Bats and Their Flowers

Although not usually ranked among the most charming of earth's creatures, bats nonetheless comprise one of the important groups of animals that can pollinate plants. Species belonging to about one-third of the bat genera have been observed visiting flowers in darkness, to lap the sugary nectar and eat protein-rich pollen. Nearly all species of these nectarivorous pollinators belong either to family Pteropodidae (the fruit bats and flying foxes, especially in the subfamily of macroglossine bats) or family Phyllostomidae (the leaf-nosed bats, especially in the subfamily of glossophagine bats). Flying from sundown to sunrise, these aerial mammals are able to find appropriate flowers, even on moonless nights. Thus, they exploit a nighttime food resource different from that of the night-active hawkmoths (see MEMBG Newsletter 4, Winter) and the many different daytime pollinators, such as hummingbirds (see MEMBG Newsletter 3, Summer).

Currently 966 species of bats are recognized, making this the second largest radiation of mammals, after rodents. Textbooks classify all bat species as belonging to the order Chiroptera (meaning hand-winged), composed of two major groups: suborder Megachiroptera or megabats, including just family Pteropodidae; and suborder Microchiroptera or microbats, consisting of all other bats now classified into 18 families, including Phyllostomidae. There have been suggestions that bats evolved independently twice, but recent analyses of molecular data, derived from sophisticated analyses of nucleic acids, all indicate that bats evolved only once.

Most bat species are insectivorous, i.e., they are insect eaters. From insects they obtain protein. It is clear that flower visitation is a derived behavior, having evolved separately, therefore convergently, in the two suborders. Most likely, it developed as a specialization from frugivory, i.e., fruit eating, but possibly a specialization directly from insectivory. Even within suborder Microchiroptera, nectarivory has evolved numerous times. Microbats shifted to visiting flowers instead of catching flying insects to obtain protein and often lipids from pollen, but they gain protein also by consuming insects found on the flower parts and in the nectar. Most nectarivorous species live in the tropics or subtropics; bat pollination does not occur in Europe and temperate North America, nor in South Africa, with the exception of the morning glory Ipomoea albivena. Bat pollination is virtually absent in Australia south of Queensland. Nectariferous bats have been observed pollinating species just below the puna vegetation of the high Andes.

Flowers that are pollinated by bats may have a very strong, fruity, nocturnal fragrance or are often described as smell musky or "batty," or like products of fermentation, and nectarivorous bats tend to have a moderate- to well-developed sense of smell. The fruit-eating bats of Megachiroptera, especially, utilize olfaction to locate flowers as well as fruits, and they key on odors of esters, alcohols, aldehydes, and aliphatic acids, particularly butyric acid, as cues for locating food resources. Many microbats use smell to identify prey items; the phyllostomids seem to have the keenest olfactory sense, which is most developed in the flower-visiting spear-nosed bat, Phyllostomis stenops.

If you can picture the shape of the bat's head, with a projecting muzzle, you can imagine that a bat flower might properly be firm, large, wide-mouthed, and bell-shaped or dish-shaped. The bat forces its head into the flower through a mass of pollen-bearing anthers, trying to reach a deep nectary with its long tongue. The night-opening flowers of certain giant cacti, such as saguaro (Carnegiea gigantea) and organpipe cactus (Stenocereus thurberi) in Arizona, and cardón (Pachycereus pringlei) in nearby Sonora, Mexico, are perfect models of this design. These cacti are pollinated by Leptonycteris curasoae, a species entirely dependent on nectar and pollen for its existence and feeding on a broad range of plants from Central America through the American Southwest. Such bat flowers are white, cream-colored, or green, but may also be purplish to red or pink. Bats apparently have only rods in their eyes, and hence are considered to be colorblind and therefore are attracted to drab-colored flowers. The animal may hover, but also can perch and then hold onto the strong, sterile parts of the flower while feeding. One evidence of bat visitation is the present the next morning of thumb claw marks on the flower.

Another flower design for chiropterophily (pollination by bats) is termed the brush type, pincushion, or shaving brush type. This may be a large flower with many stamens, as in the African baobab (Adansonia digitata), which may have 2,000 stamens, or an inflorescence of many clustered flowers with showy stamens and essentially no corollas (petals), as in the legume Parkia clappertoniana in tropical Africa. Many brush-type, bat-pollinated flowers are found among the myrtles (Myrtaceae), sapotes (Sapotaceae), and mimosoid legumes (Mimosaceae), and the bats eat the stamens of these flowers. Some of the chiropterophilous species of Agave, which are said to smell somewhat like cabbage, are more like brushes than bell-shaped, and the bat-pollinated feh'i banana (Musa fehi), probably originally from New Caledonia and cultivated in the Pacific islands, has bracts that spread apart at night. As you might suspect, not every bat-visited flower fits the syndrome of characteristics.

Bats would have difficulty navigating within cluttered vegetation when flying in the dark at high speeds. Nectarivorous species tend to have a broad wing form with long tips that enable slower travel and even hovering at the face of the flower. Plants that use bats as pollinators most often place flowers in locations where the bat is less likely to be injured. Some bat flowers are formed at the tip of the canopy, as is the case with tall cacti. Some hang below the foliage on long, pendulous inflorescences, a condition termed flagelliflory; examples, among many, are the sausage tree, Kigelia pinnata, of Africa and species of the legume Mucuna in the American tropics. Some bat flowers are formed on the trunk and large lower branches, a condition termed cauliflory; examples are the species of calabash (Crescentia) in the American tropics. Because bats may have difficulty navigating through foliage, some bat flowers are formed when the trees are leafless. Bat-pollinated cacti often have no spines on flowers; spine growth is delayed until after pollination so that the bats are not impaled during their visits.

The well-known ability of bats to navigate adroitly in total darkness has of course been a subject of intense study by biologists, especially since the late 1950s. In the animal kingdom, bats are the quintessential example of a sonar system and echolocation. This requires many ingenious adaptations, including keen audition to detect location and size of prey and to avoid hazards and obstacles while flying. Bats produce and sense loud ultrasonic frequencies that we cannot hear. Such high frequencies cannot be confused with sounds emitted by other animals or with background noises, and the sound waves travel, albeit at 340 meters per second, only relatively short distances (meters) as short wavelengths, so they do not interfere with the echolocation of other bats in the vicinity.

The sounds are produced in the larynx as the bat breathes out late in the wing upstroke. The ultrasonic high-pitched sounds, emitted through either the mouth or the nostrils, result when the thorax is compressed. Some evidence suggests that the leaflike nose apparatus on phyllostomid bats focuses sound into a narrow beam, much like an acoustic lens. Emitted sound segments are short bursts at particular amplitudes and and duration, consisting of two different classes of sound waves two to five milliseconds in length. After emitting the segment, the animal waits for an echo, calculates the distances, and then emits a new set of sound bursts to recalculate the change in position of the nearest prey item or the obstacle. Amazingly, the frequency is adjusted by the individual bat to compensate for the Doppler frequency shift, whereby the sound is distorted depending on whether the individual is moving toward or away from the target. The bat's sound receivers are the enlarged external ears (pinnae), which, like microwave dishes, are a certain size and are oriented (fixed or movable) in such a way as to intercept the returnign echoes. To avoid being deafened by its own emitted ultrasonic sounds, the bat uses a particular muscle to disengage the sound-detecting apparatus in the middle ear (specifically, the stirrup or stapes) when the sound is emitted, and then repositions the bone to interpret the echo.

Nectarivorous megabats perch while feeding. They lack echolocation and therefore use only visual and olfactory cues to locate flowers and fruits.

Megabats have substantially larger eyes than do microbats, but even the microbats have very good discrimination of white versus black surfaces and can discern different shapes very precisely at extremely low light levels. Nonetheless, nectarivorous microbats have eyes ten to 40 times larger than their insectivorous cousins---an adaptation that results in greater depth of focus for near orientation and discriminating flowers. The nectarivorous microbats seem to utilize sonar echolocation to find flowers at long distance and then visual and olfactory cues to find and choose individuals flowers to visit. Bats also receive sensory information from fine whiskers around the long rostrum (nose) when it is inserted into the flower.

The bat may eat the pollen, anthers, and stamens of the flower, but it also can carry huge loads of pollen on its face and rough fur. So much pollen can attach to a bat during one visit that some of it is bound to be deposited on the next flower. One investigator estimated more than 1.5 million pollen grains per flower of cardón, and an individual bat may visit more than 30 flowers per night! Bats fly relatively long distances swiftly. Thus they can be effective transporters of pollen between widely spaced flowers and thereby perform effective cross pollination in the process. Although local visits are more frequent, bats may forage long distances from their daytime roosts. In some instances, bats travel long distances to a specific plant population; one fruit bat species in Africa flies two and a half hours nightly to visit a particular chiropterophilous plant species.

The nectar reward of bat flowers can be copious, even reaching numerous milligrams for a single flower of balsa (Ochroma) in the American tropics. A bat flower contains substantially more nectar than does a flower pollinated by insects. The large quantities of relatively thin nectar are sucrose rich or sucrose dominant in most bat flowers. Birds such as hummingbirds use these flowers when they are open the following morning. The extensible tongues of bats have grooves and minute projections to capture the liquid food. Some bat plants offer other sweet rewards, such as the bracts of Freycineta insignis, eaten by fruit bats, and the sweet flower parts of Madhuca and Bassia. Pollen-feeding bats tend to have fewer teeth than their insect-catching cousins.

No complete worldwide list of bat-pollinated plants has been published, but records exist for more than 40 plant families and species in hundreds of plant genera. Bat pollination is especially important in certain families, such as Old and New World Bombacaceae, including the baobab, kapok, and floss-silk tree species. Classical examples of bat flowers occur in the Bignoniaceae, including the sausage trees cultivated on the UCLA campus, and calabash, among others. Cobaea scandens, a cultivated vine of the phlox family (Polemoniaceae), has flagelliflory and is bat pollinated in western South America.

Although in Westwood we have some bat-adapted flowers, and these plants form fruits, we have not determined yet whether bats are responsible for fruit formation on campus. This sounds like a good nighttime homework assignment for a UCLA undergraduate student, one not afraid of bats and vampires.


[Return to Volume 4(4) Menu]