Stories and thoughts about small animal reproduction and ultrasound

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Twitter and reproduction of rodents. Part 1.

Happy new year!

Time for the first post of 2016!

Last week (back in 2015) I was asked via tweeter ” How do female porcupines give birth?”. My first thought was “very very very carefully”! Later, with the help of doctor Google, I found some interesting information at an excellent post by Dr. Quinn Shurtliff.

But, if we just remember that porcupines are rodents, we will realize how many differences they do have when compared to other rodents, such as beavers, rabbits, rats, mice etc etc etc!

So in the next paragraphs we will try to sum up the most important -and maybe funny- facts about reproductive physiology of the rodents.

Let’s begin with the big ones…



Female porcupines are solitary animals except during the breeding season, which begins during the fall. Females may be polyestrous and re-cycle in 25 to 30 days if fertilization does not occur at the time of ovulation. Ovulation is spontaneous and may alternate between the left and right ovaries. The female porcupine is in heat for 8-12 hours. The testes of male porcupines descend into scrotal pouches during late August and early September. Spermatogenesis reaches its highest level during October.

When breeding season begins, females secrete a thick mucus which mixes with their urine. The resulting odor attracts males which fight for receptive females. They use loud vocalizations, violent biting, and each uses his quills as weapons. Although the competition happens in trees, mating exclusively happens on the ground. Once a dominant male is successful, he approaches the female and uses a spray of his urine on the female. Only a few drops touch the female, but the chemical reaction allows the female to fully enter estrus. When porcupines are mating, they tighten their skin and hold their quills flat, so as not to injure each other. Mating may occur repeatedly until the female loses interest and climbs back in to the tree. After mating, a copulation plug is formed.

The North American porcupine has a long gestation period when compared to other rodents. Pregnancy lasts 210 (205-217) days with the young being born from April to June. Porcupines give birth to a single young. At birth, they weigh about 450 g, which increases to nearly 1 kg after the first two weeks. They do not gain full adult weight until the end of the second summer about 4.5 kg. Offspring are precocial with open eyes when born. Senses of smell and hearing develop as the young grow. The quills of young porcupines are soft at birth but harden within one hour. Parental care is provided by the mother. Mothers remain with their young for up to six months. Youngs are not sexual mature until the age of 25 months for females, and 29 months for males.




Beavers are monogamous animals! The breeding season begins in January and February in both American and Eurasian beavers. Estrus lasts 10-12, maximum 24 hours. Copulation most commonly takes place in the water and in some cases can take place in the lodge. The male will approach a female floating in the water from the side and copulation may last from 30 seconds to 3 minutes! Most copulations occur at night. If not fertilized, female beaver may come into estrus again two weeks later.

The female is sexually active after 1.5 to 2.5 years old. Most beavers do not reproduce until they are three years, but about 20% of two-year-old females reproduce. Pregnancy of the American beaver lasts around 107 days and of the Eurasian 60-128 days. The average size litter is 3 to 4 young  (1-4) for the American and 1 to 3 (2-6) for the Eurasian beaver.

Baby beavers or “kits” are born well-developed, with open eyes and ample body fur. Newborn weight is 230 to 630 g. They are comfortable with the water in the span of a single day post-birth, and can even swim.



In fact rabbits ARE NOT RODENTS! They were considered to be but not anymore! They are lagomorphs. Nevertheless, we will demonstrate some things about them in the following paragraphs:

Rabbits are well-known for their reproductive capacity! Mating in rabbits is generally polygynandrous (=a reproductive strategy that occurs when two or more males have an exclusive sexual relationship with two or more females. The numbers of males and females need not be equal.), though males will attempt to monopolize particular females. Forget monogamy in this case…

Sex determination is difficult and easier with puberty (4-9 months). Small breeds mature earlier than larger breeds (body weight is more important than age). Also does (females) mature earlier than do bucks (males, that achieve optimum sperm production 40-70 days after puberty).

Does are induced ovulators with no defined estrous cycle, although a cyclic rhythm in sexual receptivity exists (can show swollen vulva). Females experience postpartum estrus and thus may have several litters per year (maybe more than 400 per year), though spontaneous abortions and resorption of embryos are common. Also they can be lactating during pregnancy.

During copulation bucks rapidly mount receptive females, reflex ejaculation follows immediately on intromission and copulatory thrust is so vigorous that buck falls backward or sideways and may emit a characteristic cry!

Gestation length is 30-32 days, and the average litter contains 5 to 8 young. Rabbit placenta allow an unusually high degree of contact between maternal and fetal bloodstreams, a condition they share with humans. Parturition occurs usually in the morning and lasts less than 30 minutes.

Neonates, called kittens, are blind, without hair and helpless, they weigh 50 g, their ears are functional from day 7 and their eyes open on day 10. They do not require stimulation to urinate! The mother visits the nest for only a few minutes each day to nurse them, but the milk is extremely rich. Young are weaned at 5 weeks of age and can live up to nine years. Males are not involved in caring for young. Mortality rate in the first year of life is very high, sometimes more than 90%.

Guinea pigs


Guinea pigs no longer exist in the wild, therefore, mating systems in natural environments are unknown. In domestic populations, mating is heavily influenced by humans. Both monogamous and polygamous systems occur, depending on how animals are housed.

Prior to mating, males smell a potential mate’s genital area and scent mark their mates with urine. Males are very protective of their mates, particularly when multiple males are housed with a single female.

Male guinea pigs reach sexual maturity at about 3 months while females earlier at about 2 months of age. Estrus occurs 3 to 4 times per year and lasts approximately 16 days. Females are polyestric animals as they cycle every 15-17 days. They are spontaneous ovulators and estrus lasts 6-11 hours.

Mating and fertilization usually occur at night, within 20 hours of ovulation. A solid mass of coagulated ejaculate (plug) that falls out of vagina several hours after mating and confirms copulation; hard and waxy product of male secretions. Guinea pigs do not exhibit seasonal mating patterns in domestic populations.

Gestation lasts 69 days (59-72). The average age at first pregnancy is 175 days. It’s important to breed females at a young age, ideally at about 6-12 weeks old (400-600 g body weight) and definitely before 6-8 months old; after 8 months old the pubic symphysis fuses and dystocia will occur. The average litter size is 3 pups (1-13). Parturition is typically rapid with only a few minutes between births. Females do not built nests, but they express fertile postpartum estrus 2-10 hours after parturition!

Newborns are called pups or young (but not piglets) and they are precocious (fully furred, eyes open). Urination and defecation must be stimulated. Birth weight is 45-115 g. Lactation lasts 3 weeks, sows are not very “motherly” and weaning occurs 14 to 21 days after birth.

Enough…more to come on part 2!

Until then, I wish you all health and happiness!

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Stories from Milan and thoughts about canine prostatic neoplasia…

One more time at Milan University! The Vet School still sitting there, full of veterinary histories as it was firstly established in 1791!  And always full of memories for me as I visited it for the first time back in 2004 in order to attend the 3rd ESAVS course of small animal reproduction. From that time till now a fruitful cooperation between the Clinic of Obstetrics & Gynecology and myself began…

Our latest research project is focused on the prostate of the dog. It may sound boring but it is not! Though the prostate is a well-studied gland, a lot of things have to be made clear yet. For example, a lot of discussion between breeders, dog owners and vets has been observed all these years, concerning the prevention of cancer of the canine prostate. Some claim that castration prevents cancer development while others don’t want to hear about it!  But the real question is “how common is neoplasia of the prostate of the dog?”. The correct answer to this could be something like “It depends”! In the following lines I will try to explain why.

During the 90’s and early 00’s it was believed that intact (= not castrated) male dogs have a high probability to develop prostatic neoplasia. So the common preventive practice was castration (orchiectomy in other words) which was performed at a very young age, even before puberty in some cases! Castration of course has some other strong benefits, such as prevention of development prostatic benign hyperplasia, prostatic cysts and other pathologies of the gland. Without a doubt castration is also a great “tool” for the control of stray dog population, that has to be combined with education of the owners.

These advantages are all true. Except one: prevention of neoplasia! In the (not so) recent literature it is well demonstrated that “neutered males had a significantly increased risk for each form of prostatic cancer. Neutered males had an odds ratio of 3.56 for urinary bladder transitional cell carcinoma (TCC), 8.00 for prostate TCC, 2.12 for prostate adenocarcinoma, 3.86 for prostate carcinoma, and 2.84 for all prostate cancers.”(Bryan et al, Prostate, 2007).

And of one the Guru’s of small animal reproduction, Michelle Kutzler, makes things clear enough: “Not only does castration not protect against future development of prostatic neoplasia in dogs, but incidence of prostatic neoplasia is higher in castrated dogs” (Neoplasms of the Prostate in Small Animals, The Merck Veterinary Manual, 2013).

These infos also reflect my own feeling, deriving from my almost 20 years’ small animal practice experiences. The “true” and “real” cases of prostatic neoplasia that I have seen are less than 5 (3 as far as I remember). “True” and “real” mean “proven” by histologic or cytological examination of prostatic samples taken with biopsy or fine needle aspiration (FNA). On the other hand, diseases such as benign prostatic hyperplasia, cysts, prostatitis (= inflammation of the prostate gland) are almost everyday practice to my experience…

But in several congresses and several discussions with vets from other countries, I realize that scientific community is really worried about prostatic neoplasia prevention and treatment! How can it be??? Am I missing cases? Or maybe vets from other countries are overestimating the frequency of prostatic neoplasia development? Well, none of these…it’s a matter of …let’s say… “culture”! And I will explain you what I mean: In countries of the southern Europe such as Greece or Italy, castration of the male dogs is not so familiar as in other countries such as Great Britain, USA, Belgium and maybe France. Castration is mostly performed in cases of prostatic disease (for example benign prostatic hyperplasia or prostatic cysts), very very rarely in young dogs and almost never in dogs before reaching puberty! As a result the incidence of prostatic cancer is low!

So when we are studying or discussing the epidemiology of prostatic diseases we also have to consider the possible differences between countries.

Also we have to keep in mind that early castration (despite what was common belief) increases the risk of development prostatic neoplasia! But on the other hand has many many many advantages – that will be fully discussed in the near future! In any case, your vet knows the pros and cons and always decides the best for your dog, depending the problem, the current health status, the age etc etc etc!

Merry Christmas to all!

Keep in touch!