Superior senses in the dark

For all animals, there are three common necessities of life: finding food, finding a mate and avoiding being eaten. But some face the extra challenge of having to do all of that in the dark.

Discover the specialist skills of some creatures that have found ways to live in low light.

Infrared eyesight

Deep in caves, beyond the reach of sunlight, ecosystems mostly rely on food being brought in from outside.

Museum researcher Prof Geoff Boxshall says, 'In many huge cave systems there are mountains of bat guano, which is a massive food source, and the whole ecosystem is based around that.'

Giant centipedes, Scolopendra gigantea, are at the top of the food chain in some caves in South America. They're big, have venom glands and choose to prey on bats. Cave boas, Chilabothrus inornatus, can also be found hunting bats in the darkness. These snakes use an infrared sensitivity to catch their prey.

'The boas hang from the ceiling of the cave and they can sense the body heat of flying mammals. They detect these little spots of heat flying past them and they can catch them in mid-air.'

The darkness of caves has dramatically affected the evolution of some animals. Cave fish no longer develop eyes, but they manage to survive despite their lack of sight.

'They can find food and they don't swim into cave walls. They can do all of the things they need to do completely without eyes, because it's dark and there is no point in making them.'

The olm (Proteus anguinus) also has undeveloped eyes. These cave-dwelling salamanders have electroreceptors that enable them to sense approaching predators and prey. 

A light in the dark

Although sunlight doesn't reach the deep sea, there is still a surprising amount of light amid the gloom. A prime example of this is the luminescent lure of anglerfish, which twitches to attract prey.

'If you're swimming around in the dark and you see a light in the distance, it could be food, it could be a possible mate or it could be a predator. A light in the dark can be a triple-edged sword,' explains Geoff.

The ocean is a predator-dominated habitat, with few places for prey to hide. This has driven the biology of a lot of animals. Some species instead use bioluminescence to protect themselves. 

'Predators that are swimming around could look up and see krill-shaped silhouettes against the light coming down from the surface. So their prey produce bioluminescence on their undersurface, which breaks up their silhouette. 

'This is driven by the fact that there is nothing to hide behind in the open ocean. They need to come up to the surface at night to feed, but in the day they go back down to avoid predators, because you can hide in the gloom.'

The vampire squid (Vampyroteuthis infernalis) also lives in the dark depths. When threatened, this little cephalopod can turn itself inside out and hide in a webbed cloak of arms. This exposes sharp spines called cirri - but this isn't their only defensive adaptation.

Vampire squids don't produce ink as this wouldn't benefit them in the lightless depths. Instead they release a bioluminescent fluid to distract attacking predators so that they can flee in the opposite direction.

Creatures of the night

Some animals prefer to live by the light of the Moon. The fennec fox (Vulpes zerda) is adapted to hunt at night. During the day, they burrow underground to keep out of the scorching heat of their North African desert habitats. 

Improved eyesight is one of the many adaptions these nocturnal animals have to help them cope with the low light conditions.

Many nocturnal animals have a reflective surface, called a tapetum lucidum, behind their retinas that allows their eyes to take in more light.

'It's about enhancing their vision as much as possible. They sort of use light twice: the photons go in through the retina, are reflected off a mirror layer behind the eye, and then go back out through the retina,' Geoff explains.

But not all nocturnal animals have reflective eyes - of any animal, tarsiers have instead evolved the largest eyes in relation to body size, allowing the small primates to maximise the amount of light they capture.

Heightened senses are important in the dark as the earlier an animal can sense prey, predators or a mate, the higher the chance of survival.

'In the absence of vision, animals develop ways of pushing out the perimeter where they can sense things by having better hearing, or long antennae like cave crickets, or ways of detecting vibrations in water - they push out their sensory horizons.

'Alternative senses allow animals to manage the imperatives of life in the dark: to find food, find a mate and not get eaten. They are successful because they are well adapted - they have senses that we're much less aware of.'