Measuring Stress Hormones in Wild Animals; Different Techniques and Their Comparative Advantages and Disadvantages

Measuring circulating levels of stress hormones (cortisol, corticosterone, etc.) in wild animals can provide valuable information for many types of studies. For example, conservation biologists may want to determine if high levels of snowmobile traffic are associated with increased cortisol levels in elk. Behavioral ecologists may want to know if white-faced capuchins (a type of monkey from Central/South America) have lower levels of cortisol if they come from a troop with a high level of social support. And physiological ecologists may be interested in whether Galapagos marine iguanas have a corticosterone rhythm driven by photoperiod or tidal cycle (*see bottom of page for more details).

In order to measure levels of corticosterone or cortisol (CORT), researchers need to obtain blood, feces, urine, saliva, feathers, or hair from their animals. Here’s some information about each method and their advantages and disadvantages:

Blood

This is the most common way to measure CORT concentrations. Because hormones circulate through the blood, this method provides researchers with the most functional measure of CORT. Since CORT has a very high concentration in the blood (compared to other hormones), blood samples do not need to be very big (for degus, 30uL of whole blood is usually enough to determine baseline CORT). The downside to using blood samples is that within 3 minutes of encountering a stressor, an animal’s CORT levels start to increase. So, if a researcher wants to determine baseline levels of CORT, they must get a blood sample within 3 minutes of capture.

Another downside of using blood samples is that each sample is only a snapshot of an animal’s stress response. In order to get a more integrated picture of an animal’s stress profile, researchers usually take a series of blood samples over the course of 1-2 hours. That way, they can see the rise from baseline, the peak of the CORT response, and then the gradual decrease caused by negative feedback.

After obtaining a blood sample, the samples must be kept chilled until they can be spun in a centrifuge (you want to spin within 24 hours of collection). Once the samples are spun, the plasma or serum (the clear layer on the top) needs to be drawn off and saved for analysis. The plasma or serum must be frozen at -20˚C and can be stored for several months. These requirements are fairly reasonable, but sometimes field sites are far away from cities. Last year I worked in a national park that was pretty remote, so I had to manually spin my samples with a hand-crank centrifuge and store my samples in a cooler full of ice. Luckily, steroid hormones like CORT are fairly sturdy, so lengthy processing times aren’t a big deal.

Finally, to process the plasma samples, researchers need to run a competitive binding assay called a radioimmunoassay (RIA). Basically, known amounts of radiolabelled CORT and CORT-antibody are added to the sample. The radiolabelled CORT and normal CORT from the sample will compete to bind with the antibody. The unbound radiolabelled CORT is then washed off, and the radioactivity is measured. The more CORT you have in your sample, the more it will bind with the antibody and displace the radiolabelled CORT. Therefore, the higher the CORT in the sample, the lower the radioactivity. RIAs are fairly straightforward but require some special equipment, so they usually have to be run in laboratories that frequently measure hormone concentrations (like my lab!). The enzyme immunoassay (EIA) is another way to measure hormone concentrations and requires less equipment, but is generally more expensive to run than a RIA.

As an additional step, researchers may also want to measure levels of corticosteroid binding globulins (CBG). CORT binds to CBG in the bloodstream, and it’s thought that only unbound CORT is “free” and able to bind to cell glucocorticoid receptors. Therefore, the functional measure of CORT is the “free” CORT, so some researchers also measure CBG levels to determine the relative amounts of unbound CORT. However, it’s still under debate as to whether only unbound CORT can bind to glucocorticoid receptors, and it has also been pointed out that without CBG, CORT would be unable to circulate through the whole bloodstream and reach all the cells in the body.

My blood samples are collected in little, glass microhematocrit tubes. I put them in plastic tubes with clay in the bottom so the bottoms of the microhematocrit tubes are plugged. I then put three plastic tubes in a conical tube which are stored in a cooler full of cold packs.

Loading the centrifuge with my samples.

After spinning the blood sample, it separates into hematocrit (the bottom red stuff) and plasma (the top, clear stuff). The plasma is what has the hormone, and that's what I draw off with a glass Hamilton syringe (shown on the table).

I pipette the plasma into a plastic eppendorf tube which then goes into a -20C freezer.

Saliva

            CORT can pass from the blood to the saliva, and salivary samples are actually a common way to measure cortisol levels in humans. While captive animals can be trained to lick, chew, or drool on something, obtaining sufficient salivary samples from wild animals in non-invasive manner is most likely impossible. However, it usually takes 20 minutes after a stressor for CORT levels to increase in the salvia, so taking salivary samples could give researchers more time to collect a sample after capture.

Feces/Urine

Another way to assess CORT levels is to measure the metabolic products of CORT in the feces. However, because this method measures the metabolites of CORT, it doesn’t have quite the same functional value that a blood sample does. There are several steps involved in breakdown of CORT, and CORT metabolite formation can be affected by sex, season, metabolic rate, and diet. Therefore, experimental power is lost when using fecal samples for CORT analysis because animals cannot be compared between different seasons, locations, or sexes.

The main advantage of using fecal samples, however, is that they’re fairly non-invasive. Researchers can follow individuals and collect feces, or in the case of small mammals, check traps after a certain period of time and collect any feces from the bottom of the cage. Using feces to measure CORT levels is also great for repeated sampling, since frequent blood sampling and other more, invasive measures can change an animal’s stress response.

Using fecal samples to determine stress hormone levels makes a lot of sense if you’re working with an animal that’s endangered or difficult to capture (like an arboreal monkey). Another advantage of using feces is that they represent an integrated measure of CORT exposure; if you know how long it takes for an animal to form a fecal pellet, then you can estimate their total CORT exposure over a period of several hours.

Collecting feces is usually pretty easy, but investigators should always try to obtain fresh samples from a known individual. If this isn’t possible because the study animal is hard to find or lives in an aquatic environment, then dog trackers can be used. The Wasser Lab at the University of Washington (my alma mater!) uses canine trackers to find feces from whales, tigers, wolves, and other, elusive endangered animals (http://conservationbiology.net/conservation-canines/#scat).

 Storing fecal samples can be tricky; temperature, storage liquid, autoclaving, and storage time can all affect fecal glucocorticoid metabolite (FGM) concentrations. Sample mass can also affect FGM concentrations, as very small samples have disproportionally higher FGM levels. Getting an adequate sample mass can be difficult when studying small animals like songbirds, mice, etc., so many studies have to combine several fecal samples in their assays.

After homogenizing the fecal samples, FGM concentrations can be measured via RIA or EIA. Another downside of using fecal samples is that the CORT-antibody may not bind with all the different CORT metabolites. Therefore, researchers usually need to do a validation experiment to determine that increased CORT levels in the blood correspond to increased FGM concentrations in the feces. One advantage of using feces, though, is that by measuring levels of metabolites the researcher is essentially measuring “free” CORT, so there’s no need to measure levels of CBG.

Like feces, urine also contains glucocorticoid metabolites (and some unmetabolized CORT). In the field, however, it’s very difficult to collect urine samples in a non-invasive manner, so urine collection for CORT analysis is rarely used outside of the laboratory.

Feathers/Hair

             Believe it or not, you can actually measure CORT in feathers. During molt (feather growth), each individual feather is vascularized, and CORT from the blood can be deposited in the feather. After the feather stops growing, the blood supply is cut of and, supposedly, no further CORT can be deposited in the feather. Therefore, feathers provide a good, integrative measure of CORT during the period of molt.

            Plucking a feather is easy to do and there’s no time limit, unlike blood sampling. Using feathers can also be non-invasive if the researcher obtains naturally molted feathers. And another big advantage of using feathers to determine CORT levels is that feathers from dead birds can also be assayed (like museum specimens!). Also, feathers do not need to frozen or stored in any specific manner.

            The big downside to using feathers for CORT analysis is that we’re still not too sure what form of CORT we’re actually measuring. Different antibodies have been found to have different levels of success during feather CORT analysis, which has made researchers unsure whether they’re measuring CORT, CORT metabolites, or other molecules similar in structure to CORT.

Another downside to using feathers is that, like feces, there’s a sample mass bias where smaller feather samples have disproportionally higher CORT. Getting a large enough sample can be difficult because researchers don’t want to compromise a bird’s flying ability, and there’s also the problem that different feathers are grown at different times, so pooling certain feathers together may not be appropriate. Feather mass requirements can also prevent researchers from examining portions of the feather (sections near the end of the feather are older, and thus represent a different time period than sections near the bottom), but a recent study found that different feather sections didn’t correspond to the CORT concentrations in the blood at the time of their growth, anyway. Feather CORT can be measured via RIA, but the preparation is a real pain (you have to mince the feather into little, tiny bits).

            CORT can also be deposited in hair during follicle growth. One advantage of hair samples is that they can represent CORT exposure over a very long period of time. The downside to hair samples is that it’s hard to collect a large enough sample in a non-invasive manner. And, like feathers, there are still a lot of questions about what the antibody is actually binding to in the assay. Another disadvantage to using hair is that the sample could be contaminated with other, CORT-containing secretions, like saliva or sweat.

            So, I’ve tried to give a basic, if somewhat extensive overview of the different ways to measure CORT in wild animals. After I was almost done writing this blog, I came across a great review paper that includes everything I mentioned and more: Sheriff, M.J. et al. (2011) Measuring stress in wildlife: techniques for quantifying glucocorticoids. Oecologia 166:869–887. Also, here’s an excellent review on fecal CORT: Goyman, W. (2012) On the use of non-invasive hormone research in uncontrolled, natural environments: the problem with sex, diet, metabolic rate and the individual. Methods in Ecology and Evolution 3, 757–765. And finally, my favorite feather CORT paper: Lattin, C.R. et al. (2011) Elevated corticosterone in feathers correlates with corticosterone-induced decreased feather quality: a validation study. Journal of Avian Biology 42, 247-252.

*(Photoperiod vs. tidal rhythms: most animals have daily stress hormone rhythms that are related to foraging opportunities. For humans, the morning is an optimal time to forage, so our cortisol levels often peak right before we wake up, and these increased cortisol concentrations help us go out, run around, and collect some food. For marine iguanas, corticosterone concentrations are influenced by time of day AND tidal cycles. This is because marine iguanas need to eat algae in the inter-tidal zone and the algae are most available during low tide, but because iguanas are ectotherms and need to bask in the sun before and after swimming in the inter-tidal zone, foraging is only possible during the day. However, this relationship still needs be teased apart, since the observed CORT peak during low tides could also be influenced by food intake.)

Radiocollars

Most of my degus have given birth! Which means that things are busy in the field and I have been spending lots of time implanting cortisol pellets and collecting baseline blood samples. After I finish implanting my cortisol pellets, I will need to put radiocollars on my female degus so I can determine social group composition. While you can usually tell which degus are in which social group by seeing what burrow they're trapped at everyday, this isn't totally reliable and the best way to determine social group composition is to see who spends the night with each other in the same burrow.

The radiocollars I have been using have good signal strength and long-lasting batteries, but the collar itself irritates the degu's necks and I usually have to remove the radiocollar after a few days. So, in order to protect my degus necks, I have been remodeling some of my radiocollars. My talented friend Cecilia figured out how to do this, here's the step-by-step process:

Original radiocollar- the thin wire digs into the degu's neck, and the edges from the crimp can cut the skin.

To improve the radiocollars, we decided to replace the wires with wide, brass bands. So, for the first step, we cut strips from a sheet of brass.

Next, we marked where we wanted to put the holes. We first used a small hammer and chisel to start the holes and then an electric drill to finish.

We then smoothed the edges of the holes with a sander attachment.

Next, we soldered a small screw to the end of the brass band and then sanded down the soldered area.

We had to remove part of the transmitter to reduce weight and create a better fit. The original transmitter is on the left and the altered transmitter is on the right.

Using superglue and Poxilina (a type of cement-putty), we attached the brass band to the transmitter.

And finally, we put heat-shrink-tubing over the transmitter and lower parts of the band.

This actually took quite a bit of time, but I always enjoy the opportunity to make and alter my research equipment. You never know what sort of new skills you'll learn when you go to graduate school....

The Matorral

The habitat I work in is called the "matorral." Similar to the chaparral of Southern California, the matorral is characterized by a Mediterranean climate (meaning, wet winters and hot, dry summers). The field site that I work in used to be an experimental agricultural station owned by the Universidad de Chile. Now, the site is used by sheep, cows, a variety of different biological researchers.

View of my field site on a clear day (you can see the Andes in the background).

The exact type of matorral habitat at my field site is called "Bosque Espino," which is Spanish for "spiny forest." It certainly is a spiny forest, mostly because of the prevalence of acacia trees called "espinos." Espinos are actually thought to be relatively new to Chile; legend has it that Inca traders brought the espino seeds into Chile only a few hundred years ago. When the Spanish first arrived in Chile espinos were not very abundant, but by clearing land for farming and introducing cattle (which spread espino seeds during grazing), the advent of agriculture has helped the spread of the espino. Spiky, spiny and stunted, espinos are ugly and a pain to work around. However, I grudgingly appreciate the espinos when spring comes and they blanket the entire landscape with their delicate, yellow-green leaves and golden blossoms.

An espino beginning to leaf out

Up-close picture of an espino, the spines are hard to see, but believe me; they're there.

While the matorral is not exactly a biodiversity hotspot, I've still found my field site to contain an amazing number of animals. At my field site I have seen 34 different species of birds, 7 different species of mammals, and 4 different species of reptiles. The wildlife guide I've been using doesn't have a great plant or insect section, so I haven't been able to keep a good checklist for those organisms.

I've recently been reading Charles Darwin's "Voyage of the Beagle" so I can compare Darwin's perceptions of central Chile's fauna with my own. Unfortunately, Darwin didn't write much about central Chile, probably because he was more smitten with the flora and fauna of Southern Chile, the occurrence of a large earthquake while he was on the coast, and the geology of Northern Chile. However, I did enjoy this quote concerning my favorite bird, the Moustached Turca:

"Of birds... the Turco is not uncommon...with its tail erect, and stilt-like legs, it may be seen every now and then popping from one bush to another with uncommon quickness. It really requires little imagination to believe that the bird is ashamed of itself, and is aware of its most ridiculous figure. On first seeing it, one is tempted to exclaim 'A vilely stuffed specimen has escaped from some museum, and has come to life again!'" (pg. 287)

Darwin's description is pretty true, although I highly doubt that turcas are ashamed of their themselves. Here's a picture of a moustached turca that I caught in one of my degu traps:

I actually catch quite a few birds in my traps, here are a few more pictures:

Common diuca-finch, known as "Diuca"

Chilean mockingbird, known as "Tenca"

Long-tailed meadowlark, known as "Loica"

Rufous-collared sparrow, known as "Chincol"

I hope to use my new videocamera to film some of the different animals at my field site, so check my future posts for some cool video clips!

Living the Chilean lifestyle

It had been 9 months since I’d left Santiago, and while I knew that things would seem much more familiar than when I’d first arrived in the city in June of 2011, I hadn’t expected that it would be so easy to get back into a comfortable routine. I suppose everything is easier the second time- this year I had a better idea of what supplies I should bring down with me and what items would be easy to purchase in Santiago. Finding stores, speaking in Spanish, and navigating the subway were a lot easier than I remembered. I was worried that it would take some time to refresh my stick-shift driving skills, but thankfully it seems those skills are more akin to riding a bicycle. Running errands and getting things organized went so smoothly that I was ready to head out to the field after only two days in the city. 

Shopping at "Jumbo," the Chilean version of Costco.

Of course, having helpful Chilean friends also made it easier to get back into the swing of things. When I arrived in Santiago, I took a taxi to the university where my good friend Juan met me and helped me store my supplies in the lab. Juan then promptly took me to his apartment where he and my other friend, Paola, cooked me lunch. My friend Cecilia then picked me up and took me to her house, where I will be living for duration of my field season.

Cecilia and her family live in the community of La Reina, which is one of 37 different communities (comunas) in Santiago. La Reina is an upper-middle class neighborhood that is East of the city center.

Cecila's family's house

One thing I’ve noticed about Santiago is that people tend to be concerned about security. Most apartment buildings have someone controlling the main doors 24 hours a day. At the university, I need to pass by a guard outside the university, pass by another guard to get into the Departamento de Ecología, and then use a keycard to enter the third floor of the building before I can finally access the lab. As for the neighborhood I’m currently living in, most houses have bars on the windows and are surrounded by walls and electronic gates. So why is security such a high priority in Santiago? I don’t think the crime rate is particularly high in Santiago, so I think it may be more due to the fact that Chile is proud to be the most prosperous country in South America and thus tries to emulate other wealthy nations. This may also explain why there’s so much bureaucracy in Chile. For example: when I renewed my visa last year I had to wait in line to get a number, then wait till my number was called, and then fill out a form that I had to take downstairs for photocopies. Then I had to go back upstairs to get a bill, take my bill to a bank that was a block away, pay the fee, and then bring that receipt back so they could finally stamp my visa.

Although there’s lots of security and bureaucracy in Chile, the people here are really quite warm and friendly. Most Chileans are very patient with me as I speak in garbled Spanish and wildly gesture around. One of the other things I really like about Chile is that the “machismo” culture, which is still very prevalent in many other South American countries, seems to be absent here (except for construction workers). 

The best part of this field season, though, is that I get to live with a Chilean family. My host family consists of my friend Cecilia and her father, mother, and younger sister. Cecilia is a veterinarian and has been working with wild and captive degus for many years. Both of Cecilia’s parents are now retired, and her younger sister is currently finishing law school. Cecilia’s family also has a golden retriever named Canela (the Spanish word for cinnamon) and black cat named Cuchipipi (which is a nonsense word).

Me, Cuchipipi, and Canela

The whole family sits down for dinner every evening, usually around 8:00pm. Dinner fare is pretty similar to what you find in the United States, and I’ve eaten several tasty meals like chicken with mashed potatoes, fish and rice, and ground beef casserole. I’ve also had some dishes that are more unique to Chile like empanadas, pureed garbanzo beans with a fried egg and hotdog, and jello with sweetened, condensed milk. Dinner conversation is always a lot of fun and we often spend time comparing English and Spanish words and phrases. My Spanish is slowly getting better, or at least it seems like I can talk a lot faster than I used to!

I’ve included some picture of my host family and their house. Check back next week for a post on the natural history of the mattoral!

My bedroom

The living/dining room

Backyard

My host father (Jorge) and mother (Sylvia)

My friend Cecilia. This picture was taken at a restaurant called "Fuente Alemana," which serves sandwiches piled high with meat, mayonnaise, and other tasty things.

I had a "lomito italiano." This pork sandwich is called an "italiano" because the mayonnaise, tomato, and avocado represent the colors of the Italian flag.

 

Chile 2012!

Watch out, degus. I'm coming back.

It's true, I'm returning to Chile for more delightfully degu-dominated field research! When I left Chile at the end of October in 2011, I wasn't sure if I'd be able to return to finish some of my projects. Luckily, my grant writing paid off and I now have sufficient funds to pursue another awesome field season! I am so thankful for the grants I've been awarded, and I'd like to thank these organizations; the Tufts University Graduate School, Sigma Xi, the National Science Foundation Doctoral Dissertation Improvement Program, the Animal Behavior Society, the American Mammalogist Society, and the Tufts International Research Program. In addition to these funding sources, my continuing field research is also made possible by the support of my collaborators; Dr. Loren Hayes (University of Tennessee at Chattanooga) and Dr. Luis Ebensperger (Pontificia Catolica Universidad de Chile).

I'm arriving in Santiago on August 7th, and then I'll be spending the next four months trapping, sampling, and observing degus at the same field site I used last year. I've spent the last few weeks ordering supplies, drafting up research schedules, and figuring out how I'm going to pack everything. I'm almost ready to go, and I'm especially looking forward to seeing my Chilean friends. I should also mention that I'll be living with a Chilean family during the next four months; my good friend Cecilia has offered to let me live with her family, and I'm looking forward to improving my Spanish, eating delicious Chilean dishes, and getting more exposure to Chilean culture.

Stay tuned for more posts!

Getting Ready to Head Back to the U.S.

Well, my days in Chile are numbered- I'll be flying home on Monday, October 31st (and yes, Chileans do celebrate Halloween, so I'm hoping for some candy on the plane ride home). The last few days have been crazy busy since I've been scrambling to collect last-minute data, steadily writing up my thesis proposal which I'm defending a week after I get back to Tufts, and cleaning/packing/organizing for the trip home. This means I'll have to wait till next week to write about the endocrinology of parental care, so for this post I'll just be uploading a bunch of photos from my Chilean field season, enjoy!

Cueca dancers, the "cueca" is the traditional Chilean dance. 

Me in front of the Moneda (the president's house). Plus a stray dog, which are everywhere in Santiago.

Hiking in the Andes.

Black-billed shrike-tyrant

Parque Nacional La Campana

Pablo Neruda, Chile's famous poet and humanitarian.

Chilean swallow.

Magellanic horned owl

Street art in Valparaiso

Enormous male sea lion in Valparaiso (it smelled bad, too)

More cool street art, Valparaiso.

I've witnessed the sun rise pretty much everyday for the past two months.

And some days it's foggy and cold.

View from my apartment balcony after a student demonstration.

I caught an iguana with a TV antenna and some string a few days ago.

What a cutie!

Degu Sex

Reproductive physiology is amazingly super cool and fascinating. With the exception of stress physiology, I’d say that reproduction is the most interesting topic in biology. So, in honor of reproduction, let me tell you about the endocrinology behind mating, pregnancy, and lactation in the degu.

The mating period for degus is typically around the month of June. Because degus have a three-month gestation period, this means that the baby degus will be born around late August or September. Degus, like many seasonal animals, optimize their time for breeding so their offspring will be able to develop and grow in ideal conditions. The austral spring is a good time for degus to give birth because there’s plentiful food due to the rainy winter. The mattoral (that’s the habitat type I’ve been working in, it’s similar to the chaparral of Southern California) is very dry during the austral summer, so it’s important that the degus time their reproduction so all energetically expensive processes (growth in pups and lactation in mothers) will be completed before the dry season hits.

For male degus, testosterone is a key player during the mating season. Testosterone is an important hormone for spermatogenesis (sperm production) and several different mating behaviors such as gaining access to a mate, wooing a female, and performing the physical act of copulation. For species where mate access is competitive (elephant seals are a classic example), the more dominant and aggressive males are usually the most successful when it comes to reproducing. This aggressive behavior is partially modulated by testosterone, and dominant males oftentimes have higher levels of testosterone compared to subordinate males. Aggressive behavior is enhanced when testosterone binds to brain receptors (either by converting to 5-alpha-dihydrotestosterone and binding to androgen receptors, or by aromatizing to estradiol and binding to estrogen receptors). And while most of us think of aggressive behavior as fighting, aggression can also be displayed through song and other non-physical behaviors (ex: marine iguanas bob their heads up and down to caution other males encroaching on their territory).

Like many other seasonal vertebrates, male degus have high levels of testosterone during the mating season and low levels of testosterone during the rest of the year. Why do we see seasonal differences in testosterone levels, and if testosterone helps increase reproductive fitness through aggression and other behaviors, then why not pump out a ton of testosterone all the time? Well, the answer is that it’s costly; high circulating levels of testosterone create an energetically-expensive lifestyle, and it has been shown that testosterone-implanted animals can have low body mass, a suppressed immune system, and a higher rate of mortality.

The Challenge Hypothesis, posited by Wingfield and colleagues, states that the seasonal changes we see in testosterone levels are due to a tradeoff between aggression and parenthood. In bird species where males provide significant parental care (ex: providing chicks with food), testosterone levels usually fall after the chicks are born. After the chicks fledge, and if the parents are able to breed again, the male will then up-regulate his testosterone levels. For species where males don’t provide much parental care (like degus, who may do a little bit of thermoregulatory huddling, but that’s about it), the drop in testosterone is often longer and slower.

So what’s going on with the females during the mating season? Well, they’re probably checking out the males and judging them, but in addition to that, females are preparing themselves for pregnancy. Compared to male reproduction, the female reproductive cycle is quite complicated, but let’s see if I can break it down into the major steps (note: most of the information I’m providing is typical of guinea pigs, which are close relatives of degus):

Proestrous:

This stage lasts a day or two, and the female is agitated and may mount other con-specifics. During this time period, the ovarian follicles are growing and the endometrium (lining of the uterus) is slowly building up. This follicle development is fueled by follicle stimulating hormone (FSH).

At the same time, the some outer cells in the follicles (specifically, the granulosa cells) are producing estrogens. Estrogen production is due to the work of two hormones: FSH and luteinizing hormone (LH), both of which are secreted from the anterior pituitary. LH stimulates the production of androgens from the theca interna (an inner layer of cells in the follicle), and FSH stimulates the production of an enzyme that takes the androgens and converts them to estrogens. This increase in estrogen levels is very important for the next stage:

Estrous:

First, the vaginal membrane opens, and after a few hours the estrous period begins. Estrous lasts 9-11 hours and this is when the female is sexually receptive (or “in heat”). In guinea pigs and other mammals, a female needs to be in estrous in order to perform “lordosis,” which is when the female arches up her back so the male can mount from behind. Copulation cannot occur without lordosis, so the males can only successfully mate with the females during this short time window. The binding of estradiol to certain neurons in the brain controls lordosis (note: I’m not sure if female degus perform lordosis, but it’s likely since guinea pigs do).

Ovulation:

Right at the end of estrus, ovulation happens. Ovulation occurs when the estrogen levels reach a certain high point and activate a special control center in the hypothalamus. This control center causes a positive feedback cycle to start: the hypothalamus increases secretion of gonadotropin-releasing hormone (GnRH) which goes to the anterior pituitary and causes increased secretion of LH and FSH, which then go to the ovaries and cause the developing follicles to rupture and release the ova. The ova then begin their journey down the fallopian tubes where fertilization must take place (there’s about a 20 hour time window for fertilization to occur). The vaginal membrane may stay open for several more days, and then it will close until mid-pregnancy.

Immediately after ovulation, the ruptured follicles in the ovaries turn into corpus lutea. The corpus lutea produce high levels of estrogens and progesterone, which inhibit FSH and LH production (thus, preventing more follicles from developing). After about three days the corpus lutea start to degrade, and within six more days the corpus lutea are gone and the animal is ready to start the whole cycle again unless fertilization occurs and the ova successfully implant in the uterus lining, which brings us to:

Pregnancy:

The ova reach the uterus about three days after fertilization and take another three to four days to implant in the endometrium. The fertilized ova then begin to develop placenta, which produce and secrete a hormone called chorionic gonadotropin (CG). CG prevents the corpus lutea from degrading, so estrogens and progesterone levels stay elevated and no more follicles develop for the duration of the pregnancy. The estrogens and progesterone help stimulate development of the mammary glands for lactation, but lactation cannot begin until after the animals give birth because of the high levels of progesterone. (Question to think about: Birth control pills prevent ovulation from occurring. What hormone(s) do you predict are in birth control pills and how do these hormones prevent ovulation?)

Parturition and Lactation:

Degus have a gestation period of about 90 days (which can be longer or shorter, depending on the number of developing fetuses) and typically give birth during the night. During parturition (giving birth), each pup takes 10-30 minutes with 1-16 minutes between each pup. When parturition begins, the hormone “relaxin” helps the relax the cartilage holding the pubic bones together, and then oxytocin, released from the posterior pituitary gland, causes cervical distention and smooth muscle contraction in the uterine wall. Twelve to fifteen hours after parturition the mother has a post-partum estrous period of about 3 hours, which means she can get pregnant again right away. In humans, nursing and lactation are an effective means of inhibiting ovulation, but degus and some other small mammals can nurse their current young while carrying another litter. Milk production is stimulated by prolactin and glucocorticoids, while the effects of oxytocin cause milk letdown. During nursing, the stimulation of the nipple causes increased secretion of prolactin and oxytocin from the hypothalamus.

While I don’t have any photos of nursing degus, here are some pictures of degu pups:

The first degu pup capture of the year.

Getting ready to be released.

It will be hard to not take one home.