Hawaiian bats: new data reveals a more complicated history

The story of Hawai`i’s bat populations just got more complicated.

The Islands have long been assumed to have just a single land mammal, the `ope`ape`a or Hawaiian hoary bat. 

(Image: The bones of a newly reported but extinct Hawaiian bat, which has been named Synemporion keana, discovered in Maui’s Mahiehie Cave. Credit: © American Museum Novitates)

Our marine mammals, the seals and porpoises and whales, pretty clearly got here by swimming. But for a land animal, arriving across a couple of thousands of miles of ocean is more problematic. So, no rats, mice, dogs, cats, wolves, cows, or any other land mammals in pre-contact Hawai`i. 

But bats, particularly migratory bats capable of long-distance flight, could presumably have made the cross-ocean transit.

And new evidence says they did, several different times.

A study by the late Bishop Museum mammologist Alan Ziegler, Bishop Museum entomologist Frank Howarth and Nancy Simmons of the American Museum of Natural History, reveals that the Hawaiian hoary bat is actually two populations, descended from two separate immigration events more than 9,000 years apart. 

Their paper, published in the journal American Museum Novitates, is here, and there’s a press release from the American Museum of Natural History is here. 

 The first Hawaiian hoary bat arrived in the Islands about 10,000 years ago—long before humans were present. The second immigration was just 800 years ago. Both of those immigrations involved the same species, although there are differences among zoologists whether they have since evolved into new species. 

The parent species was Lasiurus cinereus, and authors take the view that the Hawaiian bat is a subspecies, Lasiurus cinereus semotus.  But they agree that further study could change that designation.

But they also noted something that has been understood for decades: that fossil evidence shows that there was another bat, now extinct, flying Hawaiian skies. 

It is smaller than the `ope`ape`a and different in several ways. It has been given the scientific name Synemporion keana.

Like the hoary bat, it appears to have been an insect eater, and to have been spread throughout the Hawaiian Islands. It is not clear where the parent Synemporion came from, but it evolved into a distinct species in the Hawaiian Islands.

The paper’s co-author, Frank Howarth, first found the bones of this bat in a Maui cave in 1981. 

"The initial specimens included skeletons embedded in crystals on the lava tube wall and thus were likely very old. Ziegler eagerly guided me through the bat collection at the Bishop Museum to identify the bat and show me features to look for in order to find additional material for study," Howarth said.

Since then, fossil bones from the bat have been found on a total of five Hawaiian islands. Ziegler, a noted and respected Hawaiian biologist, died in 2003 before completing his study of the new bat. His work was completed by Simmons.

The fossil record suggests this bat was Hawai`i’s first bat. 

The oldest fossil bones date back 320,000 years, but the authors say the limited fossil record doesn’t allow an accurate arrival date. It could be a few millions of years earlier.  

Synemporion keana appears to have gone extinct about 1,100 years ago—roughly the time when both humans and rats first appeared in the Hawaiian Islands.

The authors suggest that Polynesian rats, Rattus exulans, which arrived with the first Polynesian settlers, may have either directly or indirectly led to the bats’ extinction.

“If Synemporion roost sites were accessible to rats, it is possible that Rattus exulans may have had a direct impact on the bat populations by preying upon roosting bats and/or their young. Alternatively, the effects of rats on the local environment may have indirectly contributed to bat population reductions and extinction,” the paper says.

It’s possible that the bats survived into the historic period, when Europeans brought in roof rats, cats, mongooses and other predators, which would have completed the extinction if the Polynesian rats hadn’t, they write.

© Jan TenBruggencate 2016