Extinctions are the worst, man. It's bad enough that because of them you'll never get to see a Baiji River Dolphin, or a Hawaiian Crow; and of course you can forget about finding a best Edmontosaurus contest at the state fair this summer. But more importantly extinctions can cause whole ecosystems to fall apart; once they happen the global environment is never the same and according to research published last week in the journal Science the rate of extinctions happening now around the world is beginning to look like what scientists are calling earth's sixth mass extinction event. Now, it's not as bad as the Permian-Triassic extinction that happened 250 million years ago, the worst that the world has ever seen and it's not even as severe as the one that wiped out the dinosaurs (which we'll be posting an episode on in a couple of weeks). But it's still looking pretty grim out there and scientists say it's probably our fault. The research surveyed from scientific literature from around the world and over time to document where species have disappeared as well as how surviving animal populations have changed: and the results show that since the year 1500 more than 320 terrestrial vertebrate species have become extinct, most of them in the past 200 years. That's not even counting the water ones. The data showed that the remaining vertebrate species are suffering an average 25% decline in total numbers. For invertebrates, like insects and arachnids and worms, it's more like 45%. But much of this damage is being done to larger animals, or megafauna, like tigers, and rhinoceroses and polar bears because they tend to produce fewer offspring and also require larger habitats. This has led to a special kind of extinction that scientists call defaunation, the loss of large vertebrates both predators and herbivores which causes the species they eat to take over whole ecosystems and the most rapid decline in animal numbers turns out to be where human population density is highest so the scientists conclude that the extinctions are being driven mainly by loss of habitat and climate change both of which are caused by people.

But we're not totally out of options; the raft of research released last week also includes papers that suggests how we could if not stop the extinction entirely at least lessen the damage. Aside from obvious but difficult things like not paving everything and using fewer fossil fuels some researchers recommend translocation, basically moving existing animals around, reintroducing captive creatures into wild populations and recolonizing others in places where they've gone locally extinct. And then there's de-extinction which we've talked about before: bringing back species that have disappeared through selective breeding with existing species or maybe even cloning. Ultimately the researchers conclude we're clever enough as a species to have caused all of these problems so we just might be able to use our big brains to think our way out of them.

And there's another development in animal research that's equal parts spooky and amazing. Researchers in California have made the first transparent animal. The animal to be clear is a dead animal and only its outer layers of tissue are transparent, but still this see-through creature- a lab mouse- is just the latest development in tissue clearing, a process biologists use to make animal tissues clear so that they can be studied in microscopic detail without dissecting them, thereby keeping all the cellular structures and connections intact. Making some of these tissues transparent and infusing others with dye, for example, could allow scientists to turn an animal specimen into basically a 3-D model of itself, revealing things like the exact wiring of the brain, or the organization of cells in tiny organ structures.

It's like if you ever had one of those visible man or visible woman toys except way more detailed. This has been done before on a small scale by injecting a tissue sample with hydrogel, a firm but flexible material that preserves the three dimensional structure of the tissue right down to the cellular level. The gel is then infused with what's basically a really strong detergent that breaks down the lipids or fatty molecules in the tissue that make it opaque. In the latest development, researchers at Caltech managed to make a whole mouse transparent enough to reveal the inner structures of its organs by injecting the hydrogel directly into its circulatory system, which then diffused the clarifying detergent slowly through the body. First its kidney and heart and lungs, followed by the brain and then the rest of its organ systems. Scientists say their see through mouse has all kinds of implications for the future of histology or the study cell's anatomy. But for the rest of us it's just like really cool.

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