The rhythm of the forest changes as dusk approaches. The hot exhaustion of the late afternoon dissipates, and the forest comes alive again, loudening with the calls of birds and monkeys. A few mammals like the tamandua (ant-eater) and ocelot may make a brief appearance, and many insects begin to seek refuge for the night. When the darkness deepens the quiet returns, but the forest does not sleep. Imagine taking a trip into a lowland tropical forest by night. Imagine looking for eyes reflecting the faint glint of moonlight, and listening for a soft rustling in the leaf litter.
This crab, for example, hunts by night. I found it in a small stream in close proximity to a number of large crayfish. But evidently life is not completely easy – you can see this individual is missing one of its claws. (And it was keen on pinching me with the other!)
Lizards can also be found at night. Most seem to be resting in inconspicuous locations, but perhaps some are taking advantage of new predation opportunities. It’s difficult to tell the difference between a lizard that has been startled by an overly curious biologist and one that was already active!
Many invertebrates continue their activity at night. I found this snail carefully navigating across a leaf, in search of some unknown destination.
However, some insects do not fare so well. This scarab beetle was attracted to an artificial light source, mistaking it for a natural one. After a short flight, and a sudden collision, I found it upside-down on the ground, trying very unsuccessfully to right itself. I waited five minutes but it was no closer to a solution than at the beginning. Taking some pity, I righted it, only to see it fly into the light a few short seconds afterwards. Bright lights at night are not something these creatures are adapted to.
Walking slowly deep in the forest, I heard what sounded a lot like the footsteps of an animal trampling over leaves. It turned out to be not a mammal, but a spider – in this case, an enormous tarantula, probably six inches long. Being overly curious I pushed it with a stick, and felt it push back with its very strong legs. It was enough to convince me to leave it alone and keep walking.
But some night-time creatures are more amusing than scary. On the way home I stopped by the laboratory greenhouses, which are bordered by moat to keep out snails. In the water lives a species of frog that puffs up its body with air and floats, buoy-like, on the water’s surface. It reminded me of a balloon, and I was not disappointed to find out how it reacted to being touched!
It is difficult, as a human, to imagine what life would be like in the darkness, because our bodies are not adapted to these conditions. But for other species, being nocturnal has many advantages. Many predators are no longer present, and new sources of prey are available. Competition for resources is less strong. Finding other individuals of the same species to reproduce with is easier. Temperatures become more tolerable. These conditions have led to many species like the ones you saw here being well-adapted to these conditions, and in some cases strongly challenged when the darkness of their nights has been interrupted.
A final worthwhile note is that the darkness may not be as deep as we imagine. Many creatures can see quite well at night. Though a dark night may be 100 million times less bright than a sunny day, there are creatures with eyes suited to the challenge. Some nocturnal bees are active well past dusk, and dung beetles are capable of seeing by only the starlight of the Milky Way (Dacke et al., in Current Biology this year). Though these hikes can bring us closer to this hidden world, there is a whole night-time world we will never ever be able to see.