Eat or Be Eaten

It is not uncommon for our guests to ask us this one interesting question: Where are all the animals? When people imagine a tropical forest, they often picture a place so teeming with life that everywhere you look you see an animal. This isn’t a wrong assumption- the tropical forests are home to the highest diversity and biomass of animals of all the Earth’s terrestrial biomes. The problem is that most of the animals here are hoping not to be seen. Many of the animals are so well camouflaged that we walk right by without even noticing. 

On the other hand, tropical forests are home to some of the most brilliant and outlandishly colored animals in the world too. Butterflies, beetles, birds, and amphibians come in every imaginable hue. 

What determines the way an animal will look? Today we will explore three major driving evolutionary factors that can influence this outcome. 

The first is the fear of being eaten. One of the best ways to avoid being eaten is to camouflage. Many animals have adapted to blend in with their environment. 

The small fish in our rivers and streams are brown on top and white on bottom. If you stand above them on the bank, much like a predatory heron might, their brown dorsal side blends in with the rock, sand and gravel of the riverbed. If you are swimming under them, like perhaps a hungry dragonfly nymph, their white belly blends in with the light filtering through the surface of the water. 

Nightjars are a family of birds which are famous for their complex and effective camouflage. Their cryptically colored feathers so closely resemble dead leaf litter or a branch that even keen-eyed birders often walk within feet of them without seeing them. These birds are nocturnal, so this plumage helps protect them while they sleep during broad daylight.

Surprisingly, predation is also responsible for producing some of the brightest creatures as well. Some animals have evolved the ability to either produce or sequester toxins in their bodies, making them dangerous or at least unpalatable for predators to eat. These animals don’t worry about hiding, but instead use their bright coloration as a method of warning would-be predators to stay away. Giant bright red millipedes emit cyanide gas, fuzzy yellow caterpillars advertise their poisonous skin and spines, and the jewel-colored skin of dart frogs are all examples of this kind of adaptation. 

We all know that a flying insect with yellow and black stripes might be a bee or wasp, so when we see an insect that looks like that we give it a wide berth. Many species of harmless flies have evolved to look superficially similar to bees or wasps so that other animals leave them alone, even though they themselves have no venomous sting. This is called Batesian mimicry. This same phenomenon is seen in butterflies, snakes, frogs, and toads, which have evolved to look like their more dangerous counterparts.

The second driving factor is the need animals have to eat. The hunters, like the hunted, tend to rely on camouflage to better be able to sneak up on and attack their prey. The spots and stripes of the ocelot help it hide amongst the shadows of branches and leaves in the underbrush. Mantids disguised as leaves remain motionless, just waiting for a tasty morsel to wander within reach of its deadly appendages. Many hawks have the same dark-above light-below pattern as the fish, for much the same reason. From above they blend in with the ground and from below they blend in with the sky.

The third and last driving factor is sexual selection. This is responsible for many of the absurd and most colorful appearances in the animal kingdom. Due to their ability to fly and quickly escape from predators on the ground, many birds have evolved bright colors. On the Reserve we have species of tanagers that pass through the treetop in an assortment of colors, parrots, toucans, and of course the unmistakable crimson plumage of the cock-of-the-rock. All these bright colors are advertisements of sexual fitness. 

It is interesting to note that in many bird species it is predominantly the males that present these incredible colors. The females are often drab browns, olives, and grays. In this way we can see how predation’s selective pressure is not entirely absent in these species. Males, who are able to provide much genetic material with minimal investment, and who on average spend less time on the nest than their female counterparts, are more subject to sexual selective pressure than defensive selective pressure. The opposite is true of the females.

In blue morpho butterflies we see these pressure interplay in a different way. The upper side of their saucer-sized wings are an exquisite bright blue. It is hard to miss it when these beauties float over tropical streams and fields. When they land their wings fold up vertically over their backs, completely hiding the blue. The undersides of their wings are a cryptic brown and black design which helps them blend in with the foliage. Thus these butterflies are able to accomplish both goals. When they are active, they are able to advertise their presence to potential mates, but when they are at rest they are better able to hide from predators.

Life in the tropical forests is full of examples of bizarre and strange adaptations brought about from the interplay of these and other selective pressures. Every time we walk the trails here at Reserva las Tangaras, if we keep a careful eye out, we have the opportunity to see these forces at play.