Bats - Food & Feeding
On a global scale, bats take a wide variety of food, including fruit, leaves, bark, nectar, pollen, winged insects, beetles, bugs and termites, spiders, small mammals (especially rodents) birds, lizards, amphibians (especially frogs), scorpions, other bats and fish. Some bats, i.e. the notorious vampire bats, will feed on the blood of mammals and birds.
As their name suggests, fruit bats are frugivorous, feeding on fruit, berries, leaves and bark, sometimes taking nectar and pollen, and invariably a few insect larvae that dwell on leaves and fruit. These bats often pick the fruit from the tree and return to a feeding roost where they will eat it. The fruit is crushed and the juices and soft parts swallowed; some seeds will be spat out, others are eaten and pass out later in the bat’s droppings. Some Fruit bat species, such as the Mexican long-nosed bat (Leptonycteris nivalis), have a specialized brush on their tongue to help in the collection of nectar. As in humans, the lack of the gulonolactone oxidise means that Fruit bats cannot produce ascorbic acid (vitamin C) and this important vitamin must be obtained from their diet.
All British bat species, indeed 70% of the world’s bats, are insectivorous and/or arachnivorous (eat spiders), although microbats elsewhere in the world will eat other prey. UK bats have teeth typical of insectivores: sharp incisors and canines for gripping and biting with cusped cheek teeth to cut and break-up the food. During the summer months, our native bats will emerge shortly after sunset and hunt actively for a few hours, returning to their roosts shortly before dawn. This shift towards nocturnality is thought to allow bats to capitilise on a relatively untapped resource of night-flying insects (particularly moths), while reducing their exposure to predators and reducing competition with the day-flying birds.
Favoured feeding grounds may be several kilometres from their roosts and some species spread out and hunt in a defined territory. The small size of many microbats means that they have a high surface area to volume ratio and consequently a high metabolism. In a 1969 paper, Brian McNab at the University of Florida reported basal (resting) metabolic rate for the greater spear-nosed bat (Phyllostomus hastatus) as 70.7ml oxygen per hour in an 84g (about 3 ounces) individual. A healthy young adult male human has a basal metabolic rate of about 286mg oxygen per hour per kilogram; doing the conversion mathematics indicates that P. hastatus has a metabolic rate twice that of its human counterpart (almost 560mg oxygen per hour per kg). Consequently, bats need to obtain copious quantities of food; common pipistrelles have been known to take up to 3,000 midges during a single feeding bout, equating to the entire colony consuming tens of thousands of insects each summer.
Some bats will tackle prey larger than most fly insects. In the forests of Panama, for example, the spectral bat (Vampyrum spectrum) will take small mammals (mainly rodents) and the greater bulldog bat (Noctilio leporinus) catches and eats fish. Some bat species are even known to feed on their diurnal counterparts, birds, although such phenomena are rare. In a 2001 paper for the Proceeding of the National Academy of Sciences, Carlos Ibanez, Associate Professor of Research at the Doñana Biological Station in Spain, and three colleagues report on bat predation of nocturnally migrating birds. The biologists analysed 14,000 faecal pellets of the greater noctule (Nyctalus lasiopterus) and report that this species catches and eats large numbers of migrating Passerines. Passerines are a group containing almost half the world’s bird species, passerines are often referred to as “perching birds” and include sparrows, thrushes, flycatchers, robins and starlings. Ibanez and his team found that while insect remains were found in the bats’ faecal pellets all year round, there were two seasonal peaks in the appearance of feathers that corresponded to major bird migrations (i.e. March to May and again from August to November).
The question of how the bats catch these birds is still something of an enigma. However, netting of noctules in the La Rioja region of northern Spain during Ibanez et al.’s study, recovered two freshly severed passerine wings, later identified as belonging to a robin (Erithacus rubecula) and wood warbler (Phylloscopus sibilatrix), along with a greater noctule holding robin feathers in its claws. Such findings suggest that these bats catch and eat birds in flight, in a similar manner to the “aerial-hawking” seen in bats that catch and eat insects ‘on the wing’.
Perhaps the most notorious feeding method seen in the Chiroptera is that of sanguivory, or blood-drinking. The taxonomy of this, often maligned, group of bats is still a matter of some debate; although most people now agree that vampire bats belong in a subfamily (Desmodontinae) of the Phyllostomidae (the American leaf-nosed bats), some maintain that the differences between the leaf-nosed and vampire bats are sufficient to justify them a familial rank (i.e. Desmodontidae). Whoever you choose to believe, and based on work by Professor Robert Baker of the Texas Tech University and Rodney Honeycutt of the Texas AandM University I opt to follow the Desmodontinae scheme, the one thing that is clear is that there are three species of “true” vampire bat: the common vampire (Desmodus rotundus), hairy-legged vampire (Diphylla ecuadata) and the white-winged vampire (Diaemus youngi).
Vampire bats are native to the Americas, from Mexico to Brazil, Chile and Argentina and, contrary to popular misconception, have never been found in Transylvania. Although vampire bats were known in popular mythology long before the arrival of the Irish novelist Bram Stoker’s Dracula in 1897, it was perhaps the idea that Count Dracula could morph into a large “vampire bat”, capable of exsanguinating the average human, that immortalised vampire bats in our psyche. As is so often the case with popular mythology, vampire bats don’t actually live up to their folklore reputation. Not only do vampire bats not actually “suck” blood, instead making a small cut and lapping up the trickling blood, they’re also pretty small critters. A mature common vampire bat, for example, has a wingspan of only 35 to 40 cm (14 – 16 in), is seven to nine centimetres (about 3.5 in) long and weighs in at between 15 and 50g (0.5 to about 2 ounces). Moreover, reports of vampire bats feeding on humans are very rare; vampires generally opt for livestock (horses, cows, pigs etc.), with the hairy-legged vampire showing a penchant for wild and domestic fowl, particularly chickens.
Vampire bats have three infrared pit organs in their nasal sac, which have a heat-sensing function and help them gauge the best place to cut. Experiments by Uwe Schmidt at the University of Bonn in Germany in 1985 found that the heat-sensing organs of Desmodus rotundus are sufficiently sensitive to detect human skin from at least 13cm (5 in) away. Once the cut has been made with their razor-sharp teeth, the bat laps at the slow trickle of blood, consuming about 30mL (2 tablespoons) while the donor continues to sleep.
Vampire bat saliva is known to contain an anticoagulant, nicknamed “Draculin” by Venezuelan biochemist Ana Fernandez and her co-workers in their 1999 paper in Biochimica et Biophysica Acta but more formally known as desmoteplase, which prevents the blood from clotting while the bat feeds. Normally, when mammals get cut an enzyme in our blood called thrombin reacts with a protein called fibrinogen in the presence of air to form an insoluble protein known as fibrin, which forms a net over the cut, trapping blood cells and forming a blood clot. According to Lyudmila Zavalova at the Russian Academy of Science and her colleagues, the anticoagulants of “bloodsuckers” bind to the thrombin, preventing this reaction from taking place.
There are also five species of false vampire bat (family Megadermatidae) from Africa, Asia and Australia, four of which bite the necks of rodents they catch, but don’t actually appear to drink the victim’s blood. In their 1976 paper, Ronald Pine of the Field Museum in Chicago and his colleague listed several anecdotal reports that suggest other phyllostomids (American leaf-nosed bats), excluding the true vampires, occasionally feed on blood.