Original Article By Andrew Sullivan
Zoology Technician, New Brunswick Museum Department of Natural History
How do you preserve a frog for scientific study? The answer might surprise you.
Beyond the familiar specimen jars filled with ethanol, researchers at the New Brunswick Museum have revived a fascinating old technique—skinning and pressing amphibians like botanical specimens. This approach not only preserves DNA and natural coloration better than traditional methods, but also creates scientific specimens that can be easily stored, studied, and shared worldwide.
The New Brunswick Museum research collection houses over 11,000 individual wet-preserved amphibians and reptiles stored in jars of 70% ethanol. Wet specimens provide anatomical and biogeographic data and can be a source for many methods of analysis. For example, several species of fungi that cause high mortality in frogs and salamanders have been tracked historically using wet collections of amphibians. Images 1 and 2 show rows of the New Brunswick Museum wet-preserved amphibians and reptiles collection.
Before going into ethanol, wet specimens are injected and “fixed” in formalin (a diluted form of formaldehyde). The formalin stiffens the animal by cross-linking protein molecules in the cells, creating a rigid scaffold within the animals’ tissues. Unfortunately, this method results in the rapid loss of skin coloration, and because of the protein cross-linking, DNA analysis is usually not possible using current technology. Images 3, 4 and 5 show researchers fixing frogs for preservation.
Historically, skinning and drying amphibian skins, particularly from frogs (as salamander anatomy complicates skinning), has been an uncommon method for improved preservation of their color patterns. However, as technology has evolved, it is now clear that the method offers new opportunities for both specimen collection and use. In a recently published Herpetological Review article, Dr. Donald McAlpine, Head of the Department of Natural History and Research Curator of Zoology at the New Brunswick Museum, and his colleague Dr. Frederick Schueler detailed an expanded methodology for preserving snake specimens. This new approach incorporates the skinning of snakes alongside adapted herbarium botanical techniques to improve the preservation and organization of the resulting skins (see McAlpine, D.F., Schueler, F.W. (2018). Herpetology Meets Botany: Using Herbarium Methods to Archive Dried Skins of Frogs and Snakes. Herpetological Review, 49: 236–238). Image 6 features Dr. Frederick Schueler’s diagram of the skinning process.
So what do a frog and a flower have in common? Well, both can be dried flat. Frogs are surprisingly easy to skin; the skin is tougher than it looks and is only firmly attached to the body around the hands and feet and over the bones of the head. Ventral incisions are made down the middle of the frog, and along each limb, then the skin is stripped by hand with little need for additional cutting. Snakes are incised down the belly as well, but off-centre along the edge of the ventral scales. And of course there are no legs to worry about. Image 7 shows a split frog in the process of being skinned.
While the immediate outcome seems like a flimsy, crumpled wad of frog or snake skin, the next step is to spread and flatten the skin. With a blunt tool like a pen cap, and some forceps, the skin is pressed and tugged and straightened on a piece of wax paper until nearly all the wrinkles are smoothed and it’s nice and symmetrical—like cartoon roadkill. Another piece of wax paper sandwiches the skin. After that it’s placed in a plant press to dry, or in a pinch, pressed and dried under a rug or even a bed mattress. The final step is to attach the dried skin, now freed from the wax paper, to acid-free, lignin-free herbarium paper using acid-free linen tape. Image 8 shows what a wad of skin looks like before it is pressed and flattened.
While this preservation method does not result in a natural pose, to say the least, it does have several benefits. Skins can be stored in file-folders in herbarium cabinets, occupying a fraction of the space that is required to store the same number of specimens in jars of alcohol. As with plant specimens, mounted skins can also be easily scanned and shared electronically. Images 9-12 show a completed frog specimen mounted on paper.
As previously noted, colours are better preserved in dried skins, and unlike wet specimens, so is DNA! These days, many museums, including the New Brunswick Museum, routinely collect small tissue samples for later DNA analysis. While these are mainly stored at low temperature (-80° C), separately from other wet or dry specimens, dried skins stored at room temperature have also proven to be a very useful source of DNA. Image 13 shows a scanned snake skin and Image 14 displays a scanned frog skin, demonstrating how these specimens can be digitized for sharing with researchers worldwide.
Sometimes specimens are not in good enough condition to be preserved whole. The specimen may have been found dead and crushed on a road, or it may be represented only by predator remains. Sometimes snake records are represented by only a cast skin (still a source of DNA). Or a researcher may wish to dissect the fresh specimen or prepare a skeleton. If the bones are of particular interest, skinning allows for the skeleton to be cleaned and the skin saved. Salvaged specimens, such as roadkill, are increasingly important to scientists as permission to actively collect and kill live animals becomes more difficult to secure or species simply become so rare that collecting cannot be justified. Although roadkilled frogs can be in pretty rough shape, the skin can often be saved. Image 15 shows a cleaned frog skeleton and Image 16 features a roadkill snake skin that has been preserved using this technique.
