The rainforest is a place of extravagance: beautiful, brightly colored and sometimes downright bizarre plants and animals of every conceivable form await those intrepid enough to travel to the tropics to seek them out. Here, the diversity of form is matched by a diversity in the way in which organisms survive, what biologists call their ecological ‘niche’. Many rainforest mammals, for example, live much the same way their temperate counterparts do—hunting for other animals in the dark of night, browsing for young leaves and shoots, or digging for roots and tubers. But in the tropics, where temperatures are higher and more constant, and where plants are orders of magnitude more diverse and abundant than in temperate regions, resources are available to support lifestyles that are impossible elsewhere.
In the photo above, a tiny pygmy marmoset, the world’s smallest monkey, clings nervously to the bark of a Parkia tree, taking turns between nervous vigilance and intense interest in a goopy sap that oozes from pockmarks scattered over the entire surface of the tree’s massive trunk. This sap, rich in sugars and nutrients, forms the basis of this tiny primate’s diet. What the marmoset does not receive from the Parkia’s sap it supplements with insects and whatever other small animals it can capture—weighing about a fifth of a pound, suitably-sized prey are few and far between.
Pygmy marmosets are not the only rainforest animals that feed on the nutrient rich gum produced by trees. Insects—butterflies, wasps, lantern bugs, cockroaches, ants, and beetles, to name a few—are attracted to the holes excavated by marmosets in the bark of trees such as the Parkia. For an insect, a visit to a marmoset’s gum hole is a risky affair, since sap flowing from a hole in the tree’s bark suggests a hungry marmoset is nearby. The nutrient- and energy-rich sap is clearly worth the risk, a fact attested to by the number and variety of insects visible at the holes.
The rainforest is also a place of extremes. In the western Amazon, pygmy marmosets feed from the same tree gum as the white witch (Thysania agrippina), a moth with the largest wingspan of any insect on earth. Measuring in at up to 13 inches across, the moth is as much as two times larger than the tiny marmoset!
Below, a lantern bug (Phrictus quinqueparitus) feeds on the sap of a Simarouba amara tree in the Costa Rican rainforest. The bug must often share its tree with the bizarre peanut-headed Fulgora laternaria, another lantern bug.
For those hoping to view wildlife, a visit to a tropical rainforest can be quite a frustrating experience. Unlike on the plains of Africa, rainforest animals can very easily conceal themselves among the dense vegetation, under a forest canopy that permits very little light to pass through. To make matters worse, many species avoid detection by resemblance to non-animal objects in their environment. Here, insects are the ultimate masters of disguise: stick insects and leaf-mimicking katydids imitate twigs and leaves—some even come complete with tiny spots meant to mimic diseased or chewed leaf bits. Still other insects sport patterns that allow them to blend in seamlessly on lichen-covered tropical tree trunks, some even with tiny frills and flourishes that bear an uncanny resemblance to mosses, fungi, and tree bark. By remaining unseen, small insects survive in a world teeming with hungry predators. Only one who is attuned to the jungle environment and the cryptic habits of its invertebrate denizens will appreciate the true, albeit hidden abundance of rainforest insects.
However, not all insects in the rainforest survive by hiding. Many biting and stinging insects—bees and wasps, mostly—invest little in camouflage, instead inviting hunters to attack with flashy colors and conspicuous behaviors. Those that do so are surprised by an unexpected counterattack and quickly learn that there are probably easier meals to be had. Importantly, predators are capable of remembering an unpleasant attempt to make a meal of an angry wasp, or an excited hive of stinging bees. So the rainforest, then, is full of predators, but those that have learned to avoid stinging insects—most predators probably learn this very early on—avoid insects that pack painful bites and stings. Given the extraordinary abundance of bees and wasps in the rainforest, this strategy appears to serve them well.
Below is a an example of just such a stinging insect: a wasp, right? Wrong. This is a picture of a katydid, a harmless relative of the crickets and grasshoppers. Biologists call such an animal a mimic: the katydid has escaped predation through protective resemblance—mimicry—of the much more noxious, stinging wasp, its model. In this case, the wasp must invest not only in a costly stinger and venom, but also in educating predators of its painful sting. The katydid knows nothing of these investments.
Amazingly, this katydid takes its trade one big step further than mere resemblance. Not only has this species foregone the typical, protective green coloration of most katydids, it has abandoned nearly every characteristic that makes it identifiable as a katydid at all. Instead of using its powerful hind legs for jumping the way katydids, crickets and grasshoppers tend to do, this individual gets where it needs to go by flying—in precisely the manner the wasp does. When it flies, the long hind legs trail behind, making the katydid nearly indistinguishable from its wasp model in flight. Even the antennae contribute to the deception, gesticulating back and forth, side to side, in a decidedly unkatydid-like, but wholly wasp-like manner. The katydid’s mimicry is exact to the finest detail.
Mimicry—the convergence, in this case, of not only the appearance but also the behavior of creatures as distantly related as a wasp and a katydid—is a potent testament to the transformative power of natural selection. And the wasp-mimicking katydid is but one example. The rainforest is overflowing with such wonder, if only we have the patience, and the eye, to look for it.
Deep within Peru’s Bahuaja-Sonene National Park, two rivers—one crystal-clear and strewn with rapids, the other swirling and muddy—converge to drain the pulsing heart of one of the world’s richest rainforests, a place known as the Candamo Valley. The indigenous Ese Ejja Indians call the region the ‘Last Forest Without Man’—a reference to the area’s extreme remoteness and the fact that no people, not even the Indians themselves, have ever settled here.
Candamo is one of the few remaining large, pristine rainforests left on the planet—tropical forests nearly everywhere else have been destroyed or degraded as tropical countries have scrambled over the past few decades to convert their natural resources into quick wealth. Pristine areas like Candamo factor heavily in an ambitious plan by tropical conservationists to bring humanity into the age of globalization while losing as little of our biological heritage as possible; only large, undisturbed areas can harbor the high numbers of species that will allow this to happen as surrounding forests disappear.
I’m here to survey butterflies—while detailed information are still lacking, some evidence suggests that as many as several hundred butterfly species might be declining throughout the tropical Andes and western Amazonian ecosystems. The reason? A rapidly growing human population, agricultural expansion, and increasing global demand for oil, timber, gold, and other natural resources have all devastated the rainforest habitats that butterflies depend on for survival. In Candamo, butterflies have a safe haven.
And while biologists currently know almost nothing of Candamo’s rich plant and animal communities, news from other, better-studied protected areas in the region are promising. Peruvian researchers working in the nearby Tambopata Reserve, for instance, have found more than 1,300 butterfly species alone, in only a few months of sampling at a handful of locations. So far, the protected areas are working.
Butterflies, of course, aren’t the only beneficiaries of these conservation areas. As I patrol a floodplain forest in the Candamo Valley with my butterfly net, the telltale signs of another rainforest inhabitant call my attention—the sound of crackling twigs and leaves and crashing as dislodged dead branches tumble onto the forest floor belie the presence of primates. Judging by the noise they’re making, I guess they are spider monkeys, among the largest arboreal mammals in this part of the world. As they approach even closer, I see that I am right.
Before I realize what’s happening, a huge dead branch comes crashing towards me, and explodes above my head and all around me in a hail of decaying shrapnel, complete with a swarm of angry, biting ants. Five seconds later, as I furiously pluck the ants away, another one. And another. I quickly realize I’m under attack.
As I look up, I notice around 10 monkeys staring down at me. One is shaking violently on the branch he’s on, staring me dead in the eyes. Another announces to the rest of the group with a special vocalization, “Get over here, you gotta see this!” These monkeys have never seen a human being.
In one of those magical moments I thought only happened to National Geographic explorers or, more likely, adventurers from a time long passed, we exchange incredulous looks—me, astonished at the unusual curiosity displayed by the primates above my head, and they, not knowing what to make of the bizarrely-clad, clumsy ground monkey. These animals have no fear of humans, which can only mean they have never seen any; elsewhere, monkeys flee intruders that would put them on their dinner plates.
After only a few short days I leave Candamo armed with a few new records for butterflies, but also with something greater. My experience here reminds me that there is still hope for conserving the greatest natural gift with which we humans have been endowed. In places like Candamo, monkeys live without fear of us, and we have a place where the raw, wild spirit of nature lives on. Indeed, we are greatly enriched by this place.
This is a captive orangutan at the Los Angeles Zoo, one of about 900 or so orangutans living in captivity across the world’s network of zoos. Although considered critically endangered by the IUCN, there are still many more orangutans living in the wild than in zoos—something which, sadly, cannot be said for many other critically endangered wildlife species.
Although the global population of wild orangutans is, at this moment, much larger than the captive population, it is crashing—and fast. Only two decades ago, essentially the entire island of Borneo in southeast Asia—including the Malaysian states of Sabah and Sarawak and Kalimantan in Indonesia, which today are home to about 50,000 orangutans—was draped in luxuriant, old-growth rainforest. The trees were dominated by one tropical family: the Dipterocarpaceae. Dipterocarp trees can grow to over 80 meters tall, among the tallest tropical trees in the world. Only twenty years ago, in the shadows of the great dipterocarps, roamed around 100,000 wild orangutans. In only two decades, the population has been reduced by half.
Today, the majority of the island of Borneo is covered in just one species of tree: Elaeis guineensis, or the African oil palm. When the palm oil fever took hold here, the rainforest quickly gave way. Since oil palms take roughly 5 years to start producing oil, the felling of the valuable dipterocarp forests subsidized the initial waiting period. Once they started producing, the profits from the sale of the oil lined the pockets of a few large producers and a wave of quick wealth spread across Borneo. Since the trade in palm oil is so lucrative, native forests that are cut for palm oil tend to stay cut. Worst of all, conversion of dipterocarp forests—the only home to wild orangutans on the planet—is currently not slowing down.
With the global population teetering at nearly 7 billion and growing along with the demand for natural resources as affluence spreads in developing countries, the plight of the wild orangutan seems inevitable. Tropical forests suffer disproportionately, due to the complexities of global trade—developing countries in the world’s tropics seem to be scrambling to convert their forests into tangible wealth, and developed and developing countries alike seem to have an insatiable demand for the products that come from the forest. The scale of the problem—including in Borneo, where palm oil is replacing orangutan habitat to satisfy global demand—can seem overwhelming, hopeless.
Until my recent trip to Borneo, I thought I had at least a rudimentary understanding of the palm oil dilemma there. When I flew over the island, however, I was taken aback by the true extent of the devastation: looking out the window of the small twin-prop plane during my one hour flight across the island, I saw almost nothing else. Palm oil plantations are interrupted only by small clearings in which their owners place refineries and, although they are very green and vast, they are essentially biological deserts—the overwhelming majority of species found in the native dipterocarp rainforests are completely absent in the plantations. Orangutans that are not killed or taken as pets by planters do not find food here and either starve or try to go elsewhere. Sadly, for many there is nowhere else.
For me, my experience flying over Borneo was the first time such a monumental environmental disaster presented itself so clearly: native forests and orangutan habitat are being cleared for just one single commodity, planted in neat rows in all directions. The solution—in theory at least—was also clear: stop buying palm oil! Unfortunately, palm oil is an incredibly versatile food product, and it’s found in nearly everything processed. In addition, the demand goes far beyond just the developed world, where people are much less likely to take interest in the abstract consequences of their actions as consumers. Frankly, a palm oil boycott is unlikely to have a decisive effect any time soon on the wild orangutan population.
Luckily, there are other things one can do. There are a number of organizations working in Borneo and southeast Asia to conserve threatened wildlife. The WWF Malaysia has a number of programs to save not only orangutans but also rhinos and pygmy elephants, and since they work very efficiently, every dollar you give goes a long way. Give this or any other nonprofit working in Borneo anything you can—500 dollars or just 5 dollars—it really will make a great difference.
Perhaps a more rewarding way in which you can help wild orangutans is to travel yourself to Borneo to see them. Borneo isn’t much of a destination on most Americans’ radar—it’s far away, and mysterious. But Borneo—and Sabah in particular, home to a large population of wild orangutans—is a very friendly place, with a well-developed infrastructure for tourism. By visiting this place and putting money directly into the pockets of the individuals and organizations working to save orangutans and other endangered wildlife, you are casting a vote for the protection of nature—ecotourism is one of the few economic incentives, if not the only economic incentive, to leave the forest standing. Without tourist dollars coming in, the forests will likely be cut. And what a wonderful reward for the traveller—to see one of the last remaining wild orangutans of Borneo, and in so doing help to ensure that our wild cousins, the ‘people of the forest’, have wild rainforests to roam, for now and forever.
When army ants swarm, you’d better watch out. And if you’re an insect, you’d better flee, far and wide. Army ants—as their name suggests—are raiders. During the day they fan out as the swarm advances through the rainforest, pinning down, dismembering, and taking any living creature they can subdue back to the nest to feed the queen and the rest of the growing colony.
Eciton burchellii, from the rainforests of Central and South America, is one of the most common army ant species in this part of the world. Colonies can be enormous, with some estimated at 2,000,000 individual ants or more. As you might imagine, a colony of voracious raiders this large can consume quite a large amount of food—the ants eat their way through any insect, spider, or other small animal not quick enough to escape their marauding hordes. How, then, do the ants ensure they do not exhaust their food supply?
When local resources are spent, the Eciton colony simply picks up and moves. They do this approximately every 35 days or so—the time it takes for the youngest ants in the colony to grow from egg to pupa, a period during which they must be fed regularly. When the brood is in pupal form, a stage in which the young ants change from larval to adult form and do not feed, it can be safely moved, but not before.
Unlike most other ants, Eciton’s nomadic lifestyle makes it impractical to build a permanent nest, potentially leaving the ants vulnerable to predators. It is, after all, strength in numbers that makes ants so successful at the game of survival. Instead of a permanent nest, Eciton come together nightly in the bivouac—a living mass made of the ants themselves, surrounding the queen and brood at its center. The ants hold the bivouac together with tiny hooks found on the tips of their feet, which they interconnect in a vast matrix. Eciton typically form the bivouac in semi-concealed places—under a fallen log, say, or between the buttressed roots of a rainforest tree—and although fairly open to attack by would-be predators, very few creatures dare hassle the pulsing mass of angry, biting ants.
What the bivouac lacks in reinforced, physical structure it makes up for with mobility. Like a great army on the move, the Eciton colony is large yet agile and ready to pack it up when the time comes, after the voracious ants eat through the local resources. All other insects be warned when the bivouac moves in to the neighborhood!
When people find out I study butterflies in the rainforest, one of several questions that I’m often asked is how long butterflies live for. My response: as long as it takes to get eaten.
Yes, the life of an adult butterfly is hard. If the purpose of the caterpillar is to eat and grow, then the purpose of the adult is to mate, and it is a struggle to mate before meeting the end. Indeed, with hordes of predators ready to make a meal of a juicy butterfly in the jungle, there is little time to waste.
For males, the struggle intensifies. Not only do they have to dodge attackers while seeking a female, but males must also compete with each other for the opportunity to mate with her. Since one competitive male can monopolize more than one female, the chance to pass on one’s genes isn’t guaranteed. So they must fight for that, too.
In a case of sexual… preemption, let’s call it, some male butterflies skip the whole seeking altogether. A male will hone in on a female pupa—where the caterpillar transforms into the adult—and wait around for her to emerge. As soon as she’s out, or perhaps even before, he mates with her. Biologists call it ‘pupal mating’.
The trouble is, all the males in the neighborhood are on to this trick, and they all want to be the first to mate with freshly-eclosed females. So what do they do? They all hang around her. They literally hang around, and even on, the female, waiting for her to emerge from her transformation. As soon as she’s out, they’re ready, but of course there can only be one in the end.
So what gives a male the advantage in this pupal mating strategy? A 1994 study by Erika Deinert et al. in the journal Nature set out to figure out just that. The authors figured there would be two forces—biologists call them selective forces—influencing the evolution of male morphology. One, larger males should be able to outcompete smaller males for a place on the female pupa. Second, and perhaps contrary to the first, males with smaller bodies should be able to mate more efficiently when the time comes. So which is it, larger or smaller males, that win the evolutionary contest?
When the researchers compared the ratio of wing length to body length in butterfly species that perform pupal mating to those that do not, they found that pupal mating species had larger wings relative to the length of their bodies. In those species, longer wings are used to shield the pupa from competitors once the male is in position and prevent others from landing, and smaller bodies are used to copulate more successfully once they’re on.
However, it’s important to note that there’s a limit to the benefits of large size—large wings might help a male secure his spot on the budding female, but they don’t do any good if he’s too big to mate. This is called ‘stabilizing selection’. And as we observe, males of pupal mating species aren’t monstrously large, only slightly so.
The observations by Deinert et al. provide strong evidence that the pupal mating strategy works, at least for male butterflies. From the point of the view of the female, it’s more difficult to see the benefit, although if being mated during or even before eclosion were very harmful it’s not hard to imagine females evolving to delay sexual maturity until fully emerged from the pupa. We need more experiments to better understand this remarkable behavior.
Going back to the initial question: How long does a butterfly live? Well, if you’re a female, it doesn’t have to be very long—you’ve likely got a male waiting to welcome you into the world, no need to waste precious time looking for him. And if you’re a male, it’s either as long as it takes to get eaten, or as long as it takes for a female to be born!
The Fakahatchee Strand in South Florida is one of the last places where the elusive and nearly-extinct Florida panther still roams. The panther, which has been extirpated from most of its former range due to hunting and habitat loss still prowls here due mostly to the Strand’s isolation and covering of dense, nearly impenetrable swamp.
But Fakahatchee’s swamps shelter another, perhaps more unexpected endangered organism, Dendrophylax lindenii—the ghost orchid. The orchid’s common name refers to the flower that appears to float in midair, since it has no visible leaves—photosynthesis occurs in the tangled roots that anchor the plant to a tree. However, the name could just as easily refer to its rarity—very few plants remain in the wild.
The ghost orchid, I should mention, is breathtakingly beautiful. The flower has delicate arms, which are actually the lower petal that bifurcates into two long tendrils that twist downwards. These are complimented by a long spur exiting the back of the flower that serves as its nectary, where pollinators acquire their reward for pollinating the flower. Indeed, the tantalizing beauty and rarity of the ghost orchid have made it among the world’s most sought-after specimens for unscrupulous orchid collectors—as a result, poachers have decimated wild populations.
But poachers are not the only threat to ghost orchid numbers. Even before collectors caught on to the orchid’s splendor, one of the very plant’s intrinsic characteristics had long ago sent it on an evolutionary trajectory towards rarity. The key to that trajectory is the long, delicate nectar spur, one of the plant’s most striking and mysterious features.
In 1862 Charles Darwin received a package from the horticulturalist James Bateman. In it, he found specimens of Angraecum sesquipedale, another species of orchid from Madagascar, off the eastern coast of Africa. Darwin expressed surprise at the ‘astonishing length’ of the nectar spur found on the flowers; he measured them at one foot long! Darwin predicted that there must be a moth with a proboscis (the equivalent of a moth tongue) long enough to reach the nectar at the bottom of the spur. As the moth extends its proboscis into the flower to feed, Darwin guessed, it would inadvertently pick up pollen on its face and deposit it on subsequent flowers, thereby pollinating the orchids.
Several years later Alfred Russel Wallace—Darwin’s contemporary and co-founder of the famous theory of evolution by natural selection—published a report in which he described a hawkmoth, Xanthopan morganii, from eastern Africa with a proboscis almost long enough to reach the bottom of the Angraecum nectar spur. Wallace predicted that such a moth would eventually be found in Madagascar.
Sure enough, such a moth was eventually found there—it turned out to be Xanthopan morganii, and members of the population in Madagascar had especially long proboscises. In fact, the hawkmoth’s proboscis is so long that it must back up over one foot from the flower before uncoiling its long proboscis to probe for nectar in the bottom of the Angraecum spur!
While incredibly fascinating for biologists, the problem with all this for the orchid is that there are few things that can pollinate it—or, rather, the flower rewards only those with proboscises long enough to obtain the nectar hidden one foot down at the end of the long spur. The only pollinator known to do this is the hawkmoth Xanthopan morganii and, without being able to receive any prize, no other pollinators visit it. If the hawkmoth disappears or becomes rare, so too does the orchid.
Which brings us back to our ghost orchid of Fakahatchee. An American species of hawkmoth—Cocytius antaeus—is the only moth on the continent with a proboscis long enough to reach the nectar at the bottom of the ghost’s nectar spur, and thus is its only pollinator. Cocytius is rare, and therefore so is the ghost orchid.
Such specialization is not unique to orchids. Posoqueria, which is found in the rainforests of tropical America, has long corollas that store nectar at their base, and which can only be accessed by animals with very long tongues or proboscises—most likely, a moth pollinates these flowers as well. These plants’ long nectar spurs have all been favored by the constant forces of natural selection—as the spur becomes longer over time and successive generations, so too does the moth’s proboscis to keep up. The flower benefits by having just one pollinator that is very efficient at its job—since nothing else visits the flower to seek its reward, very little pollen or nectar are wasted. The moth benefits as the sole beneficiary of the plant’s nectar reward and thus faces no competition from other pollinators.
Nor is specialization unique to plants and insects. Indeed, recent studies have shown that specialization is a common trait across groups as varied as fish, birds, and mammals.
And while specialization might be beneficial in terms of improved pollination efficiency, foraging, or other essential survival skills, ultimately the road to specialization is the road species take to extinction. Without versatility, specialized organisms survive only at the whim of finicky pollinators, resources, and changing environmental conditions.
Hopefully, the beautiful ghost orchid of Fakahatchee is not yet at the end of that road.
I always assumed that the ‘passion’ in ‘passion flower’ somehow referred to the plants’ exotic, almost sensual beauty—a combination of layered stigmas and anthers, colorful radial filaments, and delicate petals and sepals make the passion flower almost too beautiful to be real, especially in a place as non-exotic as the southeastern United States. Here, the passion flower Passiflora incarnata can be found growing as far north as Pennsylvania.
Imagine my surprise when I read that ‘passion’ actually referred to the crucifiction—or passion—of Jesus! That had to be a joke. But, as it turns out, that is where the name came from.
The passion flower—Flos passionis—got its name from seventheenth century descriptions of the flowers by Spanish priests in South America. Back then, the plant was known as ‘La flor de las cinco llagas’ or ‘The flower with the five wounds’. Those five wounds are the five sacred wounds suffered by Jesus during his crucifixion, represented by the five stamen with anthers. Above those, the three stigmas are the nails used during the crucifixion.
The coronal filaments—arguably the most striking feature of most Passiflora—represent the crown of thorns. Under the filaments, the five petals and five sepals are the 10 faithful disciples, minus Judas and Peter of course. This symbology can be seen in a 1610 drawing by Eugenio Petrelli for a book by the Jesuit Antonio Possevino.
And passion flower isn’t the only holy moniker to be used through the ages based on this symbolism: ‘Espina de Cristo’ (‘Christ’s thorn’), ‘Dorn-Krone’ (‘crown of thorns’), ‘Christus-Krone’ (‘Christ’s crown’), ‘Christus-Strauss’ (‘Christ’s bouquet’), ‘Marter’ (‘passion’), ‘Jesus-Lijden’ (‘Jesus’ passion’), and ‘Muttergottes-Stern’ (‘Mother of God’s star’) have all been used.
Personally, I see all this symbolism as a stretch—the passion of Jesus isn’t nearly the thing that comes to mind when I look at Passiflora. But whatever symbolism one ascribes to this flower, I think it’s safe to say that it is inarguably beautiful.
The Kapok tree, known to science by its Latin name Ceiba pentandra, is a true rainforest giant, perhaps the largest of all Amazonian trees, and among the largest trees in the world. Biologists call the Kapok an ‘emergent’ tree—that is, it emerges from the forest canopy and, with individual trees being measured at 150 ft. or more, easily towers over the rest of the forest.
Being very tall presents a number of problems for a tree. First of all, the emergent crown of the Kapok, unshielded by its diminutive neighbors, stands ready to catch any large gust of wind that might come along. And, with a shallow root system common among tropical rainforest trees, the Kapok faces the very real danger of toppling over during storms that occur frequently across the tree’s geographic range.
The Kapok solves the wind problem with buttresses—large, triangular projections that connect the tree’s trunk with the roots in the ground. Indeed, the buttresses of a mature Kapok can be impressively large, and they are very effective. In a study of other buttressed rainforest trees on the tropical island of Borneo, trees that had their buttresses experimentally removed by researchers quickly fell.
But wind isn’t the only factor potentially limiting a tree’s size. Emergent rainforest trees, like all other plants, require water, and extreme height poses a monumental challenge in the uptake and transport of water from the roots—from where water is drawn—to the leaves, where it is needed most, high in the tree’s canopy.
Given its towering height, how does the Kapok get water 150 feet or more off the ground?
The answer is best explained by physics. More specifically, it is largely capillary action that allows the tree to transport water so high.
Two fundamental phenomena underlie the capillary mechanism. The first is cohesion—the weak chemical bonds that allow individual water molecules to attract one another. Cohesion is responsible for the fact that a drop of water resting on the top of your hand, say, retains its rounded shape. This can also be thought of as surface tension—the reason that small insects can glide across the surface of a body of water. The second phenomenon underlying capillary action is adhesion—similar weak bonds that allow water molecules to attract, or adhere, to other substances. The reason, say, that the same drop of water will cling to the top of your hand when turned upside-down.
At the very top of the Kapok, in the tree’s crown, tiny pores known as stomata on the surface of the leaves open directly to the atmosphere. Rays from the sun strike the leaves and provide the energy needed by water molecules there to evaporate. The stomata are the exit points of the woody xylem—essentially long, thin, tube-like vessels that extend from the leaves, unbroken, all the way down through the tree trunk to the roots. As a water molecule breaks the surface of the water column in the leaf’s stoma and evaporates, cohesive forces cause it to tug upwards on the molecule or molecules directly behind it, which in turn pull on other molecules, and so on all the way down each xylem vessel to the roots.
Through capillary action, the tree is able to fight the enormous influence of gravity and transport the water it needs to incredible heights—all without the aid of any active pumps or large amounts of energy input. And the process is incredibly efficient. A single Kapok tree might be able to transport as much as 1,000 liters of water to its crown in a single day!
With the problem of water transport solved, the Kapok is free to invest in other problems that require its attention and energy. And in the rainforest, those problems are numerous—herbivores, diseases, and growth in a nutrient-poor environment are just a few. Thanks to some simple yet extraordinary physical properties of water, the Kapok is able to meet these demands and thrive, towering over the rest of the rainforest and inspiring us with its awesome size and beauty—what little of it we can appreciate from just its giant roots, anyway.
By Geoff Gallice
Bright colors in nature generally indicate danger. A brightly colored animal, for instance, typically uses its flashy hues to warn potential predators of a threat it might pose. Usually that threat is chemical: animals, ranging from insects and other invertebrates to frogs and snakes, have evolved a bewildering array of toxic poisons, venoms, and other chemical surprises that await would-be enemies. Bright colors help predators remember unpleasant experiences—they quickly learn to avoid the colorful meal that made them sick or stung them.
The passion-vine butterflies are a group of brightly colored or aposematic butterflies found throughout the rainforests of Central and South America. The most diverse genus is Heliconius and, as its common name suggests, this group of butterflies feeds on passion vines that are loaded with compounds known as cyanogenic glycosides—you don’t need to be a biochemist to guess that the cyan at the beginning of that name means it isn’t good to eat.
The glycosides are not harmful to Heliconiusbutterflies though. In fact, the butterfly larvae greedily devour vines that are high in these compounds. From them they are able to synthesize derivatives—similar chemical forms—that can be stored in the butterfly’s tissues.
Thanks to its diet of passion vines, a Heliconius butterfly is a mouthful of toxic, foul-tasting poison to a predator, which, in the day at least, is usually a bird. And since Heliconius are brightly colored, birds quickly learn to avoid them. Thus, the butterflies are chemically defended and, once the local birds have been educated, have essentially no diurnal enemies.
But what happens at night, when visual signals are less effective as a warning to nocturnal predators that hunt without good eyesight? Bats, for instance, hunt at night using echolocation or sonar, in which high-frequency sounds are produced and distance and direction to prey are judged largely by the time taken for the sounds to bounce, or echo, back to the bat’s ears. With echolocation, eyesight is unnecessary, and bats that echolocate are able to hunt very effectively even in total darkness. Many have monochromatic or otherwise very poor vision.
One peculiar aspect of Heliconius butterfly biology is their roosting behavior—that is, at night, they sleep collectively in aggregations. Roosts may contain as many as 10-15 individuals, although typically the number is less. The butterflies begin to gather at sunset and take as much as one hour to settle in for the night; they use tiny hooks at the ends of their feet to hang, upside down, from nearby small branches and the tips of dead twigs.
Many biologists believe that animals aggregate in order to dilute the effects of predation. Think of the chances of a buffalo being picked off by a lion alone versus in a large herd. The same should be true for aposematic butterflies in the daytime—considering that their bright colors most likely serve as a warning to birds, daytime Heliconius roosts would make a lot of sense. But the butterflies roost together at night, and it’s not clear why.
As a graduate student interested in tropical butterfly biology at the University of Florida, Christian Salcedo wanted to understand why Heliconius roost at night. Do the butterflies have any predators? Could they be using unseen signals to fend off attack, much in the way they use color in the day?
Salcedo decided to employ technology to study the question of Heliconius roosting at night. He created a camera system—he called it a “stand-alone nocturnal infrared camera system”—that would catch Heliconius predators in the act by activating video when an infrared beam was crossed.
After a number of nights the remote cameras had recorded several disturbance events, during which an animal—there was an agouti, an armadillo, a rabbit, and even a few stick insects—disturbed the butterflies, causing one or more of them to rouse momentarily from their sleep. In a jungle bumping with nocturnal creatures, this wasn’t very surprising. However, the most interesting event occurred when a bat was observed picking a Heliconius butterfly from its roost. Several moments later, the butterfly returned, apparently unharmed, to its roost-mates. Of course, Salcedo captured the event on video.
Watch videos of Heliconius roost disturbances here:
In some ways, these experiments shed light on the mysterious Heliconius night roosts. Based on the footage gathered with the remote camera system, we now know that Heliconius probably do have nocturnal predators. But the fact that the cameras recorded a butterfly returning to the roost after an attack, unharmed, actually raises many more questions.
Did the cameras capture an instance of predator education? That is, the bat clearly tried but did not eat the butterfly—in the video, do we witness the learning event that results in other Heliconius avoiding attack by at least this individual bat? If so, how can the bat recognize future roosting Heliconius and know to stay away?
Some bats are able to see ultraviolet light, which is reflected by parts of the wings of Heliconius. Could that be the key mechanism? Or perhaps predators like bats are able to smell pheromones or other chemical signals produced by the butterflies that act in a similar way as bright colors in the daytime, preventing a repeat bad-taste experience. Do bats typically capture prey and assess their palatability before deciding to devour them or let them go unharmed?
Video footage of a nocturnal attack is tantalizing, but we still have a lot to learn about the roosting behavior of Heliconius butterflies. The rarity of nocturnal attacks on the butterflies—Salcedo’s experimental roosts were attacked only several times in hundreds of hours of filming—make the matter of studying them more difficult. But, as is increasingly the case, technology coupled with a curious, motivated young graduate student willing to brave long, late hours and harsh tropical conditions can make great discoveries regarding the fascinating phenomenon of the roosting passion-vine butterflies.