Canine reproduction
Canine reproduction is the process of sexual reproduction in domestic dogs, wolves, coyotes and other canine species.
Canine sexual anatomy and development
Male reproductive system
Erectile tissue
As with all mammals, a dog's penis is made up of three pieces of erectile tissue. These are the two corpora cavernosa and the singular corpus spongiosum which continues in the glans. A notable difference from the human penis is that the visible part during an erection consists entirely of the glans.The retractor muscle is attached at the shaft of the penis. It is a paired smooth muscle that is used to retract the penis back into the sheath.
Glans
A dog's glans consists of two sections: Behind the lower, long part lies the "knot" which expands only after penetrating the vagina and causes the male dog to remain inside the female for some time after ejaculation. This increases the chance of fertilisation and prevents, albeit for a short time, other suitors from mating with a particular female.Behind the knot the penis is very flexible in the horizontal direction allowing the male to unmount while remaining tied.
Shaft
The shaft of a dog's penis is not visible, even during an erection; however its pathway can be felt starting at the knot passing between the hind legs and carrying on up to the anus.Baculum and urethra
Inside the corpus spongiosum lies the baculum. This allows the male dog to enter into the vagina before the erectile tissue is swollen. The urethra is located inside of a downward facing groove on the baculum and ends at the tip of the penis.During an erection a small dip just above the urethral process can be seen. This is because the skin at the tip of the penis is connected via cartilage to the baculum. When the erectile tissue swells, the size of the baculum and connective tissue remains constant, pulling back the skin at the tip.
Sheath
The penile sheath entirely surrounds the glans while not erect. The back part is intergrown with the abdominal skin. The front part, almost reaching to the navel, is free. The inner sheath, just like the glans, is covered with a mucous membrane and the outer sheath is covered with normal, hairy epidermis.Female reproductive system
Development
In domestic dogs, sexual maturity occurs between the ages of 6 and 12 months for both males and females, although this can be delayed until up to two years of age for some large breeds. Pregnancy is possible as soon as the first estrus cycle, but breeding is not recommended prior to the second cycle. As with other domesticated species, domestication has selectively bred for higher libido, and earlier and more frequent breeding cycles in dogs than in their ancestors.The female reproductive cycle
Female cycle
The average length of the reproductive cycle for females is 4-48 days. Every breed has their own cycle built into their genetics, making their time in heat unique to their own breed, therefore making crossbreeds open to new cycle lengths; 4-48 days in heat is confirmed to be the most accurately recorded range of times and proven to be the limits for the lengths of time for females to be in heat. Females reach sexual maturity as young as 3 months of age and rarely longer than 9 months for larger breeds. There is a tremendous variability in the maturation age between breeds, and even within a breed of dog.1. Proestrus, in which eggs in the ovaries begin to mature and estrogen levels begin to rise, is the first stage of the reproductive cycle. During this stage females, though non-receptive, attract males. Initial changes include swelling of the vulva lips, which become pliable, small amounts of bloody vaginal discharge, frequent urination, and signs of restlessness. Proestrus generally lasts nine days.
2. Estrus follows, in which estrogen levels are high, mature eggs are released from both ovaries, and females become receptive both physically and mentally to copulation. Only during estrus will copulation result in pregnancy.
During proestrus and estrus, females may have a clear, blood tinged, or bloody discharge. Dogs during these stages are often informally referred to as being in heat. The length of these cycles varies greatly among breeds and even between individuals of the same breed. Proestrus and estrus can last anywhere from 5 days to 21 days.
3. Diestrus is the period following mating. Diestrus lasts approximately 56 to 60 days in a pregnant female, and 60 to 100 days in a non-pregnant female. During both of these periods, progesterone levels are high. Because the hormonal profile of a pregnant female and a female in diestrus are the same, sometimes a non-pregnant female will go through a period of pseudopregnancy. At that time she may gain weight, have mammary gland development, produce milk, and exhibit nesting behaviours.
4. Anestrus is the remaining period, the time of reproductive quiescence. The female has no attraction to mating. Anestrus generally lasts four to five months.
Copulation
As with most tetrapods, canine copulation involves the male mounting the female from behind, a position that is colloquially referred to in humans as "doggy style" but does not have a specifically known origin. When a male canine is interested in mounting a female, he will sniff the female's vulva, which, for females in the estrus stage, has been secreted with Methyl p-hydroxybenzoate. If the female is unreceptive, she may sit, lie down, snap, retreat, or otherwise be uncooperative. If the female is receptive, she will stand still and hold her tail to the side, a stance referred to as "flagging". The male will often continue examining the female's rear, before mounting her from behind while attempting penetration with his penis.Unlike human sexual intercourse, where the male penis commonly becomes erect before entering the female, canine copulation involves the male first penetrating the female, after which swelling of the penis to erection occurs, which usually happens rapidly. At the time of penetration, the canine penis is not erect, and only able to penetrate the female because it includes a narrow bone called the "baculum", a feature of most placental mammals. When the male achieves penetration, he will usually hold the female tighter and thrust deeply. It is during this time that the male's penis expands and it is important that the bulbus glandis is sufficiently far inside for the female to be able to trap it.
Male canines are the only animals that have a locking bulbus glandis commonly known as bulb or knot, a spherical area of erectile tissue at the base of the penis. During copulation, and only after the male's penis is fully inside the female's vagina, the bulbus glandis becomes engorged with blood. When the female's vagina subsequently contracts, the penis becomes locked inside the female. This is known as "tying" or "knotting". While characteristic of mating in most canids, the copulatory tie has been reported to be absent or very brief in the African wild dog, possibly due to the abundance of large predators in its environment.
When the penis is locked into the vagina by the bulbus glandis, thrusting behavior stops and the male will usually lift a leg and swing it over the female's back while turning around. The two stand with their hind ends touching and the penis locked inside the vagina while ejaculation occurs, decreasing leakage of semen from the vagina. After some time, typically between 5 and 20 minutes, the bulbus glandis disengorges, allowing the mates to separate. Virgin dogs can become quite distressed at finding themselves unable to separate during their first copulation, and may try to pull away or run. Dog breeders often suggest it is appropriate for handlers to attempt to calm the mating dogs if they show anxiety once this stage is reached. After mating, the male usually licks his penis and prepuce.
Gestation and litters
Gestation in a dog is 63 days in length, if measured from the day of ovulation. Since it is difficult to determine the exact date of ovulation, errors are often made in calculating gestation period. Canine sperm can live for 10 to 11 days in the oviducts so if a female is bred 10 days before the oocytes can be fertilized, she will appear to have a gestation length of 70 days. If she is bred on the day the oocytes can be fertilized, her gestation length will appear to be 60 days long.During gestation, many physiological changes are similar to other mammals like humans. This results in similar shifts in nutrients in the blood of dogs, especially affecting glucose, fatty acids and amino acid levels.
A rule of thumb is that a mammal will produce half as many offspring as the number of teats on the mother. This rule is altered in domesticated animals since larger litters are often favoured for economic reasons and in dogs, particularly, the great range of sizes and shapes plays a role in how many healthy puppies a female can carry. A female dog usually has 10 teats, though this does not mean she can necessarily provide sufficient nutrition for 10 puppies in one litter.
An average litter consists of about five to six puppies, though this number may vary widely based on the breed of dog. Size of the breed is correlated with litter size. Miniature and small breeds average three to four puppies in each litter, with a maximum litter size of about 5–8. Large and giant breeds average 7 puppies per litter but can have a maximum litter size of about 15. In one study, the Rhodesian Ridgeback had the highest average litter size with 8.9 pups per litter while the Pomeranian and Toy Poodle had the lowest with 2.4 pups per litter.
The number of puppies also varies with the mother's age. In smaller breeds, both young and old age are associated with smaller litter size. In larger breeds, only old age is associated with smaller litter size. Use of artificial insemination is also associated with smaller litter size, with frozen semen having a stronger effect than fresh semen.
The largest litter size to date was set by a Neapolitan Mastiff in Manea, England, on November 29, 2004; the litter was 24 puppies.
Some breeds have been developed to emphasize certain physical traits beyond the point at which they can safely bear litters on their own.
A large scale study in Norway showed that across all breeds, about 4% of pups will be stillborn and a further 4% will die within the first week. Between 8 days and 8 weeks, 1% will die. Litter size, breed size and age of the female is associated with increased risk. High risk breeds for stillborn includes the Dogue de Bordeaux, St. Bernard, Chow Chow, Pembroke Welsh Corgis and Dalmatian. The Basenji, Italian Greyhound, Australian Terrier, Irish Soft Coated Wheaten Terrier and the Bichon Havanais had few to no stillborns. High risk breeds for early neonatal mortality includes the Rhodesian Ridgeback, Dogue de Bordeaux, Dalmatians and Icelandic Sheepdog while the Basenji and Tibetan Terrier had no early neonatal mortality and the Border Terrier and Danish-Swedish Farmdog had <1% early neonatal mortality.
Common causes of early neonatal mortality are bacteria infection, fetal asphyxia and fading puppy syndrome. Other causes may include elective euthanasia because of congenital defects or failure to meet breed standards.
Other multi-breed studies have put stillborn rates at 6.5–7% and early neonatal mortality at 11.5–19.8%.