Lasiurus cinereus – Hoary Bat

Lasiurus cinereus (hoary bat)

I’m privileged to work with bats. It’s always a thrill to approach a mist net and see if I’ve been lucky enough to have caught a bat, but it’s especially exciting to realize that there’s a hoary bat (Lasiurus cinereus) stuck in the net.

Hoary bats are the second-largest bat we encounter in California. only Eumops perotis, the western mastiff bat, is larger. I’ll write about Eumops perotis in a future blog entry.

Hoary bats belong to the genus Lasiurus, which means “hairy-tailed.” They have beautifully-furred wings and uropatagiums, with silver-coloration that gives a frosted or “hoary” appearance. This group’s thick coats are necessary since they tend to be solitary foliage-roosters. In other words, they don’t huddle together for warmth in close crevices or under exfoliating bark like other bat groups.

Lasiurus cinereus (hoary bat)
Angry hoary bat mouths are very impressive!

These bats are strong flyers and voracious insectivores. They’re rather intimidating to work with, with huge mouths filled with sharp teeth. They’re not a “beginner bat” when one is learning to work with bats due to their powerful bites and strong wings. In my experience, they do calm down quickly after being disentangled from mist nets if they’re handled gently and appropriately. I took the video below of a hoary bat that we had just retrieved from a mist net. The bat has not yet calmed down, and its angry vocalizations are most impressive.

This angry hoary bat was just freed from a mist net. It's very upset, but will calm down with gentle handling.

I’ve had the privilege to work on radio-telemetry studies of this species. After radio-tagging and releasing lactating female hoary bats, we release them (as quickly as possible!) to return to their pups. Then, the next day, we track to their maternity roosts where they sleep during the day with their nonvolant (non-flying) pups.

Radiotagged Hoary Bat (Lasiurus cinereus)
Hoary bat with radio transmitter glued to its back. The radio transmitter will fall off naturally when the bat molts in a few weeks.

Here, I'm holding a much calmer hoary bat that we've tagged with a radio transmitter. Later in this video, the bat flies out of my hands into the night. But, first, she pees on me.

Unlike other most bat groups, Lasiuruan bats have four nipples, visible in the image below. This allows them to nurse multiple pups. I suspect that the survival rate of Lasiuran pups is lower than the survival rate of pups from other bat groups because they are foliage roosters. Not only are the pups exposed to the wind shaking tree branches, but they’re also vulnerable to predation by corvids & raccoons.

Lactating Female Hoary Bat (Lasiurus cinereus)
Hoary bat nipples. This female is lactating, and the nursing pups have worn the fur off around her nipples.
Hoary Bat Maternity Roost
The same radio-tagged hoary bat shown above. We've tracked her to her roost, where she's snuggled with her two (large) pups. These pups were volant within a day or two of this photograph.

Per the Western Bat Working Group, hoary bats strongly prefer to eat moths, but are known to eat other insect orders as well. There’s some thought that hoary bats may themselves be carnivorous on other, smaller bats such as Parastrellus hesperus, but there is not (that I’m aware of) anything more than anecdotal evidence for this. From what I’ve heard from bat researchers, playing hoary bat calls is known to cause other bat species to disperse from an area.

Speaking of hoary bat calls, they’re rather low-frequency, usually between 20 and 25 kHz. They have a characteristic “U” shape that is similar to their small cousins, the western red bats (which call at ~ 40-45 kHz).

Recorded hoary bat call, showing characteristic "U" shape and 20-25 kHz range.
Recorded hoary bat call, showing characteristic “U” shape and 20-25 kHz range.

As a side note, if anyone is interested in seeing this fantastic bat in person, come out to the Sutter Buttes Regional Land Trust / Middle Mountain Foundation’s bat hike! My mentor, David Wyatt, offers these events several times per year. They’re a lot of fun, and we are usually able to catch, display, and release a representative assortment of California bat species.

See also:

Hoarybat (Homo sapiens) and hoary bat (Lasiurus cinereus)
The author holding his first hoary bat.
Lasiurus cinereus – Hoary Bat