Living successfully in the tropics is no mean feat. Stuart Blackman looks at the living conditions at the equator - and the coping methods animals adopt to survive
What is the equator and how long is it?
Passing through Africa, Indonesia and South America, as well as the Pacific, Atlantic and Indian oceans, the Equator represents the circumference of the Earth midway between the North and South Poles and marks the boundary between the northern and southern hemispheres.
It's an imaginary line about 45,000km long. The Earth turns on an axis running through the poles, so the Equator is something like the rim of a spinning top. It also marks the mid-line of the tropics, a region bounded by two more imaginary lines, the Tropic of Cancer to the north and the Tropic of Capricorn to the south.
What's the weather like at the equator?
It is only within these lines that the sun ever passes directly overhead. When sunbeams hit the Earth perpendicularly, the heating effect is more intense than when they hit at a shallower angle, and are therefore more diffuse. This explains why the tropics are so hot. The high temperatures, in turn, make it wet, because warmer air holds more water, which falls as more rain.
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Not all tropical habitats are sunny, wet and warm. Rainforests are often cloudy, the Sahara is very dry, and there are glacier-capped mountains in tropical South America. But on average, the tropics are warmer, wetter and sunnier than elsewhere.
It’s a combination that presents special challenges. For example, sweating is of little use for cooling down when the high humidity prevents evaporation.
How do animal cope with the conditions at the equator?
Seeking shade or limiting activity to the hours of darkness is more effective. Verreaux’s sifakas, a type of lemur endemic to Madagascar, avoid overheating by hugging tree trunks, which are themselves cooled by water drawn from underground. Meanwhile, fat-tailed dwarf lemurs can avoid the heat by hiding away in tree-holes for months at a time, living off fat stored in their tails.
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But what if there is no shade? An aquatic mammal such as a hippo has little problem with overheating, but is still exposed to the sun’s powerful rays, so it exudes a biological sunscreen, known as ‘blood sweat’, from its pores, which blocks the harmful UV component of sunlight. In tropical marine environments, shallow-water corals produce a similar compound.
Animals living closer to the Equator also have greater amounts of a dark pigment called melanin in their skin, fur and feathers, which protects against UV-damage. The melanin also functions as an antibiotic, killing bacteria that would otherwise thrive among fur and feathers in the hot, damp, tropical conditions.
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