The ObserverWildlife This article is more than 5 years oldMeet the creatures that thrive in the dark
This article is more than 5 years oldAn exhibition at London’s Natural History Museum looks at how animals move, hunt and feed in places where no light ever shinesThe pale-throated sloth, from the northern Amazon forests, has evolved in an unusual way to survive the dangers of swinging through trees in total darkness. The nocturnal bear-like creature has developed a sense of smell so sensitive it can tell whether branches nearby are emitting whiffs of sap or not.
“That allows them to swing only on to branches that are healthy,” said Professor Geoff Boxshall of the Natural History Museum in London. “They can avoid grabbing one that is sapless and dead, which might break, causing them to fall out of a tree and injure themselves. Thus they can swing safely through forests in complete darkness.”
The sloth’s adaptation to the dark side is one of many remarkable responses of living beings to nocturnal or lightless existences and features in a new exhibition, Life in the Dark, which opens at the museum on 13 July. The show will use the Kensington museum’s vast collection of specimens to demonstrate how life can thrive in the absence of light. Installations will include recreations of bat-filled caves and the spectacular luminescence of deep-sea creatures.
“At any one time, half the world is in darkness, and sunlight is also excluded from the deep sea and from underground caves,” said Boxshall. “Living creatures thrive in all these places – though they have been able to do this only by evolving in remarkable ways to overcome the problem of lack of light.
“Our own senses are utterly dominated by information from our eyes, but countless other creatures can happily survive without this input. Their approaches to lack of light give us a new way to explore nature.”
Another remarkable creature of the night is the blind aquatic salamander Proteus anguinus – or olm. These animals spend their lives in total darkness in caves in central and south-eastern Europe, especially the Postojna cavern complex in Slovenia. They grow up to 30cm long, can live for up to a century and have no vision – instead they have evolved a technique for detecting the bioelectric fields of their main prey, cave shrimps, so they can hunt in total darkness. A display recreating the olm’s electricity-directed attacks is included in the exhibition.
“Living in total darkness might protect you from predators but you still have to find food – and the olm has evolved an intriguing way of doing that,” said Boxshall.
A similar trick is adopted by the Puerto Rican cave boa, though it uses heat sensors, not bioelectric sensors, to locate bats in caverns. “These slender snakes hang from cave roofs and detect the infrared radiation bats emit,” said Boxshall. “They just snatch them and eat them.”
Scientists separate lightless environments into three categories: night-time, cave systems and the deep sea. Crucially, it is only creatures adapted to underground caverns, like the olm, that lose their ability to see. For nocturnal animals and deep sea creatures, the ability to see – even though they dwell in almost complete darkness – remains useful. An example is provided by spookfish, which swim at depths of around 1,000 metres and have upward-looking eyes that can detect the silhouettes of smaller fish above them. “That is quite an achievement, given that hardly any light makes it from the surface to this depth,” said Boxshall.
However, the exhibition also makes it clear that the Stygian world that shelters these creatures is under threat. As humanity spreads, we carry light pollution with us. For example, the nocturnal boat-billed heron of Peru and Brazil will not eat in the presence of any light source, and this is a growing problem as urbanisation spreads.
“Life in the dark is a delicate business,” Boxshall concludes. “We should not take it for granted.”
Life in the Dark runs from 13 July until 6 January 2019
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