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The Hawaiian islands are home to some truly unique species found nowhere else in the world. If you’re wondering what kinds of mammals originally inhabited these remote Pacific islands, you’ve come to the right place.

If you’re short on time, here’s a quick answer: the only terrestrial mammals native to the Hawaiian islands are bats – specifically two vesper bat species in the genus Lasiurus.

In this comprehensive guide, we will cover the following topics in detail:

An Introduction to Hawaiian Mammal Fauna

The Isolation of the Hawaiian Islands

The Hawaiian Islands are one of the most isolated archipelagos in the world, located over 2,000 miles from the nearest continent. This extreme isolation has significantly impacted the evolution and diversity of Hawaiian mammals.

With no land bridges, terrestrial mammals originally had no way to reach Hawaii. Any mammals that did make it to the islands likely arrived by chance – blown out to sea or rafting on debris from distant landmasses.

As a result, only two terrestrial mammals are believed to have colonized Hawaii without human intervention – the Hawaiian hoary bat and the Hawaiian monk seal.

Impact on Native Mammal Evolution and Biodiversity

The isolation of Hawaiian mammals led to some unique evolutionary adaptations. The Hawaiian hoary bat is Hawai‘i’s only native terrestrial mammal, evolving into an endemic subspecies (Lasiurus cinereus semotus) found nowhere else on Earth.

With no ground predators, the bat became more terrestrial, often seen roosting on trees rather than in caves.

The Hawaiian monk seal has also developed distinct characteristics from its Caribbean relatives. Adapted for colder waters, the round-faced seals have a thicker coat and layer of blubber. Their isolation means a limited gene pool, however, making them vulnerable to disease and population decline.

Today, only around 1,400 seals remain.

While beneficial evolution occurred, the lack of biological interchange also greatly limited the diversity of Hawaiian mammals. No native terrestrial mammals exist outside of bats and seals. All other mammals – from mice to mink and mongoose to axis deer – have been introduced by humans beginning with the Polynesian settlement of Hawaii around 400 AD.

The Hawaiian Hoary Bat

Physical Characteristics and Taxonomy

The Hawaiian hoary bat (Lasiurus cinereus semotus) is Hawaii’s only native terrestrial mammal. These bats have dark brown to reddish brown fur with white tips, giving them a frosted or “hoary” appearance. Their wingspan reaches up to 13 inches across.

Hawaiian hoary bats are a subspecies of the hoary bat found in North America.

Habitat and Behavior

Hawaiian hoary bats roost alone or in small groups in trees across the islands. They especially favor Calliandra, Terminalia, and exotic palms as roost sites. At night, they emerge to hunt insects, especially moths, beetles, crickets, mosquitoes, and termites.

Using echolocation, they can snatch prey from vegetation or on the wing.

Researchers have discovered the bats may migrate seasonally between higher and lower elevations. However, much remains unknown about their habits and distribution across Hawaii’s islands and habitats.

Conservation Status

With an estimated population between a few hundred and a few thousand, Hawaiian hoary bats are considered a threatened species. Major threats include habitat loss, pesticides, entanglement in barbed wire, and predation by owls and cats.

Government agencies and conservation groups are working to reduce threats through measures such as restricting trimming and removal of tall trees. They encourage residents to report bat sightings and roost locations to help monitor population trends.

While often unseen, the unique Hawaiian hoary bat fills an important ecological role. This species will remain a conservation priority into the foreseeable future across Hawaii’s island habitats.

The Hawaiian Sheath-Tailed Bat

Physical Characteristics and Taxonomy

The Hawaiian sheath-tailed bat (Lasiurus cinereus semotus) is the only land mammal native to the Hawaiian Islands. As its name suggests, this unique subspecies of bat has a specially adapted sheath surrounding its tail that allows it to rest while hanging by its feet.

Weighing around 10-15 grams, these small bats have brown fur on their back and lighter fur on their underside. Their wingspan is approximately 10 inches across. Genetic studies indicate that Hawaiian sheath-tailed bats are most closely related to sheath-tailed bats found in the southwestern United States and Mexico.

Habitat and Behavior

Hawaiian sheath-tailed bats roost alone or in small colonies in trees and shrubs across the islands. They prefer diverse forests with access to open areas for foraging on insects like moths, beetles, crickets, and termites.

Interestingly, some bats have been observed catching prey directly off the surface of the ocean.

These agile fliers use echolocation to hunt at night and can reach speeds over 20 mph while diving for insects. During the day, they typically roost camouflaged in vegetation. Their flexible tail sheaths allow them to hang upside down comfortably while resting.

Conservation Status

Once abundant on all the Hawaiian Islands, habitat loss over decades caused Hawaiian sheath-tailed bat populations to decline significantly. By the 1970s they were presumed extinct.

Amazingly, a small population was rediscovered on Kauai in the early 1980s. After receiving protection under the Endangered Species Act, surveys found additional colonies around the island. There are now estimated to be around 500 bats total.

While still endangered, this is a conservation success story showing that dedicated wildlife protection efforts can prevent extinction. Scientists hope that continued forest restoration projects will allow Hawaiian sheath-tailed bat numbers to rebound further across their native island habitats.

Other Mammals Introduced To Hawaii

Domestic Mammals

When the Polynesian ancestors of Native Hawaiians first arrived on the islands over 1,500 years ago, they brought several domesticated mammal species with them, including dogs, pigs, and the small Asian mongoose.

These species were introduced to help with hunting, food production, and controlling rats in agricultural fields.

Today, feral populations of pigs, goats, sheep, cats, dogs, and mongoose are found across the Hawaiian Islands. Many of these species have caused extensive damage to native Hawaiian ecosystems and species.

  • Feral pigs dig up forest soils and prey on native birds and plants. Their rooting and wallowing behaviors often destroy native vegetation.
  • Goats and sheep overgraze native vegetation.
  • Feral cats and dogs prey on endangered native birds and sea turtles.
  • Mongoose prey on native birds and their eggs but have been ineffective at controlling rats.

Controlling and removing these harmful introduced mammals is an ongoing challenge across the islands.

Wild Mammals

In addition to domesticated species, several wild mammal species have been introduced to Hawaii over the last two centuries, both accidentally and intentionally.

  • Rats likely arrived as stowaways on early sailing ships. Today, black rats, Polynesian rats, and Norwegian rats can be found across the main Hawaiian Islands.
  • House mice may have also arrived via early Western ships.
  • Small Asian mongooses were intentionally introduced from Jamaica in 1883 to control rats in sugar cane fields but unfortunately prey more heavily on native species.
  • Masked palm civets and small Indian civets were likely released by the pet trade in the 20th century and are now established on some islands.
  • Axis deer from Asia and Columbian white-tailed deer from North America were introduced for sport hunting in the 1900s. They damage native vegetation and spread non-native plant seeds.

Of these wild species, rats and mongooses have been the most problematic for native Hawaiian ecosystems. Rats prey on native birds and plants while mongooses prey heavily on native ground-nesting birds and their eggs.

Mammal Group Examples of Introduced Species Year Introduced Original Purpose
Domestic Mammals Pigs, goats, cats, dogs, mongoose 400-1200 AD Food, hunting, rat control
Wild Mammals Rats, house mice, axis deer, mongooses 1800s-1900s Accidental or sport hunting

Removing or controlling problematic introduced mammal species remains an ongoing challenge to protect Hawaii’s fragile island ecosystems. Innovative solutions like gene editing are being researched as more humane and effective approaches for the future.

Impact of Introduced Mammals on Hawaii’s Ecosystem

The introduction of non-native mammals to the Hawaiian Islands has had a devastating impact on the fragile island ecosystems. Since the arrival of humans and the associated species they brought with them, many of Hawaii’s native plant and animal species have become extinct or endangered.

Destruction of Native Plants

Introduced herbivores like goats, pigs, deer, and cattle have directly damaged native vegetation through grazing and trampling. They eat native plants and damage root systems, preventing regeneration. This destroys food sources and nesting habitats for native birds and insects.

Predation of Native Birds and Insects

Cats, rats, and mongoose prey on ground-nesting native birds and their eggs. These invasive mammals have contributed to the extinction of over 50 species of Hawaiian birds. They also feed on native insects like happy face spiders, reducing pollinator populations.

Soil Erosion and Water Pollution

Foraging and trampling by destructive introduced mammals removes vegetation that holds soil in place. This causes erosion, runoff, and damage to coastal reefs from sedimentation. Feral pigs contribute to pollution of freshwater streams and estuaries.

Spread of Weeds

Invasive mammals disturb soil and dispersed seeds, assisting fast-growing weeds. These weeds outcompete native plants and alter conditions to favor more weeds over endemic flora. Some even increase fire risk, endangering native ecosystems.

Disease Introduction

Non-native mammals have brought new pathogens and parasites to the islands, including mosquitos that transmit bird malaria, which affects native forest bird populations. Rats and cats spread diseases like toxoplasmosis and leptospirosis that can infect other wildlife.

Continued management through fencing, weed control, and eradication of destructive introduced mammals is critical for protecting Hawaii’s natural heritage. Removing these threats can allow native species to recover and bloom once again.


In summary, the Hawaiian islands originally had just two terrestrial mammalian species – the Hawaiian hoary bat and the Hawaiian sheath-tailed bat. Their existence points to the islands’ past connections to larger landmasses.

Sadly, these native Hawaiian mammals face severe threats from habitat loss and competition from introduced species. Concerted conservation efforts focused on protecting the islands’ biodiversity will be key to ensuring their survival.

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