Field Stress Techniques

One of my projects this field season is to determine the seasonal variation of cortisol concentrations in wild degus during the breeding season. As I blogged earlier, I’ll be collecting four blood samples from each degu: baseline, stress-induced, dexamethasone (DEX) challenge, and adrenocorticotropic hormone (ACTH) challenge. Here’s a quick overview of the four samples:

The baseline sample is taken within 3 minutes of capture (remember, CORT starts to increase 3 minutes after a stressor) and represents the typical levels of CORT an animal experiences throughout the day. The stress-induced sample is taken 30 minutes after capture and tells me how much an animal increases its CORT levels after encountering a major stressor. After the stress-induced sample, I then inject the animal with a DEX (a synthetic version of CORT) and wait 90 minutes before taking another blood sample. DEX binds to CORT receptors, thus initiating negative feedback, so the DEX sample gives me a good idea of how well the animal can turn of its stress response. After collecting the DEX sample, I then inject the animal with ACTH, wait 15 minutes, and then take my final blood sample. ACTH stimulates CORT release from the adrenal glands, so this sample will tell me the maximum amount of CORT an animal can release.

Now let me tell you in a little more detail of how I collected these samples. When I first came down to Chile in June I was working with another graduate student and three undergraduates (well, they’re really post-grads because all of the them had graduated from college that May). After a week or two of practicing our bleeding and handling skills, we set out 120 tomahawk traps at our first site and got down to business. Here’s how a typical trapping day went:

After arriving at our field site, we would set up a degu-processing station near our trapping area. Then, the five of us would each grab a bag of oats, spread out and start opening and baiting the traps. Once the traps were all open and baited, we would station ourselves around the trapping perimeter and begin monitoring the traps through our binoculars. Observing the degus could be boring and tedious- oftentimes we would go hours without catching a degu. The degus also liked to taunt us in various ways, usually by studiously avoiding the areas with traps. Some degus liked to enter the traps partway, gently rest their paw on the treadle, and then quickly run out of the trap. And occasionally, a few particular degus would sit by the traps for long periods of time, just staring back at us.

Degu processing station

Nevertheless, we did catch quite a few degus. When one of us saw a degu set off a trap, we would yell, “DEGU!” and then the closest graduate student (designated bleeder) and post-grad (designated handler) would run towards the trapped degu. The handler (using a gardening glove because the degus will bite) would get the degu out of the trap and hold it for the bleeder. The bleeder would use an electric razor to shave the degu’s leg and would then prick the saphenous vein (one of the main leg veins) with a needle. We first collected blood in a glass capillary tube for future cortisol analysis, and then we would switch over to an Eppendorf tube to collect blood for leptin and ghrelin analysis (these are two important hormones for energy regulation). After we collected the blood, the handler would hold a piece of cotton or gauze to the degu’s leg if it was still bleeding and then the degu would be taken to the processing center while the three other people continued to watch the traps.

Once at the processing center, the first thing we would do was ear-tag the degu so we could properly identify it (this is important because if the animal escaped during processing, then we could complete our measurements if we ever re-caught the animal). After ear-tagging the animal, we would then weigh the degu so we could figure out how much DEX and ACTH to inject. Then we would take a variety of measurements including ectoparasite levels (I covered this procedure in my last post), reproductive condition, glucose levels (we use the same monitor a diabetic might use), and anogenital distance (this is the distance from the top of the anus to the base of the penis or vaginal opening). By the time we were done taking all of these measurements, it was usually time to take the stress-induced blood sample and inject the degus with DEX. Then, we would stick a paper towel under the degu’s trap to collect any feces, cover the degu with a white sheet to keep off the sun, and head back to the trapping area to continue watching the traps.

After catching a few degus, the day would become a game of coordination between watching degus and taking further blood samples on the degus at the processing station. Because we had five people, we were usually able to keep the traps open the whole day. On the occasions when we caught multiple degus within a short period of time, we would have to close some traps for a while because there was no way we could watch all of the traps while processing the degus. This is one of the challenges of fieldwork; you often have long chunks of time where you’re essentially doing nothing, interrupted by brief periods of intense activity.

Fieldwork could sometimes be very frustrating, like when a degu would get caught in a trap and we didn’t notice. This mostly happened at the beginning of the field season- we eventually got better at preventing unnoticed captures by checking that all the traps were visible to at least one person before opening them, and also by tying a piece of orange flagging tape to the door of trap so we could easily tell from a distance whether the trap was open or closed. We also got better at watching the degus and recognizing the sound of a trap being triggered.

Other frustrations included when a degu would go into the trap, step on the treadle, but the trap would fail to shut. Some of the traps are better than others, and we oftentimes had to fiddle with the older traps to make then more sensitive. Sometimes we’d catch a degu but it’d escape from the trap while we were trying to get it out; it’s something that happens to everyone, no matter how long you’ve been working with the degus. And finally, the most frustrating thing about trapping degus is watching the birds eat all of the bait and set off the traps. Sometimes the birds would be so bad that we’d have to re-bait the traps every hour or so. Over the past few months, we’ve caught (in order of capture frequency) rufous-collared sparrows, common diuca-finches, long-tailed meadowlarks, band-tailed sierra-finches, Chilean mockingbirds, mustached turcas, mourning sierra-finches, white-throated tapaculos, eared doves, and shiny cowbirds. Here are some pictures of a few of these birds:

Rufous-collared sparrow 

My advisor, Dr. Michael Romero, holding a long-tailed meadowlark

Mustached turca

Common diuca-finch

White-throated tapaculo

Mourning sierra-finch

Chilean mockingbird

But even with the difficulties of trap-shy degus, unnoticed trappings, escaped animals, and hungry birds, we managed to get our first seasonal samples within two weeks (full stress series on 8 males and 14 females). Because we had a lot of time before gathering our next seasonal samples and preparing for my other project (examining the effects of poor maternal care on the pup stress response), we decided to do a side project to determine whether the stress response differs by habitat. We moved all of our traps up to a nearby field that had lots of boulders and began trapping up there. The boulder field, unexpectedly, was degu heaven, and we collected full stress series from 12 males and 8 females within just three days! Emboldened by our success, we decided to try trapping in an area with lots of trees and vegetation. This site proved to be more difficult, and it took a week to collect full stress series from 10 females and 5 males, after which we had to remove all of traps because it was time to start trapping at nearby national park. Which brings me to….

Parque Nacional Fray Jorge!

Fray Jorge is approximately 350km northwest of Santiago and is an interesting place because while the majority of the park is dry, dusty and covered with cactuses, there are also fragments of rain forest on top of some of the hills. The incoming fog from the ocean sustains these cloud forest fragments, which are more typically found in the southern part of Chile. Alas, we trapped the degus in the drier, scrubbier areas, here’s a good representative picture of our trapping habitat: 

Fray Jorge was a difficult place to trap because the degus rarely ventured out into the open, so we had to place most of the traps in and around the bushes and trap by ear instead of by sight. This usually worked fine, but on really windy days it was hard to hear the traps closing, so we ended up missing a few captures. The cactuses were also a real pain; by the end of the day we’d have tons of cactus spines embedded in the soles of our boots. But we persevered, and by the end of our time at Fray Jorge (about a week and a half) we managed to collect full stress series on 5 males and 13 females.

After we returned from Fray Jorge, we started prepping for my project examining the effects of poor maternal care on the pup stress response. We did some widespread trapping and radio collaring to help determine social groups (I went over these techniques in my last post), and then we started implanting the females with cortisol or placebo pellets after they gave birth.

I will start trapping and bleeding pups in a few days to determine if communal care helps buffer degu pups from negligent parenting. If this hypothesis is supported, then I expect to see high baseline and stress-induced CORT, plus poor negative feedback, from the pups belonging to social groups with cortisol-implanted mothers. You can read my first blog post for a fuller explanation of my experiment.

Buena suerte!