26.

Sonoma Tree Vole

Arborimus pomo

French: Phénacomys de Sonoma / German: Sonoma-Baumwiihimaus / Spanish: Topillo arboreo de Sonoma

Other common names: California Red Tree Mouse, California Red Tree Vole, Pomo Tree Vole

Taxonomy. Arborimus pomo M. L.. Johnson & George, 1991, “Jenner Ridge, 0-8 km (0.5 mi) north ofJenner, Sonoma County, California.”

This species is monotypic.

Distribution. Coastal forests of N California from the Klamath Mts of Del Norte County S to Sonoma County. The N border of this species and the S boundary of the Red Tree Vole (A. longicaudus) are poorly defined and this region near Humboldt County, California, warrants further study.

Descriptive notes. Head-body 95-98 mm (males) and 101-103 mm (females), tail 64— 68 mm (males) and 66-73 mm (females), ear 10-12 mm, hindfoot 19-20 mm; weight 20-45 g. Females are usually larger than males. The Sonoma Tree Vole is a small vole that differs from other voles (except for the Red Tree Vole, A. longicaudus) by its reddish brown back and light gray belly, sometimes tinged reddish orange. Tail is hairy and relatively long (more than 50% of head-body length). Lower molars distinguish it from other sympatric vole species. On labial side, reentrant angles of lower molars extend two-thirds the way to lingual side, while reentrant angles on lingual side extend only one-third the wayto labial side. Skull has distinct longitudinal depression between orbits, bordered by pronounced ridge, highly recurved incisors, and relatively wide incisive foramina. Molars are unrooted in juveniles and become rooted in adults. The Sonoma Tree Vole differs from the Red Tree Vole by being smaller, with nasals not extending posteriorly beyond maxillaries, and differences in karyotype: 2n = 40 or 42 vs. 48 or 54 in the Red Tree Vole.

Habitat. Wet conifer forests of Douglas fir ( Pseudotsuga menziesii), grand fir ( Abies grandis), Sitka spruce ( Picea sitchensis), western hemlock (1suga heterophylla), all Pinaceae, and coastal redwood ( Sequoia sempervirens, Cupressaceae). Sonoma Tree Voles prefer old growth forest, where densities are highest, but they can also be found in older second growth forests.

Food and Feeding. Sonoma Tree Voles, like the Red Tree Vole, primarily eat needles of Douglas fir and also consume needles and occasionally inner bark of twigs of other conifer trees. They obtain water from dew and fog that accumulates on vegetation.

Breeding. Sonoma Tree Voles breed year-round, but most litters are born in February—September. Females can breed within 24 hours of giving birth. Gestation varies from 27 to 48 days, possibly due to delayed implantation, and averages 31 days. Litters usually have two young (1-5). Young are weaned at 25-48 days old.

Activity patterns. Sonoma Tree Voles are nocturnal and arboreal, but activity is not well documented because they are secretive and hard to catch and study.

Movements, Home range and Social organization. Sonoma Tree Voles are thought to have limited dispersal abilities and early seral stage forests and clearcuts likely present significant barriers to their movement.

Status and Conservation. Classified as Near Threatened on The IUCN Red List. Like their sister species, the Red Tree Vole, combination of low reproductive potential, strong ties to old-growth forests, and limited dispersal capabilities make the Sonoma Tree Vole vulnerable to disturbances such as industrial scale deforestation.

Bibliography. Adam & Hayes (1998), Bellingeret al. (2005), Benson & Borell (1931), Bradley, Ammerman etal. (2014), Brylski, J.H. (1990), Corn & Bury (1988), Hamilton (1962), Johnson & George (1991), Musser & Carleton (2005).