An in Depth Look at the Natural History of Bats
Bats are amazing mammals that are not only critical to many ecosystems around the world, but are also fascinating to watch and learn about. This section takes a more in depth look at some interesting natural history articles about bats.
By Shannon Currie
Bats represent one of largest and most diverse radiations of mammals, illustrating the marvel of evolution. They are the only mammals capable of true powered flight, possess an intricate and sophisticated body structure, and are outnumbered only by rodents for total species amongst mammals. Yet, the story of bat evolution has been a source of contention for many years. Answers have been hard to come by and this great debate has been spurred along by a lack of fossil evidence. Ancient bats, very similar to their modern counterparts, were small and delicate and likely lived in tropical climates where decomposition is quick. This means that for a bat to become fossilized it needed to have died in a place where it was covered in sediment almost immediately, shielding it from scavengers and sources of decay. The fossils that have been discovered demonstrate very few structural differences to modern day bats, complicating the issue. What is agreed upon by all evolutionary biologists is that bats form a class of mammal all their own.
To appreciate how distinct bats are and understand their evolution we must look at their defining traits; their wings and the ability of most bats to echolocate. Bats underwent a unique suite of morphological modifications that enabled them to fly and echolocate and distinguished them from other mammals. Their wings are formed from elongated forearm and finger bones that support a thin wing membrane, giving them incredible manoeuvrability and a distinct form of flight. Echolocation was permitted by the alteration of three critical bones in the ear and throat. For example, the cochlea (structure in the inner ear) is enlarged relative to other skull structures, which makes bats better able to detect, and discriminate between, high-frequency sounds. It is clear that both these features contributed significantly to the diversification of bat species. However, this generated the important question- which came first, echolocation or flight?
Over the last decade three competing hypotheses have been in play. The flight-first theory suggests that bats developed flight as it facilitated speedy foraging, with echolocation following as a more accurate way to catch prey at night. In contrast, the echolocation-first hypothesis posits that protobats hunted aerial prey from perches using echolocation and later developed flight from gliding to return to these perches more easily. The final theory proposes that the two processes evolved simultaneously. This theory arose after studies showed that it is energetically very costly to echolocate when stationary and that cost of echolocation becomes negligible when in flight as the flight muscles help to pumps the lungs. Until recently, fossil evidence had been unable to definitely rule out any of these hypotheses.
Evolutionary biologists are always in search of the ‘missing link’; the fossil specimen that shows an intermediate stage of animal evolution. In the evolution of birds the discovery of the fossil Archaeopteryx improved our knowledge of the transition of modern birds from their reptilian ancestors. In the 1960’s a new bat fossil was discovered in the Green Lake Formation of Wyoming. Icaronycteris index was found to be 52.5 million years old and was the oldest bat fossil on record. For a long time I. index was considered very important in the study of bat evolution as it possessed a distinguishing feature from younger specimens. This bat retained a claw on the second digit of the wing, thought to be a remnant from terrestrial ancestors. However, the most significant thing about this fossil was that it mirrored extant species in almost every recognisable chiropteran characteristic. It possessed features suggestive of an insectivorous diet, full powered flight, and the ability to echolocate. Although an important fossil, unfortuantely I. index did not successfully complete the search for a transitional species.
It wasn’t until 2008 that a new fossil species was identified. Onchonycteris finneyi was also found in the Green Lake Formation and shows both ancient and modern bat features. As its name suggests (Latin for clawed bat) O. finneyi can be clearly distinguished from I. index and modern bats as it posses a claw on each digit of the wing, left over from its terrestrial ancestor. This specimen also shows proportionately longer hind limbs and shorter forelimbs than all other bats. In fact, their limb proportions are comparable to other arboreal mammals such as sloths and gibbons, suggesting that they may have evolved from animals with similar forms of locomotion. The arrangement of its wings suggest that O. finneyi could fly, however its wing ratio indicates it most likely flew in a combination flapping/gliding motion, supporting the idea that bats evolved from gliding ancestors. Finally, O. finneyi provides an answer to the question of echolocation versus flight. The ear and throat of this fossil lacks the modifications required for echolocation, indicating that bats could surely fly before they began echolocating.
DNA evidence has shown that all 26 families of bats (19 extant, 7 extinct) had evolved before the end of the Eocene period, 33.5 million years ago. This suggests a rapid diversification of bat species following the evolution of echolocation and likely coincided with a rise in global temperature and significant diversification of plant and insect species. New evidence has also been discovered to suggest how bats were able to take to the skies, developing wings from their hands. Morphometric analyses show that the length of the third, fourth and fifth digits, relative to body size, has not changed over course of bat evolution. This suggests that the bat wing may have evolved abruptly and very quickly. One study suggests that this swift evolution could be due to the alteration of a single gene during development. Comparing the development of bats and mice, scientists were able to pinpoint the time period during embryonic development when the critical forelimb digits elongate. With this information they predicted that the difference between limb development in these two species relates to a gene that codes for a growth factor. This gene is called bone morphogenetic protein 2 (Bmp2) and was found to be expressed 30% more in the forelimbs of bats than in mice. This intriguing insight may help to explain why we have had such difficultly finding further intermediate fossil specimens linking bats to their small mammalian ancestors.
Even with these new pieces of the puzzle, the complete story of bat evolution remains a mystery. The identity of the closest mammalian relative to bats is still unknown. They have been placed in the ancient group known as Laurasiatheria; however this group is large and very diverse. The modern relatives of these creatures include such varied animals as carnivores, whales and shrews! Nevertheless, the original Laurasiatherians were likely small insectivorous animals that walked on all fours. With advances in technology and rapid developments in scientific investigation, hopefully we will soon be able to elucidate a clearer picture of how these amazing creatures evolved to fill the night skies.
Bats and Plants
by Vanessa Rojas
The role of bats within tropical forests is one that is both irreplaceable and crucial to these unique and diverse ecosystems. The meticulous relationship between bats and plants is not fully understood, however scientists are often gaining new and important information. Tatyana A. Lobova, Cullen K. Geiselman, and Scott A. Mori have gone to great lengths combining decades of previous research, along with their new findings, to help us understand the importance of bats in their book, Seed Dispersal by Bats in the Neotropics.
Neotropics refers to the New World region, south of the Tropic of Cancer, including southern Mexico, Central and South Americas and the West Indies. In 1999, in-depth field research on plant dispersal by bats began in Central French Guiana. These findings were then also used to infer further understandings of the bat and plant relationships found throughout the Neotropics.
To give an idea of the importance of this relationship, of the 1,116 bats known to date, 29% depend partially or entirely on plants as a food source. However, this is a co-dependent relationship because at least 858 plant species in the Neotropics depend on bats for the survival of their species through pollination or seed dispersal. Bats make up a majority of mammals, totaling at 105 of the 191 known mammalian species in French Guiana. Of the 105 bat species, 37% are fruit eating bats. Within the study region of Central French Guiana, it was found that 15% of the native flowering plants are bat dispersed or show bat dispersing potential. These data and previous findings were used to approximate that 549 plants in the Neotropics display fruits that are consumed by bats, most often resulting in seed dispersal.
Bats are efficient at dispersing quality seeds due to a few key characteristics:
- Selective eating habits of only ripe fruits which contain mature seeds,
- Flying away from parent tree with fruits,
- Defecation of seeds over open areas suitable for germination, and
- Seeds are very rarely damaged during handling or digesting.
The plant-bat relationship goes beyond just that, it is also crucial to the ecosystem as a whole. This relationship is important for forest regeneration in slash-and-burn (cutting and burning of forests to create fields) areas. In one region where slash-and-burn agriculture techniques were abandoned for three years, 87% (8,280 out of 9,320) of the stems present were plant species that depend on bat and bird dispersal. Although secondary forests lack much of the biodiversity found in primary tropical forests, without bats, growth of these destroyed regions would be deficient.
Meet Neotropical bats at the Bat Zone!
Jamaican Leaf-nosed and Short-tailed Fruit bats help disperse seeds throughout the rainforest by eating up to twice their individual weight in fruit, spreading thousands of seeds every night. These bats and others like them aid in the regeneration of rainforests.
Where bats live is dependent on the availability of food. When the food supply declines, usually due to weather, bats have two options. They can hibernate to pass through the low or non-existent food supply period, or migrate to a place with a more abundant food supply. In some areas, bats will also do a combination of both.
Migration involves two parts: movement from one location when food is scarce and the return to that same location when the abundance of food returns. There is no clear distinction between migrating bats and hibernating bats. Some bat species, like the silver-haired bat, migrate and hibernate. The same is true for red bats. Red bats migrate from the northern portion of their range and hibernate in the southern portion of their range. Most temperate bat colonies start migrating in September.
The Mexican free-tailed bat (Tadaria brasiliensis) ranges from Oregon to south Mexico. Bats in the northern California/Oregon region are non-migratory, spending inclement weather in torpor or hibernation. Bats of this species in the eastern Nevada, western Arizona and Colorado regions are non-migratory as well, however do not hibernate. Mexican free-tails from southeastern Utah and southwestern Colorado migrate to western Mexico. The most well known colonies, like Carlsbad Cavern and the caves in Texas and southeastern United States migrate to eastern Mexico.
Some Myotis species also migrate to winter roosts traveling over shorter distances. Their migration is not necessarily in latitudinal direction. These bats may travel in any direction depending on the location of the hibernaculum. Little brown bats (Myotis lucifigus) migrate from 200 km to 800 km in distance between summer and winter roosts. Gray bats (Myotis grisescens) migrate several hundred square kilometers from northern Arkansas hibernaculum to Kansas, Missouri and Oklahoma. Most records of big brown bat migration distances are less than 40 km in distance, however there have been exceptional distances of 230 km recorded. Tolerance of colder weather may be one reason for the short distances.
In general, tree roosting bats are migratory since trees do not provide enough shelter for the winter. Hoary bats from all areas migrate more equatorially and are found below 37 degrees latitude during the winter. Both Lasiurus and Lasionycteris species are sometimes found in migrating groups, sometimes accompanied by migratory birds.
Long distance flights consume a lot of energy. Bats that migrate lose about 0.5 g per 100 km traveled. Navigation must be accurate and flight efficient. Migrating bats are known to use vision, echolocation and the sun as orientation and may use other factors as well, but these have not been studied. Not only are the energy demands of migration costly, other threats to the bat’s survival are more likely during migration. Adverse weather, higher chance of predation and disease are all factors that the bat may face. Accidents during migration such as running into buildings with wind gusts also occur. Another threat to migratory bats is pesticides. As bats consume insects that are poisoned by pesticides, they in turn store the chemical compounds in their fat. When the body fat is burned during migration, resins are released into the bloodstream and may cause illness or death.
How Bats Hibernate
By Shannon Currie
Hibernation is a concept often misunderstood and poorly described for the general public. It is a special physiologic process that is present in a specific group of mammals and few birds. Unlike the common misconception that hibernation is simply the change in activity of an individual between seasons, hibernation or torpor can occur year round in specialised creatures such as bats, and is not a continuous process.
Animals that are small in size must balance the expenditure of energy, through movement, thermoregulation and other physiological processes, with their small energy gains. In particular, bats face the problem of a very energy expensive form of locomotion- flight. When temperatures are low and food is scarce, bats combat this energy loss by slowing down their bodily functions. By adjusting their thermoregulatory set point, the thermostat in their brains, bats can effectively stop producing body heat and in turn slow their metabolism, heart rate and breathing rate to extremely low levels. For example, at rest the heart rate of a bat sits at about 300-400 beats per minute (bpm) depending on the species. When in torpor a bat’s heart rate can drop as low at 10bpm, depending on the species and the external temperature.
In most microchiropteran species this occurs on a daily basis during spring, summer, and autumn for a few hours, depending on the weather conditions. When temperatures get too cold most microchiropteran species will head to sheltered areas such as caves or inside buildings and hibernate. Bouts of torpor during hibernation usually last from a few days up to a couple of weeks and are interjected by periods of arousal. Scientists are not sure of exactly why bats arouse from these torpor bouts. It could be to urinate or to remove toxins from the muscles by moving around, in some cases individuals even copulate.
The amazing thing about hibernators that sets them apart from other animals is that they are capable of rewarming their bodies from very low temperatures all by using internally created heat. Ectotherms, such as reptiles, have body temperatures that can reach low levels but they are incapable of rewarming themselves without external heat sources. Also, the hearts of hibernators can function at temperatures below that of any other mammal. Bats are a particularly good example of this. Non-hibernating mammals will die if the temperature of their hearts drops below 10°C, where as hibernators heart can function below this temperature, and bats hearts continue to work at temperatures approaching 0°C!
The history of research into hibernation has been primarily focussed on rodents because they are easier to catch and keep in captivity than some other mammal species. However their use of torpor is solely seasonal and they do not have the added energy pressures of costly locomotion. Bats exist at an extreme end of the spectrum. They are much smaller than many rodents and flight prohibits them from storing large quantities of body fat. They also undergo torpor daily and need to have perfected the process to survive. More recently scientists have investigated the patterns of torpor in a few bat species but there is still much information to be gathered.
What Bats Eat
Due in no small part to their amazing adaptations, bats have become one of the most diverse and successful mammals throughout the world. Between 60 and 70% of all bats are insectivores. Almost any insect that is active at night can be food for a bat, including moths, beetles, flies, crickets, gnats, mayflies, wasps, and mosquitoes. There are other bats that eat a wide variety of food: scorpions, fish, fruit, pollen, spiders, arthropods, nectar, small mammals, and non-flying insects. Ten species of bats in Central America are carnivorous and prey on small birds, small mammals, or other bats. One of these bats, the false vampire Vampyrum spectrum, hunts for its avian prey using its excellent sense of smell. Other carnivorous bats hunt by listening to prey generated sounds.
These bats also eat a significant number of arthropods, so it is likely that the carnivorous bats evolved from insectivorous ancestors. One genus of bats, Noctilio, trawls for small fish over the water. It uses its hind legs and specialized toe nails to snag the unsuspecting fish and then returns to a roost to feed. There are three bats that feed on blood. These are the famous vampire bats, whose foraging habits are responsible for giving many other bats a bad name. Some bats eat plant material. In the Old World, the entire suborder Megachiroptera eats fruit and nectar, and in the Americas the Phylostomidae (of the suborder Microchiroptera) are also frugivorous.
In general, frugivorous bats tend to be larger than nectivorous bats. They have a more developed sense of smell and vision, and many roost in trees rather than caves. Frugivorous bats play a major role in the overall health of the tropical forests. They disperse the seeds of the fruit they eat which helps the forest regenerate after being cut down. The plants that rely on bats for seed dispersal usually have a strongly odored fruit that remains on the tree long after they are ripe. This fruit is often found on long stalks or positioned away from twigs and leaves. This allows for easy access for the bats, but makes it very difficult for birds to eat the fruit. The seeds of these plants are hard kernels that separate easily from the flesh of the fruit. Several species of bats are also important pollinators. There are more than twenty genera of plants that rely on bats to pollinate them, These plants range from blooming cacti to wild banana trees. The bats and plants have exhibited modifications, which increase the success of feeding and pollination.
Nectivorous bats have long muzzles and long protruding tongues that have a brush tip to gather pollen quickly and efficiently. Flowers of the plants pollinated by bats angle downward, and are shaped and sized just right for a bat to insert their head and shoulders. In fact, recent research has suggested that some flowers in the New World are shaped to reflect the echolocation calls of foraging bats so that they can find the pollen of that flower. Most of these flowers have an abundance of nectar, are open at night, and have a strong smell.
Bats are extremely important members of a healthy rainforest, and by dispersing seeds they help to regenerate the forest after clear-cutting or fires. Every ecosystem is integrated in a similar manner, with each part depending on other parts of the ecosystem and being depended on in turn. Ecologists are concerned about the loss of biodiversity because we know that the loss of any species will affect all the other species in the ecosystem.