When the sun goes down in the subtropical forests of Puerto Rico, hundreds of thousands of bats emerge from the caves that stud the island’s northern end. After a day of sleeping, the animals are ready for a hard day’s night of hunting insects. For some of them, though, there will be no feast of beetles and mosquitos, and they’ll instead wind up a meal themselves for the snakes that have set up an ambush at the cave’s entrance.

After providing a warm, safe place to sleep all day, the caves become death traps once darkness falls. Puerto Rican boas slither in from all over the forest to turn the bats’ exodus into their own hunting ground. At the mouth of the caves, they lay on the ground, cling to roots and vines on the walls or hang down from the ceiling, their tails finding footing in the tiniest cracks and crevices. As the bats flit past them on all sides, the snakes wait, swinging like clock pendulums. When a bat brushes by or collides with a snake’s head, the snake grab it in its jaws, squeeze it in its coils and eats it whole.

The nightly ritual is played out on its largest scale at La Cueva de Los Culebrones, or the Cave of the Long Snakes. The cave’s estimated 300,000 bats can empty out in as little as three hours, providing the boas with an all-you-can-eat bat buffet. Biologists observers have seen as many as 20 boas hanging around the cave entrance, grabbing all the bats as they care to and chowing down.

Even with the steady stream of food flying right into their mouths, the snakes can be picky, and competitive. Several species of bats can inhabit the same cave, and the smaller ones tend to exit first. Although the snakes are in position and waiting as soon as the first bats, they often wait half an hour to an hour to start hunting, when the larger bat species take flight. This might be a strategic choice to save their energy for prey that gives them more calories in reward for their effort.

Sometimes, a snake that’s not having any luck with its own hunting will attempt to steal a bat from another snake. On one occasion, a biologist watched three snakes fight over a large bat carcass for over an hour and a half. By the end of it, the snakes were exhausted and the bat went uneaten.

Last summer, Neil Losin and Nate Dappen, a pair of biologists and filmmakers who run Day’s Edge Productions, were in Puerto Rico and heard stories about the snakes, and had to see and film it for themselves. The resulting video, Snakes in a Cave, is awesome, and captures some closeup, slow motion bat snatching action.

Snakes in a Cave from Day's Edge Productions on Vimeo.

Reference: Rodriguez-Duran, A. (1996). Foraging Ecology of the Puerto Rican Boa (Epicrates inornatus): Bat Predation, Carrion Feeding, and Piracy Journal of Herpetology, 30 (4) DOI: 10.2307/1565698

All over the world, the expansion of urban areas and other human activities has pulled the natural prey base out from under many ecosystems and brought large predators in close contact with people and the resources they can provide – livestock, garbage, unattended pets, etc.. Many predators will take advantage of these food sources, often leading to human-animal conflicts (see wolves in the western U.S., lions in Africa and a range of others). Sometimes, though, humans and wild carnivores will become fairly friendly neighbors.

On the Horn of Africa, spotted hyenas (Crocuta crocuta) have adapted well to dense human populations, and the two species have a unique relationship that benefits both of them. The hyenas almost never attack people and mostly leave their livestock alone, preferring to scavenge on waste and refuse from homes and businesses. The hyena’s iron stomach helps them fit in well here, and they can choke down almost any organic matter they find at the dump – from leftover porridge, to feces, to putrid, anthrax-infected carcasses. The people aren’t exactly thrilled with the hyena presence, but many villages and towns tolerate them as walking garbage disposals. The locals call them “municipal workers,” and refrain from harassing or killing them.

Every year in the spring, this arrangement collapses when members of the Ethiopian Orthodox Tewahedo Church celebrate the fasting period of Lent. For 55 days leading up to Easter, Ethiopian Christians abstain from meat and other animal products, which causes a slow couple of months for the local butchers, and a dearth of carcasses at the dumps.

Without the fat of the land to live off of, what’s a non-Christian hyena to do? Gidey Yirga from Ethiopia’s Mekelle University and colleagues from Africa and Europe observed the animals during last year’s Lent and found that thy turn to hunting instead. Despite their reputation as strict scavegners and opportunists, hyenas are formidable, efficient hunters. Stalking alone or in packs, they favor large prey (150-400 pounds) and a lone adult hyena can take down an animal four times its own size. In East African ecosystems where scavenging opportunities are rare, they hunt and kill 60–95% of their food.

Last spring, Yirga and his team went to three different sites around the city of Mek’ele and collected all the hyena droppings they found on the first day of Lent, the last day of Lent and 55 days after that. Several months worth of dropping gave them a glimpse of what the hyenas were eating before, during and after the fasting period. There isn’t much to be found in a hyena turd, since they can digest pretty much all parts of their prey except hair, hooves and a few inorganic components found in bones. It was enough for the researchers to go on, though, and they compared the hair from the 553 droppings they found to hairs from a reference collection to ID the animals the hyenas had been eating.

The scat studying revealed that before and/or after Lent, most of the hyenas’ diet is made up of goat, sheep, horse and cow – usually scavenged waste parts. While humans are fasting, though, the hyenas scavenge less (while the butchers’ supply dried up, the local veterinary school still provided some carcasses) and fill the gaps in their menu by actively hunting donkeys. The donkey is a natural choice of prey if an urban hyena has to go after a live animal. They’re the right size and, unlike the other local livestock, they’re not kept penned in in compounds. They’re easy pickings, and during Lent, the hyena’s donkey consumption can double from surrounding weeks.

Faced with a carnivore’s dilemma, the hyenas showed how adaptive they can be to the sometimes strange behaviors of their human neighbors. Yirga’s study is not only evidence of the hyena’s versatility, but also highlights trash as a delicate factor in urban carnivore management. If you give predators an inch, they’ll take a mile, or a donkey, and better waste management would create a less inviting environment for them. In areas where wild carnivores have grown dependent on livestock and garbage for lack of other prey, though, a too-tight lid on the garbage can could thin their numbers and have an ecological ripple effect.

Hat tip to Barbara J King for bringing the study to my attention. 

Reference: Yirga, G., De Iongh, H., Leirs, H., Gebrihiwot, K., Deckers, J., & Bauer, H. (2012). Adaptability of large carnivores to changing anthropogenic food sources: diet change of spotted hyena (Crocuta crocuta) during Christian fasting period in northern Ethiopia Journal of Animal Ecology DOI: 10.1111/j.1365-2656.2012.01977.x

Images: “Portrait of a Spotted Hyena” by Marieke Kuijpers. “Spotted Hyenas, Crocuta crocuta, at carcass of an Impala” by Jerry Friedman. Both used under a Creative Commons License

With their long, serrated snouts, the sawfish might strike you as a little like aquatic versions of Leatherface. Scientists used to think the two were behaviorally comparable: sluggish and maybe a little dimwitted, just waving their saw around blindly and waiting for something to run into it. New research by Barbara Wueringer and colleagues from Australia and the US, though, shows that the fish actually wield their saws with considerable skill. They’ve also learned that the sawfish’s nose knows, too, and is a complex sensor as well as a weapon.

The sawfish family has not been heavily studied (a shame considering that all seven species are endangered), and with only a few firsthand accounts of how the fish hunt, researchers could only speculate on how the saws were used. One of the more common suggestions was that they were bottom feeders and raked their snout through the sand to snag buried prey on the saw’s teeth. Other ideas were that the fish cut into the sides of whales or slashed their way through schools of fish.

To figure out what was really going on, Wueringer and her team captured young freshwater sawfish in northern Australia and watched them feed on mullet and pieces of tuna. The fish went after the food in two different ways, depending on whether it was floating in the water or lying at the bottom of the tank. In the water, they quickly slashed at the prey to impale it on their saws’ teeth or knock it to the bottom or into position to eat. Some of these strikes were strong enough to cleave the fish in half. At the bottom of the tank, the sawfish used the underside of their saws to pin prey down and then move them into position to ingest, and usually preferred to eat the mullet headfirst. This video shows all of these maneuvers.

The sawfish weren’t just spearing wildly, either, and the study found that their snouts are part lance and part smart missile, with buit-in tracking systems. Sawfish are closely related to rays, skates and sharks, and share their sensitivity to electrical fields and their ability to use them to navigate and detect other animals.  Wueringer discovered a few years ago that sawfishes’ saws are covered in electroreceptors, and when she presented the sawfish in this new study with electrodes that mimicked the electric signals prey would give off, the fish reacted the same way they did to the food.

The sawfish’s saw has turned out to be more impressive than anyone thought, but cause the fish a lot of trouble, too. Every sawfish species is listed as Critically Endangered by the IUCN Red List, in part because their saws are prized by shamans in Asia as tools for expelling demons and disease, and are easily caught on fishing hooks and lines.

Reference: Barbara E. Wueringer, Lyle Squire, Stephen M. Kajiura, Nathan S. Hart, & Shaun P. Collin (2012). The function of the sawfish’s saw
Current Biology, 22 (5), 150-151 : 10.1016/j.cub.2012.01.055

Image: “What a saw” by Lorenzo Blangiardi. Used under a Creative Commons license

In pre-colonial Mexico, the winged serpent Quetzacoatl was worshipped as a god. In modern-day Texas, rattlers are regularly fried and eaten. And in Pennsylvania, the snakes at the Philadelphia Zoo’s reptile house have quietly gone about their business while my girlfriend stood in the corner, eyes squeezed shut, shaking with fear.

People’s feelings toward, and relationship with, snakes have varied greatly depending on the time period, location and culture. Today in the U.S., if fear isn’t the most common reaction (it was our #1 fear 10 years ago), then it’s at least very high profile…

You might assume it’s an old fear, too. Certainly our ancestors, whether two hundred or two million years ago, encountered snakes more often than most of us do. In a new study published this week in Proceedings of the National Academy of Sciences, anthropologist Thomas N. Headland and ecologist Harry W. Greene suggests that pre-modern humans and the lower primates have indeed had a long shared history with snakes, and its more complex than we had thought. Because constrictors usually swallow their prey whole and intact and venomous snakes attack soft tissue, there’s almost nothing in the fossil record to tell us anything about snakes attacking and/or eating humans and other primates. To figure out the relationship extinct hominins and pre-modern humans might have had with snakes, examining bones wouldn’t cut it, so the Headland and Greene had step back in time, in a way. [Read more]

While they’re less likely to occupy Wall Street than a barn upstate, bats are as concerned as we are about the economy. Their economy revolves around energy instead of money, though, and a problem on the balance sheet can be a matter of life and death. If they spend more energy catching a meal than that meal provides, they’ve created an energy deficit that impacts their health, growth and survival.

There’s no need to worry about bats during a recession, though. New research from biologists in Germany suggests that bats are actually pretty savvy shoppers and are able to use echolocation to make economic decisions while hunting and stay in the black, calorically speaking. [Read more]

Orb-weaving spiders are those spiders that build the spiral wheel-shaped webs that we often tend to think of as the Platonic ideal of spider webs. The ones you find draped between two dewy branches in a sun-dappled meadow, spider sitting in the dead center lying in wait for hapless flys and other insects to collide with the nearly invisible, impossibly sticky threads and get trapped.

While this technique keeps the spiders’ bellies full, it’s not without its problems. Those of us who’ve walked through a doorway or between two tree branches only to come away covered in thin, sticky web threads know that it’s not pleasant (for me, it is the stuff of nightmares). For the spider, it’s absolutely disastrous. Severe damage to the web by humans and other animals that the spider has no intention, or hope, of devouring costs them the production of more silk for a new web, exposure to predators, lost hunting opportunities and missed meals, and ultimately plays with the odds of their survival.

What’s a spider to do, then, when getting through the day requires a web that’s inconspicuous enough that prey don’t notice it, but has enough presence to warn animals that would just wreck it?

The species of the genus Argiope pull off a contradictory, but seemingly necessary, signaling paradox by constructing “decorations” of zigzagging bands of silk on their webs. They make the web very conspicuous to the naked eye, even from a distance, but don’t seem to tip off prey insects that they’re about to stumble into a trap. [Read more]

On the wide, open plains of the American West, it’s more than the buffalo and the antelope that roam. Mormon crickets (Anabrus simplex) also sweep across the land in huge migratory swarms that can stretch six miles long and three miles wide. The crickets (a misnomer, they’re actually flightless katydids) can march up to a mile and a quarter a day in these groups, devouring every scrap of vegetation in their path and devastating agriculture in areas they pass through.

It sounds like a Biblical plague*, but consider the poor crickets. While the swarming behavior isn’t completely understood, entomologists think that it’s partly a strategy to avoid being eaten. Observations and experiments have shown that crickets that become separated from the group are easy prey and a big, cohesive group minimizes the risk of predation for any individual cricket.

Not that life in the swarm is any easier. In addition to consuming any and all plants they come across, the crickets often eat each other. One reason for this should be obvious, says a new study, “huge, concentrated numbers of crickets require huge, concentrated amounts of food. If the landscape doesn’t provide it, a fellow cricket will.”

A not so obvious side effect of this crickety cannibalism is that it might be helpful, even necessary, in keeping the swarm moving as a unit. A swarm of insects is simply the sum of its parts. The group’s movement, coordination, cohesiveness and persistence are the simple decisions and interactions of millions of individuals scaled up to the population level, and some of those decisions and interactions happen to involve one insect eating another. Another swarming insect, the desert locust (Schistocerca gregaria), tends to cannibalize traveling companions that have stopped moving or can’t keep pace with the group, and the threat of cannibalism influences their marching behavior. Individuals keep moving and maintain proper direction and pace to keep from becoming lunch for the guy behind them, and this helps maintain coherent swarm motion. [Read more]

A lot of people mistake harvestmen for spiders, but there are two big differences between the two orders of arachnids. One, harvestmen do not scare the living shit out of me and I do not need to my girlfriend to kill any that wander into our house. Two, the eight-legged freaks commonly called daddy longlegs are awesome beyond your wildest imagination, whereas spiders are demons from Hell and are not awesome.

Among the 6,400 known species of havestmen, there are females who can give birth without the need for a male to fertilize thier eggs. There are males who mate with multiple females and then guard all the eggs, sometimes from egg-eating females they’ve recently mated with. There are harvestmen who enjoy each other’s company so much that they live together in groups of 70,000+ individuals. Then, there’s the granddaddy of wieners, willies, dongs and johnsons, the 400-million-year-old fossilized harvestman that possesses the world’s first known penis.* [Read more]

One if by land, and two if by sea/And I on the opposite shore will be/Ready to ride and spread the alarm/Through every Middlesex village and farm/For the country folk to be up and to arm.

On April 18, 1775, Paul Revere told three Boston patriots to hang two lanterns in the steeple of the city’s Old North Church. A militia waiting across the Charles River in Charlestown kept an eye out for these signal lanterns and were prepared act appropriately as soon as they saw one or both of the lights stab out at the darkness. The meaning of the two lanterns has been memorized by countless American schoolchildren in the century and a half since Longfellow published “Paul Revere’s Ride.” One lantern told the militia that the British Army would march over Boston Neck and the Great Bridge, and two meant that that the Redcoats would take boats across the river to land near Phips farm.

Many, if not most, birds and mammals that live in groups have their own signals and alarms that alert members of the group to predators and other dangers. An alarm call can mean the difference between life and death for animals who didn’t detect the threat on their own and younger animals who are especially vulnerable to predation. Belding’s ground squirrels take a cue from Revere and use two different alarm calls to warn of two types of danger. Whistle alarm calls signal aerial predators and trill alarm calls signal terrestrial ones. A squirrel needs to react differently to each type of call and to each type of predator. Listeners respond to whistles by entering a burrow or another hiding spot, and adopt a “posting” stance on their hind legs in response to trills.

When young Belding’s squirrels first emerge from their burrows when they’re a month old, they don’t respond appropriately to the two different calls and if they respond at all, they typically just freeze. They pick up on the appropriate behavioral responses very quickly, though, often within five days of coming above ground. Watching the responses of mom, dad and the other squirrels could teach a youngster what they need to know pretty quickly, but Jill Mateo, from the Department of Comparative Human Development at the University of Chicago, wondered if there was also a physiological factor. For many species, the sight, sound or even odor of a predator spurs physiological changes that make individuals better prepared to track predators and the responses of other animals, hide and be still, defend themselves or run/fly/swim like hell. Maybe a squirrel’s body reacts differently to a whistle than it does to a trill – to two lanterns than it does to one, if you will – and helps prime the squirrel for one response or another.

For the first five days after they come aboveground, juvenile ground squirrels show a higher level of cortisol (a steroid hormone released in response to stress) than during the days before emergence or the weeks after. To see if the hormone had some role in ground squirrels learning appropriate anti-predator behavior, Mateo tested how the levels of the hormone changed in response to different alarm and non-alarm calls. She caught pregnant female squirrels at a few sites near the Sierra Nevada Aquatic Research Laboratory (SNARL) at Mammoth Lakes, CA and brought them back to the lab so they could give birth and rear their young. Around the time the babies would normally leave the burrow, Mateo placed them, in pairs, in a large, dark wooden box once per day and played either a recording of ground squirrel whistle alarms, trill alarms, squeals young squirrels use during play or a silent control.

Every time a squirrel heard a recording, Mateo took a blood sample from it. These tests continued until she had one blood sample for each of the four recordings from a squirrel or until the squirrel turned 35 days old (in some cases, she was not able to get complete samples from a squirrel before it reached the age limit or did not have a large enough sample to analyze). After two rounds of tests in 2006 and 2008, Mateo had partial samples from 32 squirrels and complete samples from 17 of those.

Mateo analyzed the samples and, using a squirrel’s cortisol concentration following the silent stimulus as its baseline, looked at the hormone’s percent change in response to the alarm calls and play noises. Because multiple squirrels from several different litters were tested, Mateo averaged the cortisol responses to each recording for each litter.

For all litters, cortisol concentrations were higher following playback of trill alarm calls than after the other recordings. The change in cortisol levels compared to the baseline was only significant in response to the playback of the trill alarm calls. The whistle alarms did not increase cortisol concentrations, but earlier research by Mateo showed that they do elicit bradycardia, a slower than normal heart rate.

So the squirrels do have different physiological responses to the two alarm calls. What relationship do these changes inside the body have with behavior, though, and what do they have to do with air versus ground attacks? Mateo hypothesizes that cortisol might not increase in response to whistles because attacks by avian predators often only last a few seconds and most birds don’t make repeated attacks if their first one is unsuccessful; the attack would be over before circulating cortisol increased. Bradycardia, however, is associated in young squirrels with decreased motor activity and enhanced information processing. If the heart slows in response to whistles, the squirrels can stay still and pay attention in case it needs to make a break for a hiding spot.

On the other hand, the terrestrial predators that squirrels respond to with trills usually spend a significant amount of time either moving around squirrel burrows or waiting near one to attempt an ambush. Increased cortisol makes glucose available as fuel to the squirrels’ bodies for sustained vigilance in posting stances and, if needed, multiple escape attempts.

Both of these physiological reactions increase arousal and attention in a variety of species, so both might also just aid young squirrels in noticing and paying attention to the responses that nearby adults have to the alarm calls, making for a faster association between the alarms and their appropriate responses.

References: Mateo JM (2010). Alarm calls elicit predator-specific physiological responses. Biology letters, 6 (5), 623-5 PMID: 20236965

Mateo JM (1996). Early auditory experience and the ontogeny of alarm-call discrimination in Belding’s ground squirrels (Spermophilus beldingi). Journal of comparative psychology (Washington, D.C. : 1983), 110 (2), 115-24 PMID: 8681525

Image: “Belding’s Ground Squirrel in the Sierra Nevada Mountains, California, USA” by Justin.Johnsen via Wikimedia Commons. Used under a Creative Commons Attribution 3.0 license.

There are some 1,400 described species of scorpion in the world, and while only 25 of those have proven they can take down a human being with their venom, many more of them can easily injure and kill smaller creatures. Given that, you’d expect scorpions to be important predators in desert food webs, but you might not expect them to be equally important as prey.

Yes, despite the pincers and the stinger and the venom, plenty of animals – among them, centipedes, tarantulas, lizards, owls,shrews and bats – regularly chow down on scorpions. Hemprich’s long-eared bat (Otonycteris hemprichii, at right), found in deserts in northern Africa, the Middle East and south-central Asia, considers scorpions a major food group, along with beetles, centipedes and spiders. In Israel’s Negev Desert, the proportions of these groups in the bat’s diet changes throughout the year, with scorpion fragments found in only 10% of bat droppings in the early spring and in a whopping 70% in the late summer.

That’s a whole lot of scorpions they’re eating, and while some of those are certainly weakly toxic Large-clawed Scorpions, the bats also prey on the Palestine yellow scorpion (Leiurus quinquestriatus, below). Everything you need to know about L. quinquestriatus can be summed up in its nickname, the deathstalker. These scorpions are considered some of the most dangerous in the world and possess a highly toxic venom that contains a grab bag of neurotoxins. Their sting can cause extreme pain, fever, convulsions, paralysis and death (via heart or respiratory failure), even in humans. These scorpions are also “sit-and-wait predators” that hunt by remaining quiet and still and lashing out at unsuspecting prey that wanders too close.

They certainly sound appetizing, but how do the bats deal with prey that present such a challenge to both detecting and disabling them? Carmi Korine, of Ben-Gurion University of the Negev in Midreshet Ben-Gurion, Israel, and colleagues found that Hemprich’s long-eared bat is what’s known as a gleaner, which means it takes prey from surfaces and not right out of the air, as many bats do. Korine and his team hypothesized that, like gleaning Pallid bats that prey on scorpions in North America, Hemprich’s bat might rely on passive gleaning to find its meals, simply listening for the prey to make noise instead of actively using echolocation to detect it based on its echo signature. The researchers figured that, if that was the way the bats hunted, then they would select prey based on 1) body size (bigger scorpions being easier to detect), and 2) toxicity (less venomous scorpions posing less risk of injury).

The team captured eight O. hemprichii individuals, set them up in a room with a scorpion buffet that included both living and dead Large-clawed Scorpions (Scorpio maurus palmatus), Israeli common scorpions (Buthus occitanus israelis) and, of course, deathstalkers.

When the bats took off to search for food, they spent a few minutes circling the room and dropped down directly onto a scorpion once they noticed it. They only went after the live scorpions, ignoring the dead ones and even walking right over motionless live ones if they missed on their initial divebomb, confirming that they glean passively and rely on prey noises.

Once they landed on a scorpion, the bats immediately started biting the scorpions’ heads. The scorpions did not take this lying down and fought back, stinging the bats on the head and face and, in one case, under the eyelid. The bats made no observable attempts to either avoid or disable the stingers and once they had killed the scorpions they often ate the whole thing, including the stinger and poison gland.

This is how the bats hunted all the scorpions. Contrary to the researchers’ predictions, the bats showed no preference among scorpion species based on either on size or toxicity, diving on their prey immediately after detection without any further inspection and going only on limited acoustic information. In 49% of the test sessions the bats actually went after the more poisonous of the available species (and in 24 direct comparisons, the deathstalker was chosen 50% of the time).

The researchers did not have an explanation for the bats’ indifference to the danger their meals pose, but speculated that either scorpions aren’t able to pierce the bat’s skin, or that the bats have at least a partial tolerance to the venom. Given that the bats regularly ate the stingers and venom glands, it seems more likely that their just tough enough to handle what the deathstalkers can dish out.

For other takes on this study, see Michael Marshall’s post at Zoologger and Zen Faulke’s post at Neuro Dojo .

Reference: Holderied M, Korine C, & Moritz T (2010). Hemprich’s long-eared bat (Otonycteris hemprichii) as a predator of scorpions: whispering echolocation, passive gleaning and prey selection. Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology PMID: 21086132

Images: “Otonycteris hemprichii” by Wikimedia Commons user Charlotte Roemer, used under a Creative Commons license.

“Deathstalker near Tzehelim, Israel” by Wikimedia Commons user Yair Goldstof, used under a Creative Commons license