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Physiology of Dog Reproduction

The sexual cycle in dogs includes the time of heat and the times of resting.

Female Dog
Female dogs have 2 periods of heat a year, every 6 months that lasts about 2 to 3 weeks each.

Their first heat appears between 6 and 15 months after birth and they can be fertilized since that time. There will be a delay in the larger breeds as they have a longer growth.

The female dog has spontaneous ovulation, i.e., ovulation is independent from being mounted. It occurs every six months though there will be a slight delay from one to another and depends on the breed also. It will be usually between 150 and 300 days. This cycle has 4 phases: the proestro and the estro, which constitute the period of heat, and the diestro and anestro, the phases of sexual resting.

The behavior, physiology and anatomy of the animal go through different changes with each phase of heat.

Proestro: It indicates the beginning of heat and lasts for 7 to 10 days during which male dog is attracted. It can observed by slight bulging of vulva and slight oozing of blood. Females reject the male though the later is attracted at this phase.

Estro: Lasting 5 to 10 days, this is the next phase of proestro. Ovulation happens in this phase and male is accepted for mating. Blood oozing stops during this period.

Diestro: Third phase that lasts from 110 to 140 days. This is the period where the female dog is fertilized or is in resting phase being non-fertilized.

Anestro: This is the phase of new heat and differs in duration. As mentioned earlier, it heats two times a year.

Male Dog
Unlike females, males does not have any specific time or cycle. It arouses when the female is present. Those matured recently does not have a good sperm count or fertility rate, so it wold be better to avoid the males for mating till it reaches 10 or 15 months of age. That would be the ideal age for its first intercourse.


Hormone is secreted by a gland that acts on an organ. The sexual cycle depends completely on sexual hormones, which are different in the male and female.

In the female

Estrogens: These are secreted by the ovaries during heat. Their function is to stimulate the hypothalamus, which secretes other hormones to provoke ovulation.

Progesterone: It is secreted by ovaries and placenta during ovulation and is required for gestation.

Gonadotropines: These are secreted by the hypophysis gland. Again, it is divided into two, follicle stimulating hormone (FSH), which stimulates the growth of the follicles and luteinizing hormone (LH).

Oxiticine: Secreted by the same gland as above, it causes the dilation of tissue during birth. It is also used to induce labor.

Prolactine: Secreted by hypophysis gland, it stimulates the secretion of milk after delivery.

In the Males

Testosterone:  Secreted by the testicles, it is responsible for appearance of secondary sexual characters that determines the physical aspects of the male dog.


The first cycle can be irregular, but after that they can be observed separate from 5 to 8 months regularly. Abnormal cycles can cause sterility in a female.

  • Long lasting heat: The duration is above 12 days. The ovulation is very late.
  • Too frequent heats: Called the hyperestro where the interval between two cycles is less than 4 months.
  • Too separate heats: Called the hypoestro where the interval between two cycles is more than 12 months.
  • Non-existent heat: Lack of development of sexual organs or lack of secretion of hormones in the females.

All these anomalies should be studied by the veterinarian, who might consider the necessity of a hormonal treatment to regularize the heat.

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