Ecological Field Techniques

As you can tell from my previous posts, my primary interest in Chile is to collect a lot of blood samples so I can determine cortisol concentrations. But my time down here isn’t just spent bleeding degus; in order to get some of my desired blood samples, I have to employ several ecological field techniques. And in addition to collecting these blood samples, I am also gathering other types of data to complement my primary research. Let me describe some of the techniques that I have been using this field season:

Widespread trapping:

For my primary project investigating the effects of poor maternal care on the pup stress response, I have to find and determine several social groups. To do this, my collaborators and I set out 300 live Tomahawk traps over a large area during early August. We tried to place the traps in areas where we found active burrow systems; a good way to determine whether a burrow system is active is to look for fresh feces and dust-bathing sites. If a fecal pellet is fresh, then it should be soft and green when you break it open. Dust-bathing sites are near burrow openings and are little sandy areas that are free of rocks and vegetation. Watching degus dust-bathe is rather cute; they quickly flip onto their side, roll in the dust, and jump back into an upright position all within a second.

An open Tomahawk trap

Anyway, after setting out our traps, we trapped for several hours a day for about 3 weeks. We would typically open and bait the traps in the morning (we use rolled oats for bait, the degus absolutely love it) and then check and re-bait the traps every hour or so. If we caught a degu, we would take it back to the truck, put an ear tag on each ear, weigh the animal, and then check their reproductive condition if they were a female. We would then return the degus to their respective burrows at the end of our trapping period. We weren’t getting many degus at first, so we spent a few days perched on the hillside, watching degus through our binoculars so we could figure out how to better place our traps.

Ear-tagged degu

Male degu

Female degu

Getting weighed

Nighttime Radio-telemetry:

To determine which degus belong to which social groups, it is necessary to supplement trapping numbers with nighttime radio-telemetry data. Figuring out social groups is tricky because degus use multiple burrows, and some degus move around more than others. The accepted rule for determining social group membership is to establish whether a group of animals spend 80% or more of their nights in the same burrow together. So, in order to figure out my social groups, I had to radio collar and track my females for two weeks. Earlier this month I re-trapped my degus and removed the collars so it wouldn’t be an extra burden during their last few weeks of pregnancy. After the degus give birth (about two-thirds of my degus have already given birth) we then re-collar them and do more tracking because social group membership can frequently change composition.

In order to radio-collar an animal, the first thing I had to do was to make sure that the collar was still functioning properly. After checking that the collar had a strong, clear signal, I then took a piece of special wire and flamed some heat-shrink tubing onto it. I then threaded the wire through the transmitter, threaded one end of the wire through a piece of hard, plastic tubing that protects the transmitter’s antenna, and then pulled the two wire ends through a crimp. With one person holding the degu, I then flipped the collar onto the degu’s neck and used two pairs of pliers to tighten the collar. The degus necks are always smaller than they appear because of their thick fur, so I had to rotate the collar to get the fur out of the way and continually test whether I could push the collar over the degu’s head. It’s important to make sure the collar’s tight enough so the degu can’t get it’s front paws stuck in it, but it’s also important to make sure that the collar isn’t too tight because it can irritate the degu’s neck and cause an infection. Once the collar was the right tightness, I then crimped it, cut the wire ends, and returned the degu to its burrow. Every subsequent time that I caught the degu I checked to make sure that the collar wasn’t irritating the skin.

Radiocollar

And a radiocollared degu

Actually tracking the radio-collared degus wasn’t too difficult. By using a receiver attached to a tracking antenna, I would type in the frequency of the degu’s collar (each radio collar has its own, unique frequency) and then wander around listening to the loudness of the beeping. The receiver and collars are usually sensitive enough to pinpoint the degu’s location within a meter or so, but large obstacles like rocks and trees can sometimes bounce the signal.

Modeling the radiotelemetry equipment

Vegetation Sampling:

Some of the degus that I’m working with are part of my Chilean collaborator’s long-term study, and one of the measurements that he has always taken is relative food abundance around the different burrow systems. To do this, we take vegetation samples from two areas per burrow system. To determine which areas to measure, we choose a direction at random (N, S, E, or W) and then mark a spot 3 and 9 meters from the center of the burrow system. We then lay down a 20cmx20cm grid and harvest all of the plants within it. This sounds pretty easy, but we also have to take off the roots from the plants and then separate them into monocots and dicots. The samples are then weighed after being dried in a drying oven for 72 hours.

Hematocrit, Soil Hardness, and Fecal Water Content:

One of the other students (a recent Tufts graduate) is doing a side-project to examine the correlation between hematocrit and cortisol levels. Hematocrit is the proportion of red blood cells in the blood and has been linked to hydration, stress, altitude, etc. Measuring hematocrit is fairly easy; when I bring a blood sample back to the lab I have to spin it in the centrifuge to separate the plasma from the red blood cells (I collect the plasma because that’s where the hormones are). The blood samples are in little, glass tubes and after spinning them each tube separates into a bottom layer of red blood cells and a top layer of clear, yellow-ish plasma. I then use some calipers to measure the lengths of the two layers and, presto! There’s the hematocrit measurement.

Because hematocrit is linked to hydration, we’re also measuring rainfall (as determined from local weather stations), soil hardness, and fecal water content. Soil hardness is quick and simple; we just take a penetrometer (it looks like a long, metal tube with a spike sticking out the bottom), place it on our soil site, and push it all the way to the ground. If the spike digs in far, then the soil is soft and we’ll have a low reading. If the spike doesn’t sink into the soil much, then the soil is hard and we’ll have a high reading. To measure fecal water content, we first collect the feces from the degus (you just put a paper towel under the trap and let the degus work their magic, it doesn’t take very long) and then weigh the feces, dry them in a drying oven for 8 hours, and then weigh them again.

Collecting feces

Ectoparasites:

Degus, like many rodents, are often infested with fleas and other ectoparasites. Ectoparasite level is a good indicator of general health and may even be correlated to immune function. To measure ectoparasite levels, we have one person hold a degu over a yellow bucket while another person brushes the degu with a flea comb. We first comb the degu’s back in the direction of the fur 15 times and against the fur 5 times. Then we comb each side with the fur 5 times and against the fur 5 times, and finally we comb the head against the fur 5 times. Before we comb the degu we spray some alcohol into the bottom of the bucket to instantly kill the parasites, which we then collect and store in alcohol.