Microtus breweri

Geographic Range

Beach voles ( Microtus breweri ) are only found on the 2.6 km^2 Muskeget Island, which is located 8 km from the west coast of Nantucket, Massachusetts in the northeastern United States. Because of this limited distribution, these voles are considered an island endemic species.

• Biogeographic Regions
• nearctic nearctic
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

The vegetation of sandy Muskeget Island, with an elevation peak of 4.3 m, is primarily poison ivy ( Toxicodendron radicans ) and beach grass ( Ammophilia breviligulata ), dominated by the latter. Two-thirds of the island are considered suitable habitat, because they are saturated by beach grass that serves as a food resource. Beach voles are known to build their burrows underneath the loose sand of these grassy meadows. The other third of the island is generally not inhabited by beach voles because it consists of fresh and salt marshes and open dunes that do not provide favorable living conditions.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• savanna or grassland savanna or grassland

Physical Description

On average, male beach voles are 192.9 mm long, ranging between 169 to 215 mm. Tail length averages 51.9 mm (range 45 to 60 mm), approximately one-fourth of total length. The hind feet of males averages 23.8 mm in size. Generally, females are smaller than males. Their average length is 181.2 mm (range 165 to 201 mm), tail length is 48.6 mm (range 35-55 mm), and their hindfoot length averages 23.4 mm. The average cranial length of beach voles is 13.1 mm, while the cranial breadth is generally 11.2 mm. Their skull is larger, and their brain case is longer and narrower than meadow voles ( Microtus pennsylvanicus ). Additionally, their average basal metabolic rate (73.7 ± 2.9 cm^3 oxygen/hour) is lower compared to that of meadow voles. The texture of beach voles’ fur is prickly and rough due the abundance of stiff and long guard hairs (gray near the roots and brown near the tips) and the shortage of soft and short underhairs (yellow-white color). Throughout most of the year, their fur is light gray on the dorsum, hazel brown on lateral sides, and nearly white on the ventral area. Their grayish appearance is especially evident in summer when they lose most of their guard hairs on the dorsal surface. The posterior is a mixture of brown and white. Their whiskers, extending in length towards the ears, vary in color from black to a silky white. A distinguishing feature of beach voles is a blaze, often found on top of the head and less commonly on their chin or neck. The body weight of beach voles spans between 29-79 g, with females weighing less than males, except during pregnancy. Age and body weight are directly proportional. Juveniles weigh below 32g, while subadults weigh between 32-45 g. The weight of adults usually exceeds 45g and can reach more than 70g.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger

Reproduction

Zwicker (1989) noted that beach voles exhibit a polygynandrous, or promiscuous, mating system, in which multiple female beach voles are inseminated by multiple male partners, yielding potential multi-paternity litters. Their reproductive efforts are indirectly proportional to population density and sexual dimorphism is minimal. Additionally, there is an increased amount of territorial overlapping between sexes during the breeding season. Sexually-mature females aggressively defend and occupy their territories, while males often overlap the territories of both sexes when looking for potential mates. Once successfully mated, female beach voles remain in their home range for nesting. Male beach voles continue to compete for other sexual partners and rarely return to the breeding site. The competition for various sexual partners has led to development of larger testes (compared to body size) in male beach voles, presumably for increased sperm production.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

In order to determine the sexual maturity of beach voles, Tamarin (1977) used weight instead of age. Females breed at smaller sizes than males (range 21-36 g), weighing four grams less than male beach voles, who matured later in terms of weight (range 24-40 g). Their breeding season lasts from March to October. No research has been conducted regarding the breeding interval of beach voles, but all females in Microtus populations are polyestrous. From March to June, litter size averages four pups and half of the females are successfully impregnated. Female beach voles exhibit post-partum estrous because they are able to immediately reproduce following the birth of their pups. The second litter, born usually from July through October, consists of three pups with forty percent of the females being successfully impregnated. Even though beach voles weigh the most during the winter, no pregnancies have been observed from November to February. The average litter size for female beach voles is 3.4 pups (range 2.3-4.5 pups). The gestation period for beach voles is 21 days, with visible bulges developing in the uterine wall after the sixth day. Not much is known about beach vole pups. However, pups of meadow voles ( Microtus pennsylvanicus ), close relatives, are hairless and pink in coloration at birth, weighing 1.6 g to 3.0 g. By day 4, they are able to vocalize using ultrasonic communication and small hairs begin to cover the body with the whole body being covered by day 7, except the abdomen. Their ears and eyes remain closed until day 8. Weaning occurs 12 to 14 days after birth. After weaning, pups are left to survive on their own. It is assumed that beach vole pups exhibit the same characteristics.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• viviparous viviparous
• post-partum estrous

Not much is known about beach vole parental investment, but in populations of another member of the genus, meadow voles ( Microtus pennsylvanicus ), adult males do not aid in the development of pups. Females assume the primary parental role. When pups are born, they are dependent on their mother for survival and thus are altricial. During the nesting period, they remain in the females’ home range, with males being aggressively excluded. Oliveras and Novak (1986) noted that meadow voles rarely engage with their offspring and typically retract whenever approached by them, displaying low levels of direct parental care. Generally, pups spend 63% of the time alone in the nest, with mothers roaming the nearby area for food while socializing with female siblings. By often deserting them and only sporadically nursing them, mothers encourage the weaning of pups within two weeks after birth. After weaning, pups are left to fend for themselves, sometimes remaining near the natal site. It is assumed that beach voles follow a similar pattern of parental investment.

Lifespan/Longevity

There are no current data on the longevity of beach voles. However, meadow voles ( Microtus pennsylvanicus ) generally have a lifespan of two to three months in the wild. In rare cases, they have been observed to live nearly 16 months. It is assumed that beach voles have a similar lifespan in the wild. In captivity, meadow voles can live more than two years. However, beach voles are not kept in captivity.

Behavior

Beach vole behavior varies by season, with aggressiveness being prominent during breeding months (March-October). Sexually-receptive females are aggressive towards both females and males, but less towards the latter. After birth, females become more tolerant of female siblings but actively exclude males from their territory. Occasionally, females allow fertile males to enter their territory, solely for mating purposes. Males are receptive towards female conspecifics but will engage in altercations with male conspecifics. Odor selections also vary by season. During the breeding season, females choose odors of possible male mates, usually disregarding the odors of other females. Males choose the odors of sexually-receptive females over that of other males. During the non-breeding season, female beach voles will select the odor of same-sex conspecifics over opposite-sex conspecifics. The same holds true for male beach voles, who choose the odor of other non-breeding male beach voles. The decrease in intrasexual altercations and increase in same-sex odor selections in the winter months indicates social tolerance. On rare occasions, Ferkin (1990) observed that male beach voles would unite to form overwintering groups, while females remained mostly solitary. In terms of movement patterns, beach voles travel longer distances in the summer months looking for potential mates. Adults travel the farthest, closely followed by juveniles. Males travel farther than females. Most Microtus populations do not hibernate and are active most of the day throughout the whole year. During the winter, they spend more time in their burrows to retain warmth and energy. Throughout the rest of the year, they are actively searching for food or building burrows during dusk and dawn when the temperature is cooler.

• Key Behaviors
• terricolous
• fossorial fossorial
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• motile motile
• sedentary sedentary
• solitary solitary
• territorial territorial
• social social

Home Range

Zwicker (1989) noted that there are remarkable changes in the sizes of beach vole home ranges throughout the year. During the breeding season, male home ranges average 178.7 m^2 while female home ranges average 158 m^2. During the non-breeding season, home ranges decrease in size for both sexes, averaging 145.0 m^2 for male beach voles and 76.1 m^2 for female beach voles. Females maintain and actively defend majority of their home ranges throughout the year. Males do not defend territories and frequently overlap the home ranges of females and occasionally other males. This indicates a lack of territorial dominance in male beach voles. Additionally, female beach voles are more likely to build burrows beneath the ground, presumably for the protection of their litter.

Communication and Perception

Ferkin (1990) examined odor preference in beach voles and found that they use different odors according to sex and sexual maturity to mark their territories. Depending on the season (non-breeding or breeding), voles are attracted to different odors of the same species. During experimental trials, non-breeding voles chose the odor of same-sex voles, while breeding voles chose the odor of opposite-sex voles. They also use odors to signal differences between each other, such as level of aggression or sexual receptivity. For example, when deciding on which territory to enter, beach voles will choose the odor of the conspecific that is least aggressive, resulting in less direct fights compared to other rodent relatives. Not much has been reported on communication in beach vole populations, likely because they were only recently recognized as a separate species from meadow voles ( Microtus pennsylvanicus ). Meadow voles primarily rely on chemical signaling, such as scent-marking and odor release, for communication purposes. They mark their presence via self-grooming, when scent from their fur is released into the surrounding area. Hobbs and Ferkin (2011) noted that older and sexually-mature meadow voles were more likely to deposit scent marks in the territories of other meadow voles, especially males. When engaging in hostile behavior, meadow voles vocalize threats at a maximum frequency of 1.8 kHz. They also utilize ultrasonic communication during their early stages of life or when in distress. Because beach voles are closely related to meadow voles, it is likely that they exhibit similar modes of communication.

• Communication Channels
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• pheromones pheromones
• scent marks scent marks

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• ultrasound ultrasound
• chemical chemical

Food Habits

Beach voles are opportunistic herbivores that primarily consume grass and other forbs during different seasons. They most commonly feed on beach grass ( Ammophilia breviligulata ) and bayberries ( Myrica pennsylvanica ). In the spring and summer, beach voles consume mostly leaf edges, which yield the highest unrefined protein values, utilized in reproduction and growth. During the late summer and fall they feed on roots. Throughout the end of fall until the early spring, beach voles consume stems. The lack of protein in roots and stems supports water retention, which is vital to the beach voles’ survival because their saline environment often lacks access to fresh water.

• Primary Diet
• herbivore herbivore
  • folivore folivore
  • lignivore

• Plant Foods
• leaves
• roots and tubers
• wood, bark, or stems

Predation

Because of their limited geographic range, the predators of beach voles are minimal. According to Tamarin (1977), there are no mammalian predators living on Muskeget Island. Avian predators were documented, although none were observed preying on beach voles. The avian species that breed and nest on Muskeget Island include marsh hawks ( Circus cyaneus ), short-eared owls ( Asio flammeus ), herring gulls ( Larus argentatus ), and great black-backed gulls ( Larus marinus ). In order to avoid detection from potential predators flying overhead, beach voles, like meadow voles ( Microtus pennsylvanicus ), build their burrows on top of or beneath dense vegetation.

Ecosystem Roles

Beach voles host multiple parasites including flea, lice, and mites. Winchell (1977) found that Eimeria wenrichi , a sporozoan, was present in the inner intestines of examined beach voles. The bacterium Haemobartonella sp., only transmitted by lice and fleas, was also found in three-quarters of studied beach voles. Polyplax alaskensis , a blood-sucking louse (ectoparasite), was also present in the blood smears of the beach voles. Male beach voles, due to their larger home range, were more commonly infested with these lice than female beach voles. Mites were also present, especially the mesostigmatid Laelaps kochi . In some beach voles, the flea Epitedia wenmanni and a parasitic flatworm, Andrya macrocephala , were discovered. In one rare instance, one of the male beach voles had a tick ( Ixodes muris ) removed from its ear that was most likely introduced by the pet of a fisherman visiting the island. Winchell (1977) noted that beach voles probably act as intermediate hosts for parasites found in marsh hawks ( Circus cyaneus ) and short-eared owls ( Asio flammeus ), potential predators of voles. The number of parasites hosted by beach voles is considerably lower than other Microtus relatives due to limited differences in the microenvironment, and the absence of intermediate hosts on the island that allow for parasites to develop.

Economic Importance for Humans: Positive

There are no known direct positive economic effects of beach voles on humans.

Economic Importance for Humans: Negative

There are no known direct negative economic effects of beach voles on humans.

Conservation Status

The IUCN Red List of Threatened Species lists beach voles as a “vulnerable” species. Although they have no natural predators, threats that might negatively affect beach vole populations include habitat destruction by humans, island erosion, flooding, and the introduction of new predators. Beach voles also experience fluctuations in their population density that lead to large increases, and often decreases, in population size. Because of their extremely limited geographic range, they are susceptible to possible extinction events. However, there are no ongoing conservation efforts for beach voles. Furthermore, they have no special status on the U.S. Federal list, State of Michigan List, or CITES.

Additional Links

Encyclopedia of Life

Contributors

Jordan Chittick (author), Radford University, Alex Atwood (editor), Radford University, Lindsey Lee (editor), Radford University, Karen Powers (editor), Radford University, Joshua Turner (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

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Boonstra, R., C. Krebs, M. Gaines, I. Craine. 1987. Natal philopatry and breeding systems in voles (Microtus spp.). Journal of Animal Ecology , 56/2: 655-673.

Ferkin, M. 1990. Odor selections of island beach voles during their nonbreeding season. Journal of Mammalogy , 71/3: 397-401.

Goldberg, M., N. Tabroff, R. Tamarin. 1980. Nutrient variation in beach grass in relation to beach vole feeding. Ecology , 61/5: 1029-1033.

Gruder-Adams, S., L. Getz. 1985. Comparison of the mating system and paternal behavior in Microtus ochrogaster and M. pennsylvanicus. Journal of Mammalogy , 66/1: 165-167.

Heske, E., R. Ostfeld. 1990. Sexual dimorphism in size, relative size of testes, and mating systems in North American voles. Journal of Mammalogy , 71/4: 510-519.

Hobbs, N., M. Ferkin. 2011. Effect of protein content of the diet on scent marking and over-marking behavior in meadow voles, Microtus pennsylvanicus. Behaviour , 148/9: 1027-1044.

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