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Creature Feature: The Hawaiian Hoary Bat (‘ōpe‘ape‘a) is Hawaii’s Only Native Terrestrial Mammal
When the first humans arrived in the Hawaiian Islands around 1,700 years ago, they soon learned that they weren’t the only land-dwelling mammals there. Hoary bats had already colonized the place. Since the islands were born as fiery volcanoes, have never been attached to a larger land mass, and are situated some 2,300 miles from the nearest mainland habitat, the tiny creatures who first carved out a niche in this faraway place had to have made their way across vast stretches of open sea.
Exactly how and when they pulled off this amazing feat remains a mystery. Bats don’t migrate thousands of miles nonstop like some birds do. Scientists suspect that storms might have delivered wind-blown bats to the Hawaiian archipelago, but they can’t know for sure that the pioneers weren’t delivered by some other rare happenstance, such as a tree that drifted out to sea with bats aboard and eventually arrived on a Hawaiian shore. Lacking fossil evidence for the hoary bat, they can’t even pin down the time of the initial colonization, which could have been anywhere from 10,000 to several million years ago. One thing that biologists do know for sure is that the hoary bat is the only extant terrestrial mammal native to the Hawaiian Islands.
The Hawaiian hoary bat (Lasiurus cinereus semotus) is a small, mostly nocturnal insectivore with a flair for adapting to differing habitats. The latter ability is a well-documented trait of the vesper bat family (Vespertilionidae) to which this species belongs. Amazingly, various subspecies of hoary bats inhabit every continent except Antarctica, and can be found on nearly every good-sized island outside the polar latitudes. This is the kind of ubiquity that puts one in mind of people and rats.
An Interesting Name
The hoary bat was labeled hoary (frosty) because its reddish- to dark-brown and gray fur is tinged with silvery white, giving it a “frosty” appearance. The native Hawaiian name for the hoary bat is ‘ope‘ape‘a, a term that mean’s “half-leaf” and refers to the radial-spoked appearance of the bat’s extended-wing profile, which resembles the half of a taro leaf remaining after the top half is removed for cooking.
Distribution and Abundance
However it may be labeled, the Hawaiian hoary bat is a species whose distribution and abundance have been very difficult to establish. One reason is that the bats are hard to see. An adult Hawaiian hoary has a foot-wide wingspan, but weighs only about as much as a mouse and flits about the sky at 60 miles an hour while pursuing moths, mosquitoes, and other night-flying aerial prey. (It likes to feed over water and can consume 40 percent of its body weight, perhaps more, in a single night.) The bat’s diminutive size, nocturnal feeding, twisting flight, solitary roosting (usually in trees, sometimes in lava tubes or rock crevices), and other behavioral dispositions can make it very hard to get a reliable count of its numbers -- or in some cases, to even detect its presence.
Scientists conducting systematic surveys have gotten around the worst of these problems by using MiniBat-III acoustic instrumentation that detects the ultrasonic echolocation calls that bats emit to detect and home in on flying insects. (The bats also emit audible lower frequency calls for social purposes and to warn other bats away from their feeding areas.) This technology, which can detect the presence of bats invisible to observers, has added an element of precision missing from earlier studies.
Just how many hoary bats there may be in Hawaii remains open to question. Wildlife biologists guess that the number is probably somewhere in the range of a few hundred to a few thousand individuals.
All of the main islands in the Hawaiian archipelago have suitable habitat, and that even includes most areas where non-native vegetation has overwhelmed the native plants. Though now absent or rare in some of their historical range, hoary bats have been documented at one time or another on all of the main islands except Ni’ihau. These days the little night-flyers are most commonly seen on the islands of Hawai`i, Kaua`i, and to a much lesser degree, Mau`i. Sightings on O’ahu and Moloka`i are rare. It could very well be that only Hawai`i and Kaua`i have breeding populations, since these are the only two places where pregnant or lactating female bats have been documented in recent years.
Visitors can see hoary bats in all of Hawaii’s national parks if they are persistent and know where and when to look for them – generally near roosting sites and over water or the shoreline within the first hour or so after dusk. As reported in the Pacific Cooperative Studies Unit’s Technical Report 140 (April 2007), a survey conducted in 2005 documented the hoary bat’s existence in all four national parks on the Big Island (Hawai`i) – Hawai`i Volcanoes National Park, Pu`uhonua o Hōnaunau National Historical Park, Kaloko-Honokōhau National Historical Park, and Pu`ukoholā Heiau National Historic Site -- as well as in Haleakalā National Park on Maui and Kalaupapa National Historical Park on Moloka`i.
Many (perhaps nearly all?) of the Big Island's hoary bats migrate seasonally to higher elevations, spending the winter months on the windward slopes of Mauna Loa and Mauna Kea at elevations up to around 7,500 feet . Scientists speculate that bats do this because they find it difficult to find aerial prey in lowland areas during the wetter, chillier winter months and can take advantage of the energy-saving torpor (lower, more efficient resting metabolism) induced by the cool temperatures of the higher mountain slopes. The bats don’t truly hibernate, and some may not even migrate altitudinally.
It’s hard to say just how well Hawaii’s bat population is faring these days. The bat’s absence from much of its historical habitat, the reduced frequency of sightings in some of the areas it currently inhabits, and the spread of humans and their impacts in the islands all heighten suspicion that the bat population may be declining, even if it’s hard to get numbers we can use with confidence. There is no doubt that pesticide use, predation, the removal of native trees in historical roosting areas, and other factors have adversely impacted Hawaii’s bat population to some degree, even if the contribution of each impact is not precisely known.
The Hawaiian hoary bat has been a federally listed endangered species since 1970. The U.S. Fish and Wildlife Service thinks it might be possible to downgrade its status to threatened in the not very distant future, but this will require more evidence of recovery than is currently available. Technically, a downlisting is not supposed to be ordered until there is evidence that the bat population on Hawai`i has increased or stabilized for at least five consecutive years.
Critics of the current federal listing argue that it was done too hastily and without reliable evidence that the species was “in danger of extinction throughout all or a significant portion of its range" as the ESA standard requires. To support the argument for downlisting, they point out that breeding populations of bats are known to exist on at least two different islands (Hawai’i and Kaua’i), that the bat has proven it can adapt to disturbed areas with non-native vegetation, and that the species has benefited from federal- and privately-funded research and recovery programs, including the USFWS-funded Hawaiian Hoary Bat Recovery Plan (completed in 1998), the State of Hawaii's Comprehensive Wildlife Conservationist Strategy, the Hawaiian Hoary Bat Research Cooperative, and research efforts funded by The Nature Conservancy and other NGOs.
Hawai‘i's Comprehensive Wildlife Conservationist Strategy, which identified the hoary bat as one of Hawaii’s Species of Greatest Conservation Need, promotes the conservation of known occupied bat habitat, develops and implements conservation plans that guide the management and use of forests to reduce negative efforts to known bat populations, and provides continuing support for the Hawaiian Hoary Bat Research Cooperative. Since recovery efforts must rest on a solid foundation of scientific research, the contributions of the Hawaiian Hoary Bat Research Cooperative have been very welcome. Established in 2002, the cooperative is managed by Hawaii’s State Board of Land and Natural Resources to harness the resources of government agencies, private landowners, and NGOs (including the USFWS, Hawaii’s Division of Forestry and Wildlife, the Hawaii Forestry Industry Association, and The Nature Conservancy) to the task of providing funds for scientific research.
Much work remains to be done on the research front. If this bat recovery campaign is to work out the way it's supposed to, we'll need to learn a lot more about the Hawaiian hoary bat’s numbers and distribution, critical roosting and foraging habitat, diet, migratory behavior, pesticide susceptibility, predation losses, and other vital considerations.