Canine sexual anatomy and developmentEdit
In domestic dogs, sexual maturity (puberty) occurs between the ages of 6 to 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 reproductive cycleEdit
The average length of the reproductive cycle for females is 2–4 weeks. Females reach sexual maturity (puberty) between 8 and 18 months of age. 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 pseudo-pregnancy. 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.
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As with most tetrapods, canine copulation involves the male mounting the female from behind, a position that is colloquially referred to 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. 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 gland 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 or "bulb", 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. In coyotes, 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 (less than one minute) 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 (when the stud is "tied"), 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 (but sometimes longer), 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 littersEdit
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 uterine tubes (fallopian tubes) so if a female is bred 10 days before the oocytes (eggs) 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.
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.
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 (early neonatal mortality). 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 (14.2%), St. Bernard (12.3%), Chow Chow (12.1%), Pembroke Welsh Corgis (11.7%) and Dalmatian (10.6%). The Basenji, Italian Greyhound, Australian Terrier, Irish Soft Coated Wheaten Terrier and the Bichon Havanais had few to no stillborns (0-0.6%). High risk breeds for early neonatal mortality includes the Rhodesian Ridgeback (11.6%), Dogue de Bordeaux (10.4% ), Dalmatians (8.8%) and Icelandic Sheepdog (8.7%) 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%.
On the basis of an analysis of data on 42,855 dachshund litters, it was found that as the inbreeding coefficient increased, litter size decreased and the percentage of stillborn puppies increased, thus indicating inbreeding depression. Inbreeding depression is a reduction in progeny fitness due largely to the homozygous expression of deleterious recessive mutations.
Because the African wild dog (Lycaon pictus) largely exists in fragmented small populations, its existence is endangered. Inbreeding avoidance via mate selection is characteristic of the species and has important potential consequences for population persistence. Inbreeding is rare within natal packs. Computer-population simulations indicate that all populations continuing to avoid incestuous mating will become extinct within 100 years due to the unavailability of unrelated mates. Thus the impact of reduced numbers of suitable unrelated mates will likely have a severe demographic impact on the future viability of small wild dog populations.
Red wolves primarily live in packs composed of a socially monogamous breeding pair and offspring of different ages. Using long-term data on red wolf individuals of known pedigree, it was found that inbreeding among first-degree relatives was rare. A likely mechanism for avoidance of inbreeding is independent dispersal trajectories from the natal pack. Many of the young wolves spend time alone or in small non-breeding packs composed of unrelated individuals. The union of two unrelated individuals in a new home range is the predominant pattern of breeding pair formation.
Among Ethiopian wolves, most females disperse from their natal pack at about two years of age, and some become "floaters" that may successfully immigrate into existing packs. Breeding pairs are most often unrelated to each other, suggesting that female-biased dispersal reduces inbreeding.
Inbreeding is ordinarily avoided because it leads to a reduction in progeny fitness (inbreeding depression) due largely to the homozygous expression of deleterious recessive alleles. Cross-fertilization between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.
Female dogs are at risk for endometritis and pyometra in the postpartum period and after estrus or vaginitis. Signs and symptoms include fever, lethargy, loss of appetite, excessive thirst, restlessness, a foul smelling vaginal discharge which may or may not be bloody, infertility, or they may be asymptomatic.  Uterine infections should be treated expeditiously if suspected. Contrary to common belief, uterine infections can strike any intact female, whether she has been bred or not, and whether it is her first season or not although it is more common as dogs become older.
An artificial vagina is prepared, which is a conical thin latex sleeve ending in a sterile collection tube. The inside of the latex sleeve is lightly lubricated. The male is allowed to sniff a female in estrus. Experienced studs cooperate readily in the process. New studs often require encouragement in the form of manual stimulation. Generally the male will mount the female, and the collector quickly directs the male's penis into the latex sleeve. The male ejaculates and the semen is collected in the tube. The semen is then drawn up into a long thin pipette.
Designer breed dogs are Mixed-breed dogs that are intentionally bred from parents of two established breeds. Studies have shown that cross-bred dogs have a number of desirable reproductive traits. Scott and Fuller found that cross-bred dogs were superior mothers compared to purebred mothers, producing more milk and giving better care. These advantages led to a decreased mortality in the offspring of cross-bred dogs; however, the qualities of cross bred dogs are not predictable. For example, Labrador x Poodle ("Labradoodle") can inherit the coat of either a Labrador, a poodle, or a remix.
Spaying and neuteringEdit
Spaying (females) and neutering (males) refers to the sterilization of animals—usually by castration (removal of the male's testicles) or ovariohysterectomy (removal of the female's ovaries and uterus)—to eliminate the ability to procreate, and reduce sex drive. Castration has also been known to reduce aggression in male dogs (in some cases), but spaying has been shown to occasionally increase aggression in female dogs.
Animal control agencies in the United States and the ASPCA advise that dogs not intended for further breeding should be spayed or neutered so that they do not have undesired puppies. Spaying and castrating can decrease the risk of hormone-driven diseases such as mammary cancer, as well as undesired hormone-driven behaviors. However, certain medical problems are more likely after neutering, such as urinary incontinence in females and prostate cancer in males.
Studies have shown that spaying or neutering may be associated with increasing some serious health and behavioural consequences while reducing others. The American Veterinary Association (AVMA) position provides no single recommendation to spay or neuter nor for one single age for spay or neuter that is more or less optimal than another. Rather, the AVMA position is that spay or neuter be determined on a case-by-case basis to assess risks for orthopedic disease, neoplasia, reproductive disease, longevity, and population control for each individual.
Increased aggression can be shown in altered females if they have previously displayed aggression prior to surgical alteration. In a study by O'Farrell and Peachy, female dogs less than 11 months of age that had previously shown signs of aggression are more likely to have an increase in aggression after being spayed. These increases in aggression may be due to the sudden change in hormone concentrations that are the result of alteration. While spaying female dogs does not "induce" aggression, it can increase aggression and facilitate indiscriminate appetite in young altered females and can include them rapidly eating meals or eating food-associated items such as trash.
In nearly 2/3 of the cases that involve inter-dog aggression, castration can help decrease aggression. Castration also decreases other male-typical behavioral traits such as mounting, roaming, and urine marking. But a few studies have shown that male behavioral issues of mounting, roaming and urine marking still exist in altered males. Some people have reported after altering their male dogs that behavior such as roaming, mounting and urine marking has not changed the behavior. Aggression may increase, as the decrease in testosterone may lead to emotional issues and become more likely to react aggressively when feeling under threat. Male puppies that are neutered between 7 and 10 weeks are three times less likely to display behavioral problems, compared to canines neutered at 6 months or older. Most dominantly aggressive dogs are male, which causes many people to neuter their male canine companions. Removing testosterone can decrease the intensity of a canine's reaction to stimulus. Testosterone does not cause a behavior to occur, but its absence may decrease the occurrence of a "bad" behavior.
- "Reproductive performance". ilri. Archived from the original on 1 September 2015. Retrieved 4 November 2015.
- 5361/the_normal_canine.htm[permanent dead link], Dr B Eilts, Louisiana State University School of Veterinary Medicine, "Normal Canine Reproduction" retrieved 10 April 2013
- "What to Expect when Your Dog is Pregnant".
-  Archived 2013-04-21 at the Wayback Machine, Dr B Eilts, LSU Veterinary Medicine, "The Normal Canine Estrous Cycle" retrieved 10 April 2013.
- Edward C. Feldman; Richard William Nelson (2004). Canine and Feline Endocrinology and Reproduction. Elsevier Health Sciences. ISBN 978-0-7216-9315-6.
- Dan Rice (3 October 2008). The Complete Book of Dog Breeding. Barron's Educational Series. pp. 51–. ISBN 978-0-7641-3887-4. Retrieved 7 February 2013.
- Sharir, A.; et al. (2011). "The canine baculum: The structure and mechanical properties of an unusual bone" (PDF). Journal of Structural Biology. 175 (3): 451–456. doi:10.1016/j.jsb.2011.06.006. PMID 21708270.
- Trevor Turner (31 August 2011). Veterinary Notes For Dog Owners. Ebury Publishing. ISBN 978-1-4464-8976-5.
- Veterinary Medical Terminology E-Book. Dawn E. Christenson. 6 November 2019. p. 238. ISBN 9780323612081. Retrieved 10 March 2020.
- Howard E. Evans; Alexander de Lahunta (7 August 2013). Miller's Anatomy of the Dog - E-Book. Elsevier Health Sciences. ISBN 978-0-323-26623-9.
- Bekoff, M.; Diamond, J. (May 1976). "Precopulatory and Copulatory Behavior in Coyotes". Journal of Mammalogy. 57 (2): 372–375. doi:10.2307/1379696. JSTOR 1379696.
- Kleiman, D. G. (1967). "Some aspects of social behavior in the Canidae". American Zoologist. 7 (2): 365–372. doi:10.1093/icb/7.2.365. Retrieved 2011-05-14.
- Creel, S. (1998-08-27), "Social organization and effective population size in carnivores", in Caro, T. M. (ed.), Behavioral ecology and conservation biology, Oxford University Press, pp. 246–270, ISBN 978-0-19-510490-5
- Kleiman, D. G.; Eisenberg, J. F. (November 1973). "Comparisons of canid and felid social systems from an evolutionary perspective". Animal Behaviour. 21 (4): 637–659. doi:10.1016/S0003-3472(73)80088-0. PMID 4798194.
- Social Behaviour In Animals
- Biological Exuberance: Animal Homosexuality and Natural Diversity
- The Illustrated Encyclopedia of North American Mammals
- Ruvinsky, A.; Sampson, J. (2001). The genetics of the dog. CABI. pp. 564 (see p. 407). ISBN 978-0-85199-520-5. OCLC 45707635.
- Bonnie V. G. Beaver (2009). Canine Behavior: Insights and Answers. Elsevier Health Sciences. pp. 197–. ISBN 978-1-4160-5419-1.
- "Archived copy". Archived from the original on 2009-04-03. Retrieved 2009-02-27.CS1 maint: archived copy as title (link), Dr B Eilts, Louisiana State University Veterinary Medicine, "Canine Pregnancy" retrieved 10 April 2013.
- Scott, Mary A (2000). "A glimpse at sperm function in vivo: sperm transport and epithelial interaction in the female reproductive tract". Animal Reproduction Science. 60–61: 337–348. doi:10.1016/S0378-4320(00)00130-5. PMID 10844205.
- Yoon, Carol Kaesuk (19 October 1999). "Of Breasts, Behavior and the Size of Litters". The New York Times. Retrieved 27 June 2020.
- Borge, K. S.; Tønnessen, R.; Nødtvedt, A.; Indrebø, A. (March 2011). "Litter size at birth in purebred dogs—A retrospective study of 224 breeds". Theriogenology. 75 (5): 911–919. doi:10.1016/j.theriogenology.2010.10.034. PMID 21196028.
- "Largest litter - dog".
- Tønnessen, R.; Borge, K. S.; Nødtvedt, A.; Indrebø, A. (June 2012). "Canine perinatal mortality: A cohort study of 224 breeds". Theriogenology. 77 (9): 1788–1801. doi:10.1016/j.theriogenology.2011.12.023. PMID 22365700.
- Gresky C, Hamann H, Distl O (2005). "[Influence of inbreeding on litter size and the proportion of stillborn puppies in dachshunds]". Berl. Munch. Tierarztl. Wochenschr. (in German). 118 (3–4): 134–9. PMID 15803761.
- Charlesworth D, Willis JH (2009). "The genetics of inbreeding depression". Nat. Rev. Genet. 10 (11): 783–96. doi:10.1038/nrg2664. PMID 19834483. S2CID 771357.
- Becker PA, Miller PS, Gunther MS, Somers MJ, Wildt DE, Maldonado JE (2012). "Inbreeding avoidance influences the viability of reintroduced populations of African wild dogs (Lycaon pictus)". PLOS ONE. 7 (5): e37181. doi:10.1371/journal.pone.0037181. PMC 3353914. PMID 22615933.
- Sparkman, AM; Adams, JR; Steury, TD; Waits, LP; Murray, DL (July 2012). "Pack social dynamics and inbreeding avoidance in the cooperatively breeding red wolf". Behavioral Ecology. 23 (6): 1186–1194. doi:10.1093/beheco/ars099.
- Randall, DA; Pollinger, JP; Wayne, RK; Tallents, LA; Johnson, PJ; Macdonald, DW (2007). "Inbreeding is reduced by female-biased dispersal and mating behavior in Ethiopian wolves". Behavioral Ecology. 18 (3): 579–89. doi:10.1093/beheco/arm010.
- Geffen E, Kam M, Hefner R, Hersteinsson P, Angerbjörn A, Dalèn L, Fuglei E, Norèn K, Adams JR, Vucetich J, Meier TJ, Mech LD, Vonholdt BM, Stahler DR, Wayne RK (2011). "Kin encounter rate and inbreeding avoidance in canids". Mol. Ecol. (Submitted manuscript). 20 (24): 5348–58. doi:10.1111/j.1365-294X.2011.05358.x. PMID 22077191. S2CID 14682287.
- Bernstein H, Hopf FA, Michod RE (1987). "The molecular basis of the evolution of sex". Molecular Genetics of Development. Adv. Genet. Advances in Genetics. 24. pp. 323–70. doi:10.1016/s0065-2660(08)60012-7. ISBN 9780120176243. PMID 3324702.
- Michod, R.E. (1994). "Eros and Evolution: A Natural Philosophy of Sex" Addison-Wesley Publishing Company, Reading, Massachusetts. ISBN 0201442329 ISBN 978-0201442328
- Teunissen, G. H. (April 1952). "The development of endometritis in the dog and the effect of oestradiol and progesterone on the uterus". Acta Endocrinologica. 9 (4): 407–420. doi:10.1530/acta.0.0090407. PMID 13007336.
- Okano, S; Tagawa, M; Takase, K (November 1998). "Relationship of the blood endotoxin concentration and prognosis in dogs with pyometra". The Journal of Veterinary Medical Science. 60 (11): 1265–1267. doi:10.1292/jvms.60.1265. PMID 9853312.
- "Pyometra - Dogs | Thickening of the Uterus". PetMD. Retrieved December 9, 2013.
- "Endometritis Symptoms and Treatment in Dogs". wikiPet. Retrieved December 9, 2013.
- Veterinary Medicine. Veterinary Medicine Publishing Company. 1989. Retrieved 9 February 2013.
- "Semen Collection from Dogs". Arbl.cvmbs.colostate.edu. 2002-09-14. Archived from the original on 2012-02-05. Retrieved 2012-01-29.
- John Paul Scott; John L. Fuller. Dog Behavior.
- Heidenberger E, Unshelm J (1990). "[Changes in the behavior of dogs after castration]". Tierärztliche Praxis (in German). 18 (1): 69–75. PMID 2326799.
- "Top 10 reasons to spay or neuter your pet". American Society for Prevention of Cruelty to Animals. Retrieved 2013-12-09.
- Morrison, Wallace B. (1998). Cancer in Dogs and Cats (1st ed.). Williams and Wilkins. ISBN 978-0-683-06105-5.
- Arnold S (1997). "[Urinary incontinence in castrated bitches. Part 1: Significance, clinical aspects and etiopathogenesis]". Schweiz. Arch. Tierheilkd. (in German). 139 (6): 271–6. PMID 9411733.
- Johnston SD, Kamolpatana K, Root-Kustritz MV, Johnston GR (2000). "Prostatic disorders in the dog". Anim. Reprod. Sci. 60–61: 405–15. doi:10.1016/S0378-4320(00)00101-9. PMID 10844211.
- The Negative Aspects of Neutering
- Golden retriever study suggests neutering affects dog health
- Sanborn, Laura J. (May 14, 2007). "The Long Term Effects of Spay/Neuter in Dogs" (PDF). The NAIA Library. National Animal Interest Alliance. Retrieved December 9, 2013.
- Moore, G. E.; Guptill, L. F.; Ward, M. P.; Glickman, N. W.; Faunt, K. K.; Lewis, H. B.; Glickman, L. T. (1 October 2005). "Adverse events diagnosed within three days of vaccine administration in dogs". Journal of the American Veterinary Medical Association. 227 (7): 1102–1108. doi:10.2460/javma.2005.227.1102. PMID 16220670.
- "Elective spaying and neutering of pets". American Veterinary Medical Association. Retrieved 2020-11-24.
- Khalsa, Ji (2019-09-19). "A New Perspective on Determining Best Age of Spay or Neuter". Midwoofery. Retrieved 2020-11-24.
- "Elective spaying and neutering of pets". American Veterinary Medical Association. Retrieved 2020-11-24.
- Bonnie Beaver (1999). "Canine Behavior: A Guide for Veterinarians". Cite journal requires
- Peter Borchelt (1983). "Aggressive Behavior of dogs kept as companion animals: Classification and influence of sex, reproductive status and breed". Applied Animal Ethology. 10 (1 & 2): 45–61. doi:10.1016/0304-3762(83)90111-6.
- V. O'Farrel; E. Peachy (1990). "Behavioural effects of Ovariohysterectomy on Bitches". Small Animal Practice. 31 (12): 595–598. doi:10.1111/j.1748-5827.1990.tb00701.x.