Not true of all organisms, but what's the evolutionary correlation in mammals? And why just in mammals? In general smaller mammals have more frequent estrus, but this is not always true. In humans it's almost at the continuous arbitrary level (polyestrous all year - 12/year). Humans are almost like rabbits. wolves, bears: once/year domesticated canines, domesticated coyotes: twice/year sheep, goats, deer: seasonally polyestrous cats: yearly polyestrous hominids: twelve/year - yearly polyestrous (cycle all year) rabbits: arbitrary/anytime Estrus frequencies of other mammals (duration and timing - see: http://www.siu.edu/~tw3a/431estcy.htm): Cow: 21 days Elephant: 16 weeks Goat: 21 days Rodents: 4-5 days Sow: 21 days Persistent estrus induced in mice (via estradiol implants) http://nationalmeeting.aalas.org/pdf/1994-abstracts.pdf It's a basic evolutionary adaptation for reproduction, but for humans, this reproductive strategy keeps the males continually aroused - free to have sex whenever they want, except for social controls - and females are continuously vulnerable to sexual predators year round. "Human beings have lost their instinctive connection with estrus. A male can be amorous, a femae receptive, at all moments. Among human beings sex has become a recreation and a pastime. You can, in fact, argue that the human animal is largely oversexed." http://www.sunstoneonline.com/magazine/searchable/Issue92.asp "Bottle nose dolphins herd females in estrus away from the group and subject them to repeated and apparently nonconsensual copulations. The males sometimes band together in what are called coalitions to fight off other bands of male dolphins, bent on the same rape themselves." http://gssq.blogspot.com/search/label/links Cats are very unique and somewhat unusual. The feline is seasonally polyestrous, responsive to photoperiod, and can maintain sexual receptivity for a period of some days after ovulation, while the corpus luteum is forming. "Most importantly from a clinical standpoint, like the rabbit and ferret the queen [feline cat] is an induced ovulator, requiring a copulatory stimulus or exogenous hormones for ovulation and corpus luteum formation. Cyclicity of the cat has been determined to be dependent on photoperiod. Cats are "long day breeders" and require 12 hours or more of light to maintain normal cyclicity. Normal polyestrous behavior can be induced by controlling exposure to light. Cyclicity stopped abruptly and folliculogenesis was inhibited in queens exposed to less than 8 hours of light. Cyclicity in the cats resumed an average of 16 days after returning to a 14 hour photoperiod. " http://www.vetmed.lsu.edu/eiltslotus/theriogenology-5361/filne_e.htm Walruses are unique among aquatic mammals. They are polyestrous, but "the minimum interval between successful births is two years. Walruses give birth several months after the breeding season and do not have a postpartum estrus. This differs from other pinnipeds [aquatic carnivorous mammal] whose females mate within days or weeks after birthing their young. This may be due to the walruses nursing their calves for more than a year, suppressing ovulation until the calf is weaned. These factors all result in the walrus having the lowest reproductive rate of any pinniped species." http://www.akcf.org/_attachments/pdf/IceDependentMarineMammals.pdf
My guess is that sex in higher mammals becomes recreational/social rather than purely reproductive. I'm sure there's more involved, but this is a high level explanation for underlying selective pressures.
I should know the answer to this; I'm sure I've read it somewhere. My guess is that it has something to do with females being more able to choose when the best time to have a child is. Also, greater ability to hide when they are sexually receptive.
