Most visitors to the Galápagos will overlook its smallest inhabitants in favour of the larger, more charismatic species, like the blue-footed booby. But Charles Darwin was not your typical visitor. He’d had a love of beetles from an early age and described his collecting during his Cambridge University days as the activity that gave him the most pleasure. One day, out in the countryside, he’d grabbed hold of two rare beetles from beneath a chunk of bark, holding one in each hand. On spotting a third species, he wanted to free up a hand and raised his right fist to his mouth and let the beetle crawl in. ‘Alas! it ejected some intensely acrid fluid, which burnt my tongue,’ he wrote, forcing him to spit it out and return with just the beetle in his left hand.
Insects
When he first set foot in the Galápagos, Darwin was most interested in the rocks, but he kept a look out for beetles and other invertebrates too. When in the highlands, he repeatedly scoured the bushes for insects in all kinds of weather but only came across a few species, moaning that with the exception of the barren forests he’d explored in Tierra del Fuego, he had ‘never collected in so poor a country’. Of his beloved beetles, for instance, he only found twenty-nine different species. This may sound like a good return on five weeks’ work, but he would certainly have expected more. Indeed, there is a pretty tight relationship between the area of an island or archipelago and the size of its beetle fauna. The bigger the island, the more beetles. But the Galápagos sits well below this line, boasting the number of beetle species you’d expect of an archipelago half the size. This is probably down to a combination of factors: the relatively recent origin of the islands and the lack of water.
If beetles have struggled to settle in the Galápagos, so too have other insects that would normally thrive at this latitude. Take butterflies for instance. Over the water in mainland Ecuador, there are well over 2,000 different species. In the Galápagos, there are just ten. The paucity of butterflies looks even stranger when you consider that there are more than three hundred species of moth—except, that is, if moths, being principally active at night when the world is cooler, just find it easier to get established. It’s certainly a possibility.
The intensity of the equatorial sun may also explain why there’s just one species of bee in the islands. If others have attempted to colonise, they appear to have failed in their efforts. That the Galápagos carpenter bee managed to survive could well be down to its eclectic tastes. Whereas many other bees show a strong preference for a particular flower, the Galápagos carpenter bee seems happy to feed from an impressive array of flowering plants. In the Galápagos, it does not pay to be too fussy. As it forages for nectar and pollen in the coastal and arid zones, this species may play a crucial role in the pollination of many plants. Once it’s had its fill of nectar, an all-black female will carry pollen back to her larva, a single individual nestled in a hole bored into the branch of a palo santo (or other suitable) tree. American naturalist William Beebe remarked on the strange absence of insect voices. ‘Even when an occasional large, black bee appeared, it flew and drew nectar from the flowers silently, with muffled wings,’ he wrote in Galápagos: World’s End, the best-selling account of his first visit to the islands in 1923. ‘The trade winds made no sound among the thin foliage, as if they had blown so long and so regularly that no sough was left in this part of earth.’
FIGURE 5.1. Butterflies and moths collected by the California Academy of Sciences in 1905 and 1906. The large moth at the top (No. 7), Manduca sexta leucoptera, was collected as a caterpillar in Wreck Bay on San Cristóbal. Reproduced from Francis Williams, Proceedings of the California Academy of Sciences, Fourth Series 1 (1912): 289–322.
In contrast to the Galápagos plants, whose most common mode of transport to the archipelago was hitching a ride with birds, most insects—certainly the butterflies, the moths and the bee—are thought to have taken the aerial route. Even spiders, if small enough, can use strands of silk to parachute through the air. Darwin had noticed this earlier in the voyage when the Beagle was almost 100 km from land. He watched in amazement as thousands of tiny, dusky red spiders came floating on board, their silk threads trapping them in the rigging. This and observations of other spiders doing much the same led Darwin to the conclusion that ‘the habit of sailing through the air’ was ‘characteristic of this tribe’. In 1992, in the midst of an El Niño, one entomologist set up nets from boats whilst travelling between islands in the Galápagos. The stormy conditions blew more than 18,000 specimens of insects and spiders into his hands.
Larger insects not blessed with the power of flight or a silky parachute, like many beetles, bugs, mantids, weevils and crickets, probably came by sea, rafting on mats of vegetation or possibly even floating on the surface itself. Other invertebrates, like centipedes, mites and ticks, were probably carried by birds. Although most of this creepy crawly fauna is small, this is not always the case. When it comes to size, one of the Galápagos centipedes deserves a special mention. It goes by the name of Scolopendra galapagoensis and can reach up to 30 cm long. As with other centipedes, its front legs are modified into a pair of devilishly sharp pincers that it uses to inject a venomous cocktail into its prey of insects, lizards and sometimes birds. A couple of ornithologists, camping out on isolated Wolf Island in the early 1980s, found this ‘foot-long centipede’ a particular nuisance. ‘Slowly crawling about at night, when frightened they swiftly dash for cover and are usually found in the morning curled up in our cooking utensils or food cans.’ There are a couple of scorpions in the Galápagos too. Their stings, though painful, are usually not fatal to humans.
There are, of course, several entire branches missing from the Galápagos tree of life and the part of the canopy in which the invertebrates reside is no exception. There are, for instance, no mayfly, stonefly, caddisfly, or alderfly. It would be rather surprising if there were, dependent as all these insect families are on freshwater for the development of their larvae. As Bishop de Berlanga and his men found, this is just not something that you find much of in the Galápagos.
For those land-based invertebrates that did make it, the special set-up in the Galápagos has resulted in speciation, and hence the origin of new species, on a particularly impressive scale. Some 735 of 1,555 native insects, for instance, are endemic, found nowhere else in the world. The ratio is higher for spiders, with fifty-five out of around eighty species unique to the Galápagos. The Galápagos flightless weevils are more special still, with ten of thirteen recognized species originating in the islands. In fact, looking at terrestrial invertebrates as a whole, over half of all native species are endemic, made in the Galápagos.
Land Snails
When it comes to speciation in the Galápagos, though, one invertebrate family tops them all: the bulimulid land snails. All of them are pretty small (less than 3 cm) and difficult to see (often hiding out beneath boulders of lava), and they might not be the sort of thing that really excites you. But if, as has been suggested, a single ancestral colonist has given rise to around seventy different species to be found from the arid zone to the highlands, the Galápagos bulimulids might offer one of the finest examples of adaptive radiation anywhere in the world.
Darwin was interested in snails, boxing sixteen different species (including some bulimulids), and he got to thinking about how they might have reached remote islands like the Galápagos. ‘It occurred to me that land-shells, when hybernating and having a membranous diaphragm over the mouth of the shell, might be floated in chinks of drifted timber across moderately wide arms of the sea,’ he wrote. Bulimulid snails, like other land snails, can indeed seal up their shells to prevent desiccation, which would have been helpful for rafting out across the Pacific. But as anyone who has tortured slugs knows, salt is something that snails and their allied molluscs do not relish. Perhaps there was another way, ‘some unknown, but highly efficient means for their transportal,’ mused Darwin. ‘Would the just-hatched young occasionally crawl on and adhere to the feet of birds roosting on the ground, and thus get transported?’ he wondered.
It was a pretty good idea, and Darwin hit upon another of his delightful, ‘Heath Robinson’ ways to test it. Working from home at Down House in Kent post-Beagle, he took one duck’s foot (dismembered) and dangled it in an aquarium in which some freshwater snails had recently hatched. Within a matter of hours, dozens of minute snails were tenaciously stuck to the foot. No amount of waving it about could dislodge them, and a few survived for more than twelve hours out of water. In this time, Darwin figured, ‘a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point.’
We now know that land snails can do this too, worming their way in amongst the feathers of birds. Incredibly, snails might even be able to survive a journey through an avian intestine. In a recent study, researchers fed land snails to a couple of different bird species and found around one in six of them made it out the other end intact. One snail even emerged and promptly gave birth. This kind of hitchhiking makes more sense than rafting on the ocean. A snail that washed up on a shore would have struggled to make it across the salty, barren landscape to the more suitable habitat beyond. It is notable that bulimulid snails are completely absent from the coastal zone, and the case of Fernandina, where there’s a vast stretch of impassable lava between the shore and the safety offered by vegetation, is particularly telling. The snails that live on the verdant rim of its crater simply couldn’t have got there by walking. Conclusion: they must have been airlifted.
Divergence
So how could one colonising species have diverged into all the different bulimulids we see today? The two key ingredients for this to occur are isolation and time. With enough of these, two populations will diverge to become two species, such that if they find themselves reunited they no longer recognise each other as kin. For a small, water-dependent creature that moves at a snail’s pace, the landscape is full of obstacles: a freshly solidified lava flow, a patch of dry scrub, a deep crevice. With so many opportunities for isolation and several million years to play with, one species can easily become many.
In order to understand the mechanics of this divergence, it’s necessary to know a little genetics. The production of sperm and eggs is achieved through cell division. This demands the duplication, division and repackaging of the DNA contained in the nucleus of a cell. During all this DNA rearrangement, the odd mistake can creep in. So after many generations in isolation, the genetic makeup of one population is likely to have ‘drifted’ in a different direction to that of another. But how could such ostensibly meaningless changes in a genetic sequence give rise to differences in appearance and behaviour that have so much meaning? How, for instance, could this process of random genetic drift possibly account for the fact that those bulimulid snails living at altitude have a classical rounded look to their shell, whereas those at lower elevations have more slender, pointy shells? The short answer is that it can’t.
We need instead to consider a second mechanism that can lead two isolated populations in different directions, a phenomenon far more powerful than genetic drift. Snails trying to make it at different altitudes will face rather different pressures. At the highest altitudes, where there’s plenty of moisture, the default, rounded shell works just fine. At lower altitudes, however, this is not the case. As conditions become more arid, a well-rounded snail with its relatively large shell aperture will find life increasingly hard, with a very real risk of dehydration and death. A snail with a slightly pointier body whorl (a feature that is under genetic control) will tend to have a tighter opening to its shell, a characteristic that will put it at a distinct advantage in such droughty conditions. Those snails that manage to survive, therefore, will have a genetic makeup that is far from random (as it is with genetic drift). The hardy survivors will have been ‘naturally selected’, not by some forethinking, supernatural selector but by virtue of the simple fact that they didn’t die. At some point, which appears to be around 40m above sea level, the conditions become so harsh that few snails—however pointy they might be—seem able to survive.
It would be rather nice to test this explanation with an experiment, moving fat snails down to the arid zone to see how they fared. Prediction: the fat snails would struggle and die. Unfortunately, the Galápagos bulimulids have undergone such alarming declines in recent decades that this life-or-death experiment would simply not be possible. More than fifty species are now listed on the World Conservation Union’s Red List of Threatened Species, many of them considered critically endangered and several of them probably extinct. We cannot know for sure the reasons, but species that occupy and have become tied to tiny ranges are especially vulnerable. If there’s rapid change of any kind, like the transformation of highland habitat into agricultural land or the introduction of invasive predators like rats, the disturbance can be devastating.
FIGURE 5.2. Left, land snails. There is an incredible diversity of land snails in the Galápagos, almost all of them endemic to the islands. The Galápagos bulimulid family (to which specimens 12 to 16 belong) is one of the most remarkable examples of adaptive radiation. Reproduced from William Healy Dall, Proceedings of the Academy of Natural Sciences of Philadelphia 48 (1896): 395–460.
As brilliant as this family of snails is at illustrating the principle of evolution by natural selection, the land birds of the Galápagos have made the most significant contribution to the public understanding of Darwinian evolution. It is to them that we now turn.
