Townsend’s Big-Eared Bat

Townsend’s big‑eared bat (Corynorhinus townsendii) belongs to the order Chiroptera, family Vespertilionidae, and subfamily Myotinae. It inhabits western North America, ranging from southern British Columbia and Alberta through the Pacific Northwest and the Rocky Mountains to northern Mexico. The species prefers cool, humid roosts such as limestone caves, abandoned mines, hollow trees, and occasionally attics of human structures. Its most striking morphological trait is the exceptionally large, funnel‑shaped ears that can exceed the length of the head, aiding in passive acoustic detection of prey. The bat has a short, broad muzzle, a well‑developed tragus, and a soft, gray‑brown dorsal pelage with lighter ventral fur. Seasonal movements are modest; individuals may shift between summer maternity colonies in forested valleys and winter hibernacula in deeper caves. The IUCN classifies C. townsendii as Least Concern, reflecting a relatively wide distribution and stable overall numbers, although some isolated populations are declining due to habitat disturbance. Conservation actions focus on protecting roost sites, monitoring disease impacts, and mitigating wind‑energy related mortality.

Townsend’s big‑eared bats are nocturnal and form moderate-sized maternity colonies of 30‑100 females during the summer months, while males often roost singly or in small bachelor groups. In winter they aggregate in hibernacula, sometimes sharing caves with other vesper bats. Foraging begins shortly after sunset and continues for 3‑5 hours, primarily over riparian corridors, forest edges, and open meadows where insects are abundant. Individuals travel 5‑10 km from their roosts, employing a combination of aerial hawking and gleaning from foliage. Echolocation calls are low‑frequency (around 30‑40 kHz) and highly modulated, allowing detection of prey in cluttered environments. Social communication includes a repertoire of contact calls used to maintain colony cohesion and mother‑pup recognition. Territory is not strongly defended; overlapping foraging ranges are common, and individuals rely on acoustic cues rather than visual landmarks to navigate.

Townsend’s big‑eared bat is an insectivore that consumes a diverse assemblage of nocturnal insects. Stomach‑content analyses and DNA metabarcoding studies show a diet dominated by Lepidoptera (moths), Coleoptera (beetles), Diptera (flies), Trichoptera (caddisflies), and occasional Hymenoptera (wasps). Foraging occurs low to the ground (often <5 m) where insects emerge from water bodies or are attracted to lights. The bat employs both aerial hawking—capturing insects in flight—and gleaning—snatching prey from leaves or the water surface. Nightly food intake can equal 30‑50 % of body mass, reflecting the high metabolic demands of flight. Seasonal shifts in prey composition are observed; during late summer, moths become the primary prey, while early spring diets contain more flies and caddisflies. Pesticide exposure is a concern because it reduces prey abundance and can lead to bioaccumulation of toxins in the bats.

Sexual maturity is reached at about one year of age. Mating occurs in late summer to early autumn, but fertilization is delayed until spring when females store sperm over the winter. Ovulation and conception take place in April‑May, leading to a gestation period of roughly 50‑60 days. Typically a single pup is born between late June and early July; twins are rare. Newborns weigh about 1.5 g and are altricial, clinging to the mother’s fur while she nurses. The mother provides milk for 4‑5 weeks, after which the juvenile begins to forage independently but remains in the maternity colony for several weeks. Weaning is complete by early August, and juveniles achieve flight proficiency within 3 weeks, allowing them to accumulate the fat reserves needed for the upcoming hibernation period.

The IUCN lists C. townsendii as Least Concern, yet several regional threats are causing population declines. Disturbance of hibernacula by caving, mining, and tourism can lead to premature arousal and loss of fat reserves, increasing winter mortality. Habitat loss from logging, urban expansion, and conversion of riparian zones reduces foraging habitat and maternity roost availability. Wind‑energy development poses a collision risk; studies in the Pacific Northwest have documented fatality rates of up to 0.5 % of local populations per turbine per year. Pesticide use diminishes insect prey and can cause direct toxicity. Although White‑nose syndrome (WNS) has impacted many North American bats, C. townsendii appears less susceptible, but the disease’s spread remains a potential future threat. Population estimates suggest 1–2 million individuals continent‑wide, with a decreasing trend in the southern portion of its range. Conservation measures include protecting key roost sites, implementing seasonal closures of caves, and monitoring wind‑farm impacts.

• Townsend’s big‑eared bats have ears that can be up to 30 % larger than their head, enhancing passive listening for prey.
• They are one of the few North American bats capable of both aerial hawking and gleaning, giving them a flexible foraging strategy.
• Mating involves delayed fertilization; females store sperm through the winter and ovulate in spring.
• The species can enter torpor on warm days during summer, reducing energy expenditure when insects are scarce.
• Despite being listed as Least Concern, some subspecies such as C. t. ingens are federally listed as threatened in the United States.