You can technically catch shrews in Sherman traps, but you're more likely to get rodents. Pitfall traps are better, but again, the success rate for any trapping project is often pretty frustrating when you're trying to build a respectable sample size. Also, you can't tell between the northern and southern short-tailed shrews (Blarina brevicauda and B. carolinensis, respectively) by external morphology, you have to use skull measurements. This means that if you do catch an animal, it has to be skinned and the body has to be skeletonized in the beetle colony, which takes quite a while. (Just as a side note, one interesting/sad thing about shrews is that it is extremely rare to find one live in a trap, their metabolisms are so fast that they can starve in as little as two hours).
Question: What is an easier way to get shrew data?
Answer: Beer bottles.
*braces self for wisecracks about boozey college kids*
Seriously, though, checking old beer bottles along roadsides is the fastest and most efficient way to collect shrew skulls. There have even been peer-reviewed papers written about using the method (Pagels and French 1987, Gerard and Feldhamer 1990, Benedict 1999). They like going into small dark holes, and will crawl into beer bottles pretty frequently. If the bottle is on a slope, they can't get traction to climb back out again, and they starve so quickly that they're just goners. We have found about seven skulls in a single bottle before! I have only ever found one in a plastic bottle, usually they turn up in glass ones. Occasionally we find skulls from small rodents (we found a vole on our last collecting trip), but the vast majority of "bottle victims" are shrews.
It's never as simple as picking up a bottle and seeing a tiny skeleton, there is almost always a bunch of sediment/slime/goo in the bottle. If the material is not too compacted you can pour it out and use a pokey stick to uncover bones and hair, but sometimes you have to break the bottle. (There is an art to hitting the bottle with the hammer hard enough to break it but also aimed so that it won't shatter any skulls that might be inside, these things are unbelievably delicate).
Sometimes the success rate is better than others, in a good site you might get one shrew bottle for every ten you pick up, sometimes you get nothing. Places on slopes with loose leaf litter are the best bets. If you see beetle remains in a bottle that is a good indicator that you should check it carefully for skulls, because beetles frequently feed on the shrews after they've died. Also, the lower jaw is almost always disarticulated, so it's important to make sure you end up with two dentaries (right and left) for every skull you find, it's easy surprisingly easy to miss them in even a half a cup of bottle sludge.
So, there is my ultra high-tech scientific method of the day. ;) It's a good way to combine community service with field work, though, we always end up with huge garbage bags full of bottles to throw away, we can get hundreds in a single afternoon.
The shrews, by the way are really neat little creatures. They have distinct pigmented teeth, and produce a paralytic venom called soricidin (named in honor of the shrews' taxonomic family, Soricidae). They also use high-frequency calls to navigate, extremely similar to the echolocation used by our other friends from this fieldwork series, bats! (I personally think that using high-frequency calls for auditory navigation is much more widely spread among mammals than we have "discovered" so far).
There are four species of Blarina: the northern, southern, Elliot's, and Everglades (B. brevicauda, B. carolinensis, B. hylophaga, and B. peninsulae). I am studying the two that occur in Alabama, B. brevicauda and B. carolinensis. The genetics of these guys are really fascinating. It is virtually impossible to tell them apart by external morphology. There can be slight size differences, with B. brevicauda sometimes slightly larger, but this is hard to separate from clinal variation, with B. brevicauda species ranging father north. Southern B. brevicauda and northern B. carolinensis specimens commonly overlap in size.
What makes this interesting is that they have a vast karyotypic difference: Despite their physical similarities, the species differ greatly in chromosome number. B. brevicauda has 48-50 chromosomes, while B. carolinensis has only 36-46 (this is attributed to Robertsonian fan rearrangements, basically this means two small chromosomes fuse to make one larger one). It is significant that the 2n numbers are given as ranges. The standard had been set at 2n = 46 for B. carolinensis, yet one study in Tennessee failed to find a single specimen that contained the standard number, although the individuals in the population were breeding and producing fertile offspring (Qumsiyeh et all 1999). Could this great degree of variability possibly be a snapshot of "evolution in action?" Maybe, maybe not, but it's definitely intriguing.
I suppose that's all I have to say about bottling, we'll be back to bats in the next edition of the series!
By the way, we found a pretty complete mammal skeleton by the roadside while collecting bottles during the trip that the above pictures came from. Ultra gold star to anyone who can identify what mammal species it was from the bone I am "accessorized with" in the photos! The fact that I picked it up while bottling in northern Alabama probably narrows the choices down to a few simple guesses, but I still thought it might be worth a quiz question.
Benedict, R. A. 1999. Characteristics of a hybrid zone between two species of short-tailed shrews (Blarina). Journal of Mammalogy. 80: 135-141.
Gerard, A.S. and G.A. Feldhamer. 1990. A comparison of two survey methods for shrews: Pitfalls and discarded bottles. American Midland Naturalist. 124: 191-194.
Pagels, F. and T. French. 1987. Discarded bottles as a source of small mammal distribution data. American Midland Naturalist. 118: 217-219.
Qumsiyeh, M. B., S. Barker, S. Dover, P.K. Kennedy, and M.P. Kennedy. 1999. A potential model for early stages of chromosomal evolution via concentric Robertsonian fans: a large area of polymorphism in southern short-tailed shrews (Blarina carolinensis). Cytogenet Cell Genet. 87: 27-31
4 comments:
Very interesting post! Imagine littering being helpful for Science. You don't hear that everyday...
I am assuming that bottles are scattered around because other people threw them out. And they are scattered randomly enough to be useful for you, i.e., they cover a lot of territory, there is not too much variation in the distance between "batches" and there is not too much variance between sized of "batches" (i.e., number of bottles in a pile). Ideally, you would yourself place bottles in an exact grid - is the random distribution of trash similar enough to such an "ideal" experimental grid? Of course, if you placed the bottles, you would feel horrible about CAUSING the death of so many animals.
P.S. why don't you use the BPR3 code instead of just image? This way, the aggregator cannot track your BPR3 posts.
Very nice post! Kudos to you for digging around in those nasty beer bottles (and picking them up). I'm guessing the bone is from C. latrans. Maybe he was after a shrew meal!
great article but i have trolled for floaters since early high school.
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