, if the bottom were the same dark shade as the top, it would be easier to see against the background.
While a white belly may stand out when standing up, such as with a penguin, for the most part, animals are positioned belly-down, so it rarely has that effect. Countershading doesn鈥檛 require a fully white belly 鈥� a lighter-coloured shade is often sufficient.
There are also cases of reverse countershading to make an animal distinctly visible (dark bottoms and light tops), usually to act as a warning to predators in species with strong defences, such as the skunk. There are some cases of reverse countershading that is still camouflaging, for example in the Nile catfish, which tends to swim and live upside down.
Countershading is also referred to as 'Thayer's law' after the US artist who was one of the first to study it in the late 19th century
Countershading is also referred to as 鈥淭hayer鈥檚 law鈥� after the US artist Abbott Thayer, who was one of the first to study it in the late 19th century. In 1902, Thayer was even awarded a patent to paint warships with countershading to make them less visible to the enemy. The US Navy rejected the idea at the time, but in the second world war, many fighter planes used countershading to make them less visible from below.
David Muir
Edinburgh, UK
Imagine that you are a raptor flying over your hunting ground. Two prey animals are below, both pigmented to match the terrain. But one is brown all over, whereas the other is brown-backed with light flanks and a white underbelly. What does the raptor see, and which prey is more likely to be attacked?
When light falls on a uniformly coloured animal, the upper side appears lighter and the underside darker. The visibly lighter parts are then framed by the seemingly darker ones, making the animal easier to detect. An animal with colour gradation from dark above to lighter underneath doesn鈥檛 suffer this problem; instead its pattern reduces differences in brightness and colour, making it harder to detect. Therefore, the white-bellied prey is more likely to survive and pass on its genes than the uniformly coloured prey.
Reverse countershading, a dark underside and light back, maximises contrast by enhancing the effect of natural light and is the opposite of camouflage. This is used by animals such as the honey badger to signal to predators that they have potent defences, in other words, that they aren鈥檛 to be tangled with, so potential attackers better beware.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
麻豆传媒 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, 麻豆传媒, 25 Bedford Street, London WC2E 9ES.
听