Feline reproduction: An overview (Proceedings)
In veterinary medicine, cats are too often treated as strange small breed dogs. To those who work with cats every day, the fallacy of this supposition is obvious. To those who deal with cats only occasionally, this inaccuracy must be recognized since cats now outnumber dogs as the preferred household pet in the United States and cat owners expect veterinarians to be familiar with the husbandry, normal physiology and diseases of this species.
Cats appear to reproduce well without veterinary intervention, leading some to question the necessity for the study of feline theriogenology. Bastet, the Egyptian goddess of fertility, was represented as a cat. It is true that fewer cats than dogs are presented as clinical cases to small animal reproductive specialists; at the University of Minnesota, only 7/158 (4.4%) of animals presented to this clinician over a 3 year span were cats, with 71.0% of those presenting with ovarian remnant syndrome. However, increased understanding of feline reproduction will serve veterinary medicine beyond clinical practice. Studies of normal feline reproductive physiology and reproductive technology, such as artificial insemination and in vitro fertilization, can be extrapolated for use in the 36 species of wild felids, all of which are threatened or endangered. Information about regulation of estrus activity and spermatogenesis can be used in development of effective contraceptives and knowledge of pregnancy used for development of safe and efficacious pregnancy termination protocols, all of which will benefit those working to control feral cat populations and the current overabundant domesticated cat population.
The queenFemale cats go through puberty, defined by onset of their first estrus, at an average age of 8-9 months, with a range from 4-18 months. Long-haired breeds enter puberty earlier in life than short-haired breeds. Puberty onset is also dependent on season; cats reaching an appropriate age during the seasonal anestrus will not go through their first estrus until the breeding season begins.
Cats are seasonally polyestrous, cycling for an average of about 6 days every 2-3 weeks from January through mid-October. The seasonal anestrus from mid-October through December is defined by day length; cats maintained under artificial lights for 12 continuous hours daily will cycle year-round and may exhibit increased fertility. Length of estrus may or may not be affected by breeding. Some studies suggested a shortening of estrus in bred cats while others showed no effect of breeding. Bred cats that are induced to ovulate but do not conceive will undergo a prolonged luteal phase, lasting 45-60 days. Induction of ovulation in estrous queens may be used to decrease cyclic activity between desired breedings.
Proestrus is not routinely discernible in cats. Shille et al documented proestrus in only 27/168 (16.1%) of cycles followed. Estrus behaviors in the cat include monotonous vocalization, increased affection, lordosis (elevation of the hindquarters with lateral deviation of the tail) and rolling. A positive correlation has been shown between estrous behavior and cornified vaginal cytology. Cornified vaginal cytology specimens may be useful in defining estrus in queens which exhibit no overt estrous behavior. Cats may rarely have clear vulvar discharge associated with estrus.
Cats are induced (reflex) ovulators, as are rabbits, ferrets, mink, skunks, 13-lined ground squirrels, camels, llamas, short-tailed shrews and giant fruit bats. In these species an external trigger, usually coitus, stimulates release of gonadotropin releasing hormone (GnRH) from the hypothalamus. This stimulates release of luteinizing hormone (LH) from the pituitary within 2-4 hours, which will then cause ovulation in 1-3 days. The amount of LH released in dependent on the number of copulations and the time during the estrous cycle when copulation occurs. Queens bred only once exhibit great variability in serum LH concentrations and fewer than 50% will ovulate. Concannon et al demonstrated that greater than 4 copulations were required to insure LH release adequate for ovulation.
In the colony at the University of Minnesota, 14 cats were bred at 23 cycles. They were bred an average of 5.6 times per cycle, with a range of 4-8 times per cycle. The pregnancy rate was 73.9%, and those queens that did not become pregnant had prolonged interestrous intervals, averaging 61.5 ± 14.5 days in length. This suggests that breeding an average of 5.6 times/cycle was sufficient for reliable induction of ovulation.
There is some evidence that cats may occasionally spontaneously ovulate. Wildt et al failed to demonstrate spontaneous ovulation in 17 queens by laparoscopy. However, cats housed at Ralston Purina were noted to have high incidence of uterine disease, an unexpected finding in cats which were not induced to ovulate. Serial blood samples were drawn from 20 queens that were housed individually and who had no perineal contact with other cats or from their handlers. Thirty-five percent of these cats had serum progesterone concentrations suggestive of ovulation at one or more samplings. This suggests that queens may occasionally spontaneously ovulate, may ovulate secondary to a trigger other than coitus, or may undergo luteinization of follicles instead of atresia after a non-ovulatory cycle.