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ROBERT SIEGEL, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

If you want to learn about koalas, St. Bees Island is a prime location. It's a small island off the coast of Queensland in eastern Australia. Decades ago, koalas were imported there to enhance tourism at a local resort. The resort is long gone, the koalas have stayed, and their presence has led to a new kind of visitor: research biologists such as Alistair Melzer.

(Soundbite of rainforest)

Dr. ALISTAIR MELZER (Senior Research Fellow, Centre for Environmental Management, Central Queensland University): I've been studying koalas since 1988 and I came to St. Bees Island because I heard that there were some koalas brought over here in the 1930s. What I'm doing is leading a research team, trying to understand why the koalas on this island basically staying as a steady population. The question we've got is that when koalas have been introduced to islands in other parts of Australia, they become pissed. They reach plagued proportions, they killed the trees that they live on, and caused a lot of damage.

When koalas have been introduced to islands in Queensland - that's in the tropics - these populations don't run out of control. So we started to study with the intention of trying to understand what's keeping the population steady, describe the population, figure out how to count the koalas on the island, and see what they're eating, see where they go, see how they're doing. So we catch koalas, we fit them with radio transmitting collars and we tag them and then we follow them around through the bush.

We've been walking for about three hours now, hiking up and down the hill. And we did find spiders and wallabies and butterflies. If that's what we were looking for, we'd be doing a fantastic job. But right now, we're still in the hunt for the koalas. When you look at a picture of a koala when it spans out -big ears, big eyes, black nose. But put it in a tree, in its natural habitat, and it just blends into the landscape. And it's just so hard to see.

Just to convince you that the - that, in fact, the koalas are sitting up trees, waving at you as you go pass, all these scratches on this tree, that's all koala, koala scratches, yeah. That almost certainly, you've walked past, probably, 20 koalas today.

Unidentified Woman: You think so?

Dr. MELZER: Oh, probably.

Unidentified Man: I was in my third reading and we weren't able to pick up anything, really. Just a faint signal back down the hill.

Dr. MELZER: Okay. That's possibly means you have to go higher, believe it or not. It maybe you're so close under the ridge that's the signal's being blocked out, so go up to the crest of the ridge and listen again. Over.

Unidentified Man: Copy that. Right now, basically, I am holding this antenna up high in the air and trying to catch a radio signal. Right now, we've got a pretty good signal. I think we're really close, everybody.

Dr. MELZER: So you're looking for silhouettes of various koala forms. You can be looking up in a dense canopy and you suddenly see that's a patch or fur. And you don't know how you've seen that, but it's because of, maybe, the size of, of half an envelope or smaller. But there you are and you can see that and you react to it and you found your koala.

Unidentified Woman: Push your knees, too. Mommy and babies. Get up. Okay, let's, not make a lot of noise.

Unidentified Man: I could see it now.

Unidentified Woman: Which female is this? India(ph)?

Unidentified Man#2: Natasha(ph).

Unidentified Woman: Natasha? And Natasha has a young joey. I'm not sure, but it's definitely less than one year. Hello Alistair. Are you there? Okay, just to say that we found Natasha and she also has a joey.

Dr. MELZER: Oh, good. We're just going up the nearest (unintelligible). Thanks a lot. Tape what you seen.

Unidentified Woman: So we take measurements of the tree that the koala is in. And then we make a small grid just running north, south, east, west, based on the center of the tree. And then for each direction, we pick the closest tree within that grid just to give an idea of where the koalas being found in around.

Dr. MELZER: We came here to study the ecology of the koalas, to try and understand why they were different from island koalas in other parts of Australia. And we've done that to some extent. We know a lot about how the koalas are using the landscape now, and we know that the koala population is staying stable. We understand that that is probably because the koalas are dying when they are sub-adults before they're independent. But we haven't figured out why they're dying, and that is a mystery for us to pursue. But while we've being doing this, we've been starting to get some insights about koala ecology more broadly.

You have to understand that koalas are animals that - you have to keep their metabolic rate very low, putting all their energy into digesting. They target vegetation that is highly indigestible. So during the day, they - it's still -just keep the metabolic rate very low and most of their activity occurs in the cool of evenings or at night.

Male koalas are highly territorial. And during the breeding season, they spend a lot of time chasing other male koalas that are in the landscape. I do that in two ways. One is just by trumpeting at each other so that at night you can sit down on the beach and you can listen to one koala start to bellow across the hills and then that will trigger another koala that will answer and then another koala that will answer that. And you'll have this cascade of calls that will run around the hills.

(Soundbite of koalas squeaking)

Dr. MELZER: And it's a primeval sound, too. This is a sound that koalas have been making for 15 million years across the Australian landscape. And so when you're privileged to hearing something that's going on, it can never be repeated anywhere else. And let's say the koalas became extinct, there's nothing that will replace that call. There's no other species of koala that can do that. So it's a special sound to hear.

NORRIS: That Science Diary, the first in an occasional series, was produced by Jim Metzner.

SIEGEL: Here's an update on another story about animals out of their natural habitat. There is two whales that swam up a river in Northern California. They'd been moving back toward the ocean. But on Monday, they stayed put near a bridge about halfway along their journey. And this morning, they were spotted moving farther down the river closer to San Francisco Bay. Boats have been deployed to make sure that the whales didn't get lost again and swim up another river that feeds into the same delta.

Scientists hope that the saltier water closer to the ocean will help heal wounds on their skin. On Saturday, they injected the whales with antibiotics. Over the Memorial Day weekend, whale watching was a big attraction on the Sacramento River. The two whales, a mother and baby, were first seen in the river more than two weeks ago.

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