DEATH BY ESTROGEN: FERRETS

Tavishi In mammals, most females undergo a menstrual cycle or an estrous cycle. Both cycles are very similar, with the same process of ovulation. While menstrual cycles are exclusive to more advanced primates like humans and baboons (and for some reason elephant shrews), most other mammals, like dogs, cats, ferrets, rabbits, and pinnipeds experience an estrous cycle.

The main differences between an estrous cycle and a menstrual cycle include differences in what happens with the endometrium. During the estrous cycle, if conception fails to occur, the endometrium merely reabsorbs, whereas during the menstrual cycle, the endometrium is properly shed. As a result, animals with an estrus cycle (estrous can also be spelled oestrus and estrus) do not experience a "period". If you've ever had an unspayed dog, you might have noticed vaginal bleeding during heat and conflated it with a period in humans. Quite the contrary: this is still just bloody vaginal discharge, and isn't actually menstruation or an endometrium. Menstruation is split into two phases: a follicular and luteal phase. During the follicular phase, we first begin with low estrogen and low progesterone. At this point, we are shedding our endometrium in menses. Throughout the follicular phase, our ovarian follicles develop, preparing to release an egg in the process of ovulation. When the follicle develops, it releases estrogen, starting right after the period. This estrogen causes thickening of the endometrium in preparation for children. The follicle eventually releases the egg in the process of ovulation, causing a decrease in estrogen and an increase in progesterone. After ovulation, the luteal phase occurs in which estrogen and progesterone are both high. The follicle which released the egg forms a structure known as the corpus luteum, which continues to produce progesterone and estrogen, thickening the endometrium. But when fertilization never occurs, the corpus luteum is degrading, causing a shedding of the endometrium, landing us right back in the follicular phase. The estrous cycle is split into four phases, and lasts a much shorter period (get it?) First, proestrus occurs, in which estrogen is high. Breeding does not occur, but the follicle prepares for ovulation. During proestrus, the corpus luteum degrades. Ovulation occurs during estrus, which is when estrogen and progesterone both decrease. This is the only time in which successful mating can occur, something which is entirely uncharacteristic of the menstrual cycle. After estrus, metestrus and diestrus follow in which the corpus luteum forms. However, while most of these phases take a few days, diestrus takes a few months, resulting in estrus occurring only about twice a year. From species to species, estrus differs slightly. But in ferrets, estrus is... strange and potentially really dangerous. In some animals, induced ovulation occurs- where rather than ovulation just naturally occurring because of luteinizing hormone in time, mating is required to trigger ovulation. But in ferrets, this results in estrogen poisoning in which the high estrogen produced prior to ovulation in proestrus becomes a threat to ferret safety.

An important role of estrogen that is not related to reproduction is management of bone density. When estrogen in ferrets is really high, aplastic anemia results. Essentially, bone marrow density decreases, thus resulting in a decrease in blood cells because of an increase in osteoclast activity. (Bone marrow gives rise to blood cells like erythrocytes, platelets, etc.) Hyperestrogenism also results in thinning hair in ferrets. This anemia, when allowed to go on for two long, can lead to fatality- at first it may be mild, but ultimately, that ferret is suffering. That is one reason why it is more than crucial to spay/neuter ferrets- it also helps with the smell.