The formation of the Puerto Rican karst dates from the early Cretaceous to mid-Miocene. On the island’s interior, the oldest ridges were once ancient reefs that surrounded volcanoes. Coastal belts arose much later along shallow receding shores. These vast and staggering landscapes have deepened over time due to the dissolution of limestone bedrock. In the northwest region, the Aguada limestone hills are referred to as mogotes, characterized by rounded caprock tops typically asymmetrical in shape. East-facing sides are more susceptible to erosion and show gradual slopes, whereas steep vertical walls stretch across the leeward sides. Limestone erosion occurs from rainfall at the surface, but also from collapsed sinkholes that contact subterranean rivers. As a result, karst topography is extremely variable and unpredictable at small scales, with twists and turns of porous shelfs. So although the contours may seem helpful to orient yourself in the rocky forest, the branching passages subtly become complex if you move kilometers away. Stepping carefully along the troughs, you can get a glimpse of the extraordinary system of caves lying below. For me, that is probably the most alluring geological feature of the karst.
Caves offer an entirely distinct set of environmental conditions that many organisms depend on. For instance, they are thermally stable, providing refugia for nocturnal animals who only venture out under the cover of darkness… like bats! Bats dramatically alter cave ecosystems by their sheer biomass, which initiates a cascade of species interactions from detritivores that feed on guano and their amblypygid predators, to parasitic flies that function within a remarkably narrow temperature range. At Cueva de los Culebrones, over 100,000 bats are estimated to exit the cave every hour. Such high numbers flying out of an entrance 5 meters in diameter make sites like this very attractive to bat predators— in particular, the endemic Puerto Rican boa (Chilabothrus inornatus).
Cueva de los Culebrones stretches almost 200 meters from the entrance to the back dead end, sloping downwards to a depth of 47 meters. Tucked to the left, just out of reach of a daylight’s trace is the first area where bats roost. Although this area is far smaller than the rooms where the majority congregate, there is still a startling change in temperature and air composition right at the entrance. Researchers who enter these enclosed spaces are at risk of carbon dioxide poisoning, and it isn’t uncommon to start feeling lightheaded in caves, even in the absence of noticeable increases in temperature. Just to give you a sense of how potent bat byproducts can be, they can carve spaces in cave ceilings. Warmer CO₂-rich air rises, creating condensation against cooler rock surfaces, and erosion from the gas’ chemical reaction with water forms large empty pockets that culminate in a dome top. As if cave exploration wasn’t already treacherous, the high water solubility of limestone makes the ground soft and damp, prone to collapsing.
But on to the boas and bats! Every day at dusk, a spectacle happens at the cave’s entrance. Whirlwinds of bats rise through the vertical column like a tornado that dissipates at the top end. The mixture of so many bat wingbeats creates a hollow resonance echoing from the depths. The sound through the wind tunnel is more or less constant, but if a light is shone into the cave, the bats recede, becoming hushed to the point of sounding like murmurs of speech from a distance. Although bats make excellent use of echolocation, collisions are inevitable among the hundreds of thousands of individuals, and occasionally one will brush (or smack) against your head.
Five bat species roost in large numbers within the cave:
(1) Antillean ghost-faced bats (Mormoops blainvillei); insectivorous,
(2) Sooty moustached bats (Pteronotus quadridens); insectivorous,
(3) Greater Antillean long-tongued bats (Monophyllus redmani); omnivorous,
(4) Buffy flower bats (Erophylla sezekorni); primarily frugivorous, &
(5) Antillean fruit-eating bats (Brachyphylla cavernarum); primarily frugivorous.
Peak activity rates vary, so the observed species assemblage of bats changes throughout the night. Surveys suggest moustached bats fly earlier in the night, flower bats have a large spike soon after, long-tongued bats escalate about an hour after sunset, and ghost-faced bats steadily increase as time passes. During the single night I took snapshots of bat activity, ghost-faced bats were the first to emerge in mass, followed by moustached bats, whereas the other three species flew at later hours and at lower abundances. On two occasions I watched an incredibly large species, the greater bulldog bat (Noctilio leporinus), fly towards the cave from the trail. Although this species was previously considered piscivorous and insectivorous, research at Mata de Plátano has revealed that they incorporate smaller bat species into their diet. Specifically, they snatch baby bats perched on the cave walls. For many bat species, a single parent will stand guard for nourishment and protection, but their efforts are probably futile in the face of comparably large bulldog bats.
Coinciding with an increase in bat activity, Puerto Rican boas descended from their arboreal resting postures down to the cave entrance. They dangled off branches, roots, vines, and the cave wall. The climbing and grasping ability of boas is astonishing, and many took on the most precarious hunting positions. One inspired individual worked its way down and up a centimeter thin vine that wrapped eight meters into the cave above a potentially fatal drop. Another who we called the “freeclimber” would extend off an upside down ~45° incline and flex its tail end tightly against a crevice. The majority gripped their tails to vines and suspended 60-90% of their bodies downwards in ambush. Over the course of several weeks, we observed 5-13 boas per night assume these positions, and while certain boas showed strong site fidelity, others changed perches on a daily basis.
Because of the high density of boas at the cave, there is no doubt conflict for occupying perch sites where bats most often fly across. Direct competition over the same food item sometimes happens, but boas never bite or physically harm each other. Nevertheless, they seem averse to contact, and several times I saw boas accidentally touch, and the two would move briskly in opposite directions. While pondering whether the boas ever fall, a medium-sized individual plopped down about four meters onto the cave floor. Another large individual had a recent scar on its face, and it’s amusing (and morbid) to think that falling to death might be one of the principal sources of mortality for cave-dwelling boas.
Plucking a bat out of flight mid-air is a challenging endeavor, but Chilabothrus have acquired an effective strategy to reliably catch bats. Rather than strike an incoming bat, they wait until a bat accidentally touches their snout or upper body, then the boas will constrict instantly to take hold of their prey. Once a bat is immobilized, the boa will take several minutes to contort itself into the required position for ingestion. Despite the insane abundance of bats, boas may sometimes take hours before a single bat is caught. Other times, they consume 3-5 over a couple of hours. One boa who was unsuccessful started to gape and twist back and forth, in what looked like a last-ditch effort. These movements might increase the chance of prey capture but must be energetically costly over long periods of time. Shortly after, it tucked back towards the vine and slithered away.
This blog post features selection of boa-bat interactions I observed at Cueva de los Culebrones. It was a unique and ethereal experience to gaze upwards and witness such a high frequency of snake predation events in the heart of the karst.
All photographs in situ [1] unless otherwise stated
