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!
