FORAGING & DIET
As would be expected from a class of mammals that comprises almost 25% of the total number of mammalian species of the world, found in all but the extreme polar regions, bats as a group consume a wide array of food types, including...
1. insects and arthropods
insectivory, such as the Little brown bat (Myotis lucifugus)
2. flesh of other vertebrates
carnivory, such as the False Vampire (Vampyrum spectrum)
3. fish
piscivory, such as the Bulldog bat (Noctilio leporinus)
4. fruit and/or flowers
frugivory, such as the Gray-headed flying fox (Pteropus poliocephalus)
5. pollen and/or nectar
nectarivory, such as the Long-nosed bat (Leptonycteris sanborni)
6. a variety of food items
omnivory, such as the Short-tailed fruit bat (Carollia perspicillata)
7. and of course, blood
sanguivory, such as the Common vampire (Desmodus rotundus)
When scientist initially began to classify Chiroptera, it was thought that they could be easily categorized by types of food consumption. Unfortunately, that wasn’t the case. Many bats do not restrict their diets to only one type of food. Often a bat was named by what it’s perceived food type was. For example, Vampyrum, Vampyrops, Vampyrodes, and Vampyressa all erroneously refer to supposed vampire habits when indeed the latter three are frugivorous and the first is carnivorous! With the exception of the megachiropterans, who are frugivores, many common names allude to dietary preferences. While each of the following do eat the food they are named after, they are not exclusive consumers of that food type. Cuban Frog-eating bat, Hairy fruit-eating bat, Buff Flower bat, Fisherman’s bat to name a few.
Drinking
In these two pictures, Yuma bats have found a source of water at this swimming pool. They skim along the surface of the water, sticking their tongues into the water as a CL-215 water bomber would do!
Insectivores
Since 70% of all bats are insectivores, let us begin by discussing their foraging habits. There are three general categories of foraging styles used to capture insects.
Aerial
Large sized bat of the species Hipposideros, for example, display a “lazy” method of hunting. They will leave their roost in the evening, and take up their position on a perch within their hunting area and wait for their prey to come to them. While waiting, they scan their hunting area with the use of low frequency acoustic signals, emitted as long, constant cries, which permit long-range target detection. When prey crosses their beam, they track the prey item, figuring out the best intercept location. When the prey approaches the position, they leap from their perch and grab the food item, returning with it to the perch, consume it, and start over again. This is one of the more energy efficient ways to hunt, but it requires the food to come to you. If there is no food in the area, then either moving, or going hungry will result.
More costly in terms of energy use, foliage gleaning bats like the fast flying Mexican Free-tailed bat (Tadarida brasiliensis), will fly along regular flyways scanning with their echolocation utilizing a broad-band sweep through a series of high frequencies. This permits short distance detection and pursuit capabilities. When prey is detected, it will be captured in one of several ways. It may be taken directly by mouth; it may be ‘swatted’ by a wing towards the mouth or interfemoral membrane; it may capture the prey in the interfemoral membrane; or, as in the case of the Red Bat (Lasiurus borealis) there may be an acrobatic manoeuvre involving an aerial somersault while wrapping wings and tail membrane around the insect! Bats eating larger insects like beetles, will often take the food item to a perch where the head, legs and wing covers will be clipped off. The wings of large moths are usually culled before eating as well.
Foliage gleaning
This method is very similar to aerial hunting, except that the bat scans for insects as well as arthropods such as spiders sitting on the tops of leaves. When detected, the bat will alight on the foliage and capture the prey.
Terrestrial acquistion
Bats like the Pallid bat (Antrozous pallidus) will fly close to the ground scanning for crawling arthropods like beetles, crickets, and scorpions. When detected, they will land on the insect, and either consume it immediately or return to a perch to eat. These bats are often quite agile when on the ground, unlike most bats.
Carnivores
As one might expect, carnivores bats are generally larger sized bats than the average (typically greater than 40 grams), and boast the largest microchiropteran member Linnaeus’ False vampire (Vampyrum spectrum) who has a wing span of nearly one metre. Their diets may consist of any combination of small rodents, birds, bats, frogs, lizards, and frogs. Two species of False vampire bats, are well known for their consumption of other bats. They have been observed throwing themselves onto a wall where an unsuspecting bat may be roosting! Their is controversy on whether or not these bats actually take other bats on the wing.
Piscivores
The fishing bats, of which there are only two species that I am aware of, live in the tropical and subtropical regions of the New World. these bats have long legs, with very large feet and strongly hooked claws. Both species have a long calcar which helps to hold the interfemoral membrane out of the water. They forage by flying low and slow over the quiet water, echolocating against the top of the water to detect ripples that are being created by small fish swimming close to the surface. They then gaff the fish with their toes, haul it out of the water and grasp it in their mouth. They may eat it on the wing, or go to a favourite roosting site. They may catch as many as 40 fish per night. They also eat insects, and will catch various insects that are on the surface of the water. It is thought that this may be how fishing bats evolved - by eating insects off the water surface, then graduating to catching fish from beneath the surface.
Frugivores
Fruit-eating bats consume many different types of fruits, including those that are not considered palatable to man such as wild figs and the fruits of the kapok tree, and even pepper. They also consume commercially grown fruits such as mangoes, guavas, bananas, peaches, apples, papayas, oranges, coconut fruits, and many small berries. Frugivorous bats are usually slow flyers (their prey is not likely to get away from them) and are attracted to a tree or plant by the smell of its ripening fruits.
There are some important environmental issues involved with these types of bats. The seedlings of most tropical plants will not grow and mature in the shade of the parent plant. Fruit eating bats provide the means for the plant to disperse. By consuming the seeds while eating the fruit, the bat will fly away and excrete the seeds in it’s faeces in another location. It is estimated that 95% of seed dispersal of the fruit trees of the tropical rainforests are accomplished by bats. Some plants have evolved to the point of requiring bats to be involved in their reproduction. The seeds of the Ficus plant will not germinate unless they have passed through the digestive system of a bat or some birds.
Nectarivores
There are three groups of bats that feed on nectar or pollen. One of these is found only in the Old World megachiropteran family of Pteropodidae. The other two live in only the New World (Glossophaginae and Brachyphyllinae or Phyllostomidae). Generally speaking, these bats have a long muzzle, and like hummingbirds, are able to extend their tongues a considerable distance. They sometimes eat insects, but usually only because the insects are inside the plant at the time they arrive to eat. Their jaws and teeth are not suitable for chewing, since their diet is primarily liquid. Like the frugivorous bats, nectarivorous bats fly slowly. They are capable of hovering flight, and will feed at the mouth of the plant much like a hummingbird. The pteropodid bats do not hover, and will land on the flower heads, or adjacent to the flower. These bats are extremely important for the pollination they perform. Plants that live in hot, desert like climates prefer to stay closed up conserving moisture during the day. They will open up in the cooler part of the night, displaying their usually white or light coloured petals making it easier for the bats to see them in the darkness. The flowers are often positioned in such a way to accommodate the bat, by either the flower opening upwards, or hanging downwards to facilitate the bats approach. There are over 400 plants requiring bats for pollination including cactii (saguaro and agave - from which tequila is made) to bananas!
Sanguivores
This is likely the most unusual feeding strategy of all, and has caused the most concern worldwide, even though the three species of blood eating bats only live in the New World. This particular food habit is unique not only to bats, but to mammals, and perhaps even vertebrates. The Common vampire (Desmodus rotundus), the White-winged vampire (Diaemus youngi), and the Hairy-legged vampire (Diphylla ecaudata) are the three species of blood eating bats. They live in the tropical and subtropical regions of the New World, with Desmodus and Diphylla extending their range into the temperate regions of North and South America. The Common vampire (Desmodus rotundus) eats blood from mammals, and sometimes birds, while the other two are primarily restricted to birds. They hunt within 5 to 8 km of their diurnal roost, and likely due to their specialized digestive system, they will altruistically share food with members of their roost when one of the roost mates is unsuccessful while hunting! These bats have specially modified tongues with grooves on the sides that move the blood by capillary action as it laps from the wound. They do not suck blood. They have highly specialized dentition including large razor-sharp canines and incisors. They use the larger molar teeth to shave off hair or feathers that make it difficult to get to the skin. They inflict a small V-shaped wound with a quick bite that rarely awakens the prey. The would is kept open with an anticoagulant in their saliva. (The anticoagulant is estimated to be 10 times more powerful than any of the anticoagulants used today in modern medicine, in the treatment of cardiac and cardiovascular patients). This may present problems for some victims, as the wound may continue to ooze blood for some time after the bat has finished eating. While the loss of blood will be minimal in larger animals or humans, it provides a site for various types of infections to start. Also problematic, is the ever potential for rabies to be passed on. It takes about 20 minutes for the meal to be completed and the bat will consume approximately 2 tablespoons (or 20 ml) of blood daily. While the amount of blood consumed is small, it is not recommended to feed Vampire bats.
Unlike most bats, vampires are very agile on the ground. Studies have shown that Desmodus can leap into the air as high as 18” - vertically! They are also quite fast moving horizontally across the ground and have been clocked as fast as 2.4 m/sec!
Foraging
There are many different foraging strategies used by bats. Foraging typically begins shortly before, or at dusk, and continues throughout the night. Many bats will stop during the night to rest, or digest food, and then continue again. With Little brown bats (M. lucifugus), it has been found that they forage for about two hours after sunset, during which time they will consume 60 to 65% of their nightly diet. Some studies have shown that 20 to 30 % of the nightly diet is consumed in the first 20 minutes of foraging!
In the many species of bats, foraging areas are visited night after night, and such is the case of Big Brown bats (Eptesicus fuscus). These bats are quite predictable in their routing, and protective of their territory, chasing away other big brown bats that might venture into it. They are however tolerant of other species that hunt in “their” area, in particular, Myotis and Pipistrellus species. Recent studies have also shown that Spotted bats (Euderma maculatum) are also regularly visiting the same areas and flying the same circuits. Some work has shown that vampire bats will return to the same area looking for the same prey night after night!
Various species of bats will forage in various types of habitat. Edges, both vertical and horizontal, seem to be important for both commuting and foraging for food. However, recent studies are indicating that bats are not using the open areas of clear cuts for hunting. Other studies show that these same area are being used for roosting, so we look forward to more work in these respective areas. One thing that has been made clear is the fact that bats are preferring to use older growth forest for work, play and sleep!
