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):
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:
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).
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:
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.
|The first degu pup capture of the year.|
|Getting ready to be released.|
|It will be hard to not take one home.|