Little brown bat
The little brown bat or little brown myotis is an endangered species of mouse-eared microbat found in North America. It has a small body size and glossy brown fur. It is similar in appearance to several other mouse-eared bats, including the Indiana bat, northern long-eared bat, and Arizona myotis, to which it is closely related. Despite its name, the little brown bat is not closely related to the big brown bat, which belongs to a different genus.
Its mating system is polygynandrous, or promiscuous, and females give birth to one offspring annually. The offspring, called pups, are quickly weaned and reach adult size in some dimensions by three weeks old. The little brown bat has a mean lifespan of 6.5 years, though one individual in the wild reached 34 years old. It is nocturnal, foraging for its insect prey at night and roosting in hollow trees or buildings during the day, among less common roost types. It navigates and locates prey with echolocation.
It has few natural predators, but may be killed by raptors such as owls, as well as terrestrial predators such as raccoons. Other sources of mortality include diseases such as rabies and white-nose syndrome. White-nose syndrome has been a significant cause of mortality since 2006, killing over one million little brown bats by 2011. In the Northeastern United States, population loss has been extreme, with surveyed hibernacula averaging a population loss of 90%.
Humans frequently encounter the little brown bat due to its habit of roosting in buildings. Colonies in buildings are often considered pests because of the production of waste or the concern of rabies transmission. Little brown bats rarely test positive for rabies, however. Some people attempt to attract little brown bats to their property, but not their houses, by installing bat houses.
Taxonomy
The little brown bat was described as a new species in 1831 by American naturalist John Eatton Le Conte.It was initially in the genus Vespertilio, with a binomial of Vespertilio lucifugus, before it was re-categorized as belonging to the Myotis genus. "Myotis" is a Neo-Latin construction, from the Greek "muós and "oûs", literally translating to "mouse-eared". "Lucifugus" is from Latin "lux" and "fugere", literally translating to "light-shunning". The holotype had possibly been collected in Georgia near the Le Conte Plantation near Riceboro, but this has been disputed because the initial record lacked detail on where the specimen was collected.
Within its family, the Vespertilionidae, the little brown bat is a member of the subfamily Myotinae, which contains only the mouse-eared bats of genus Myotis. Based on a 2007 study using mitochondrial and nuclear DNA, it is part of a Nearctic clade of mouse-eared bats. Its sister taxon is the Arizona myotis, M. occultus.
As of 2005, five subspecies of the little brown bat are recognized: M. l. lucifugus, M. l. alascensis, M. l. carissima, M. l. pernox, and M. l. relictus. Formerly, the Arizona myotis and southeastern myotis were also considered subspecies, but both are now recognized as full species.
In a 2018 study by Morales and Carstens, they concluded that the five subspecies are independent, paraphyletic lineages, meaning that grouping them together excludes other lineages with the same common ancestor, and therefore each warrant specific status.
Results of one study suggested that the little brown bat can hybridize with Yuma myotis, M. yumanensis. The two species occur in the same area in much of the Western United States, as well as southern British Columbia. The two species are morphologically different throughout most of the range, but in some regions, individuals have been documented that are intermediate in appearance between the two. However, a 1983 study by Herd and Fenton found no morphological, genetic, or ecological evidence to support the notion that the two species hybridize.
Anatomy and physiology
External characteristics
The little brown bat is a small species, with individuals weighing with a total body length of. Individuals have the lowest weight in the spring as they emerge from hibernation. It has a forearm length of and a wingspan of. It is a sexually dimorphic species, with females larger than males on average. A variety of fur colors is possible, with pelage ranging from pale tan or reddish to dark brown. Its belly fur is a lighter color than its back fur. Its fur is glossy in appearance, though less so on its belly. A variety of pigmentation disorders have been documented in this species, including albinism, leucism, and melanism.Head and teeth
It is a diphyodont mammal, meaning that it has two sets of teeth during its lifetime—milk teeth and adult teeth. The dental formula of the milk teeth is for a total of 22 teeth, while that of the adult teeth is for a total of 38 teeth. Newborns are born with 20 milk teeth which becomes 22 when the final upper premolars emerge. Pups begin losing milk teeth once they have reached a body length of ; total loss of milk teeth and emergence of adult teeth is usually complete by the time a juvenile is long.It has a relatively short snout and a gently sloped forehead. It lacks a sagittal crest, which can be used to distinguish it from the Arizona myotis. Its skull length is. The braincase appears nearly circular though somewhat flattened when viewed from the back. Its ears are long, while the tragi, or cartilaginous flaps that project in front of the ear openings, are long. The tragi are blunt at the tips and considered of medium length for a mouse-eared bat.
Senses
The little brown bat is dichromatic and its eyesight is likely sensitive to ultraviolet and red light, based on a genetic analysis that discovered that the genes SWS1 and M/LWS were present and functional. Its ability to see ultraviolet light may be useful in capturing insects, as 80% of nocturnal moths' wings reflect UV light. It is unclear if or how seeing red light is advantageous for this species. It is adapted to see best in low-light conditions. It lacks eyeshine.The little brown bat lacks a vomeronasal organ. Relative to frugivorous bat species such as the Jamaican fruit bat, it has small eyes and a reduced olfactory epithelium. Instead, it has a more sophisticated system of echolocation, suggesting that reliance on echolocation decreases the need for orientation via sight or smell.
Physiology
In fall through spring, the little brown bat enters torpor, a state of decreased physiological activity, daily. Torpor saves energy for the bat when ambient temperatures are below throughout the year and in the winter; instead of expending energy to maintain a constant body temperature, it allows its body to cool and physiological activity to slow. While in torpor, its heart rate drops from up to 210 beats per minute to as few as 8 beats per minute. The exception to this rule is females at the end of pregnancy, which no longer have the ability to thermoregulate, and therefore must roost in warm places. During daily roosting, it can cope with high levels of water loss of up to 25%.In the winter time, it enters a prolonged state of torpor known as hibernation. To conserve energy, it limits how frequently it arouses from torpor, with individuals existing in uninterrupted torpor for up to 90 days. Arousal is the most energetically costly phase of torpor, which is why individuals do so infrequently. Despite the energy-saving mechanism of hibernation, individuals lose a quarter of their pre-hibernation body mass during the winter.
Similar species
The little brown bat can be confused with the Indiana bat in appearance. The two can be differentiated by the little brown bat's lack of a keeled calcar—the cartilaginous spur on its uropatagium. While it does have a calcar, that of the little brown bat is not nearly as pronounced. Additionally, the little brown bat can be distinguished by the presence of hairs on its toes and feet that extend beyond the length of the digits. The northern long-eared bat, another similar species, can be distinguished by its much longer ears, and tragi that are long and sharply pointed.Biology and ecology
Reproduction and life cycle
The little brown bat has a promiscuous mating structure, meaning that individual bats of both sexes mate with multiple partners. It is a seasonal breeder, with mating taking place in the fall before the annual hibernation. As a seasonal breeder, males do not produce sperm year-round; instead, spermatogenesis occurs May through August each year. Throughout the spring and summer, males and females roost separately. In the fall, however, individuals of both sexes will congregate in the same roost in a behavior known as "swarming". Like several other bat species, males of this species exhibit homosexual behaviors, with male bats mating indiscriminately with torpid, roosting bats, regardless of sex.Although copulation occurs in the fall, fertilization does not occur until the spring due to sperm storage. Gestation proceeds for 50–60 days following fertilization. The litter size is one individual. At birth, pups weigh approximately and have a forearm length less than. While they have a small absolute mass, they are enormous relative to their mothers, weighing up to 30% of her postpartum body weight at birth. Pups' eyes and ears are closed at first, but open within a few hours of birth. They exhibit rapid growth; at around three weeks old, the young start flying, begin the weaning process, and are of a similar size to adults in forearm length but not weight. The young are totally weaned by 26 days old. Females may become sexually mature in the first year of life. Males become sexually mature in their second year.
It is a very long-lived species relative to its body size. In the wild, individuals have been documented living up to 34 years.
The average lifespan, however, is around 6.5 years. Males and females have high annual survival rates, though survival rates vary by sex and region. One colony documented in Ontario had a male survival rate of 81.6% and a female survival rate of 70.8%; a colony in southern Indiana had survival rates of 77.1% and 85.7% for males and females, respectively.