Mammals reproduce sexually. There is a distinctive advantage of sexual reproduction. When sperm and ova are matured, during meiosis, some gene shuffling can occur before chromosomal numbers are halved. Beneficial alterations allow for rapid evolutionary progress. Continuity certainly exists from one generation to the next. Notwithstanding, the goal of reproduction is to perpetuate the �fittest� of species. Natural selections of superior genetic variants assure successes of some individuals over others (organisms that reproduce asexually, from a single parent, rely on random genetic mutations to adapt to changing environmental constraints). Sexual reproduction, however, has its drawbacks. Generation intervals tend to be lengthy. Only one-half of the gene complement is passed to the offspring. Mating takes time/energy (eg., finding an appropriate mate could be limiting in a sparsely populated setting) and can be damaging (injury, disease). And there is a significant cost of producing males whom do not generate ova and often do not contribute to postnatal care. Females are the limiting resource in terms of population growth.
Courtship behaviors. The sequence of courtship behaviors in mammals is simple in comparison to the elaborate performances of lower vertebrates; nonetheless, species-specific behaviors still exist (Figure 5-1). Matings between species are rare (a classical departure is the cross of a male donkey and female horse to produce a sterile mule; F1 cattle/bison hybrids are fertile).
Among most mammals the male is the sexual aggressor (exceptions are the golden hamster and spotted hyena). Males will (somewhat indiscriminately) investigate females and initiate physical contact to determine if she will "stand-to-be-mounted" (Figure 5-2).
Olfaction is used by males to identify chemoattractants emitted by estrous females. The flehmen reaction of some ungulates and carnivores is exhibited after sniffing the female's urine or perineum; the head is raised and the upper lip is curled during inspiration (Figure 5-3). The vomeronasal organ, located above the roof-of-the-mouth, contains receptors for signaling pheromones. Some species have well developed vomeronasal organs (eg., burrowing/nocturnal rodents), but do not demonstrate a flehmen reaction. Higher primates have a poorly developed olfactory system, yet male rhesus monkeys use the sense of smell to monitor receptive females. The biochemical compositions of sexual attractants are just beginning to be uncovered (one such volatile substance extracted from the vaginal discharge of hamsters is dimethyl disulfide).
Males also secrete pheromones that elicit female courtship behaviors. Recall that boars release a steroid of preputial origin. Copious secretions (presumably containing pheromones) exude from the temporal glands of bull elephants in musth (Figure 5-4).
Motor activity of estrous females is increased and directed toward the male. The female will usually align her hindquarters toward the male - if the male fails to investigate, she will pursue and often nudge at his flank. Estrous females of some species (eg., rodents and pigs) become rigid and arch their backs (lordosis) when approached by the male or touched over the back (Figure 5-5). Females will not generally permit the male to mate outside of the ovulatory phase (some primates do, though active intent by the female [to include humans] is intensified at such time).
Olfactory, tactile, and postural cues are not the only criteria used in the expression of sexual behaviors - auditory and other visual types of communication are also exploited. For example, stag deer frequently vocalize during the breeding season (Figure 5-6) - serving notice to both competing males and estrous females. The swollen and brightly-colored genital skin of some primates is considered a sexual attractant (Figure 5-7). Equids in estrus frequently urinate followed by contractions of the vulva ("winking").
Very few mammals practice monogamy (exceptions are wolves, gibbons, and some species of monkeys). The other extreme of polygamy, consisting of a dominant male and his harem, is actually more prevalent; this situation is well illustrated among seals (Figure 5-8).
Neuroendocrine factors. Control mechanisms governing sexual behaviors are fraught with complexity - seeming contradictions are commonplace.
After adult males are castrated, sexual and aggressive behaviors will persist (albeit diminished) for extended periods. Spayed females will not display estrus (unless treated with steroid hormones).
Estradiol plays a role in the physiology of both male and female sexual behaviors. Estrogens are even more effective than testosterone in augmenting libido in castrate males (androgens are converted within the brain). Differences between sexes in behavioral patterns have been attributed to gender-specific hormonal rhythmicities and neural dimorphisms (traits entrained during perinatal life). The nature-nurture debate concerning human sexual orientation rages on.
Progesterone usually has a suppressive effect on sexual activity in females (high amounts can actually cause anesthesia). The dichotomy is that progesterone produced during the luteal phase facilitates subsequent estrus-inducing effects of estradiol. In rodents, who naturally exhibit a proestrous surge of progesterone, the order of action of steroid hormones is reversed - priming with estradiol enhances expression of estrus in response to an acute secretory episode of progesterone.
Although it is commonly believed that estrous behavior completely ceases during pregnancy, this is not the case. Anovulatory heats have been observed in domestic ruminants and several species of laboratory animals. Circulatory levels of estrogens in excess of those needed to cause psychological estrus in nonpregnant animals are generally achieved during pregnancy.
Substances other than gonadal steroids also have stimulatory (eg., GnRH, acetylcholine, norepinephrine, oxytocin) or inhibitory (eg., endogenous opiates, serotonin) effects on sexual behaviors. An integrative model for the neuroendocrine circuitry underlying a lordotic reflex is presented in Figure 5-9.
Copulation. Duration of intromission and frequency of copulation vary between species. Rams ejaculate almost immediately upon vaginal insertion, but are capable of repetitive matings over a short interval. Boars can mate for as long as one-half hour, but have a rather long refractory period. Mink and sable copulate for hours at a time during which the male ejaculates on several occasions.
Other social interactions. Male-female encounters can result in profound influences on patterns of breeding activities and pregnancy. Males can have a synchronizing (Whitten effect in mice) or shortening (cat) effect on estrus, accelerate the initiation of the breeding season (ewes, goats, voles), enhance attainment of puberty (mice, gilts), and induce abortion (Bruce effect in mice). Sexual excitement of males in the presence of estrous females leads to a transient rise in testicular output of testosterone; libido is increased when novel females are introduced (Coolidge effect).
Encounters between members of the same sex also can alter reproductive behaviors. Puberty is suppressed in female mice reared in large groups (an internal check on population density). Dominant males will often suppress attempts to mate by subordinates.
Selection-of-the-fittest is clearly a factor in preference for a mate. Rodent females are attracted to urine of dominant males. When given the chance, a bitch will choice a sexual partner of greater physical stature (sometime take note of the incidence of mongrels that have police dog-like traits). Inbred strains of mice will select partners of a different strain (an assurance of hybrid vigor).
Seasonal effects. Many mammals are seasonal breeders - this is certainly true in the wild. The adaptive significance of seasonal breeding strategies is that young are born when chances of survival are optimal (ie., an adequate food supply is available and climatic conditions are favorable). Length of gestation (Table 5-1) ultimately resolves the season during which it is most advantageous for animals to mate (Table 5-2).
Seasonal cycles are by no means fixed. Domestication/confinement or displacement can lead to an eventual loss in seasonal breeding cycles. Conversely, most animals will avert to a seasonal breeder when obligated to live in a feral environment. Fertility in captive rhesus monkeys is not related to season. The common house mouse (when in the house) will mate continuously throughout the year. And breeds of sheep and rabbits that normally exhibit seasonal activities in temperate zones will become nonseasonal when relocated to a tropical climate. Kangaroos become anestrus during prolonged drought. There is even circumstantial evidence that fertility in certain human cultures (eg., Eskimos) is depressed with increasing daylength (most babies are born in the spring or summer).
Changing photoperiod is the primary external signal controlling seasonal reproduction. Influences of photoperiod on female behaviors are most conspicuous (ie., she will become unresponsive to sexual advances during the nonbreeding season). Testicular regression and recrudescence are pronounced among males of some species.
Housing animals indoors under controlled lighting conditions can induce estrus - successes have been realized in a number of laboratory, captive wild, and farm species. Mares are routinely bred out-of-season with the aid of artificial illumination. Because some breed organizations have inauspiciously set the birth date of all animals at January 1, it is customary practice to breed the mare early in the year (ie., during short days) so she will give birth as soon as possible after the onset of the new year (this will allow the offspring the utmost physical advantage when racing as a two-year-old).
A good deal of knowledge about causative photoperiodic mechanisms of seasonality has been derived from research using the female sheep and male Syrian hamster as models. The circuitous neural route via which light:dark information is transferred to the pineal gland has been traced. Retinal receptors send impulses to the suprachiasmatic and paraventricular nuclei of the hypothalamus. Information is relayed through the superior cervical ganglion. The final (endocrine) component of the relay system involves melatonin (Figure 5-10). The circadian rhythm of melatonin codes the circannual cycle of seasonal reproduction. Melatonin is synthesized by the pineal gland in the darkness (NAT and HIOMT are activated). Melatonin exerts an inhibitory action on secretion of gonadotropins in long-day breeders. In the ewe melatonin extends the breeding season. Pinealectomy disrupts the seasonal cadence of reproduction in animals responsive to photoperiod. A definitive role for the pineal gland remains obscure in mammals that do not demonstrate distinct seasonal changes in reproductive potential.
The transition into and out of the nonbreeding season in males is marked by an increase and then decrease in hypothalamic sensitivity to the negative feedback effects of testosterone. A lack of sustained testicular gonadotropic support during the off-season leads to a decline in testosterone output, fertility, and libido.
In females, hypothalamic sensitivity to the negative feedback effect of estradiol intensifies with the approach of anestrus. The reproductive cycle becomes disrupted at CL regression. Because of heightened responsiveness to estradiol negative feedback there is no increase in tonic production of gonadotropins upon progesterone withdrawal (ie., as if the decrease had gone unnoticed). The final stages of follicular maturation, a preovulatory rise in estradiol, and the positive (surge) mode for secretion of gonadotropins (spontaneous ovulators) are circumvented. Estrus, ovulation, and luteinization are bypassed until follicular growth to the preovulatory stage can once again be driven by gonadotropic support. The first ovulation after an extended period of acyclicity usually occurs without behavioral estrus ("silent" ovulation) and is associated with a shortened luteal phase (Figure 5-11). Failure of estrus is attributed to a lack of hypothalamic priming by progesterone. Shortened luteal phases are due to ovulation of a defective follicle or premature production of luteolysin.
A yearly adjustment in tactic is a case of low-frequency rhythmicity. It is important to clarify, as an aside, that higher frequency rhythms (Table 5-3) propagated by external cues, also can have profound modifying influences on reproduction. All such rhythms coordinate physiological function with the changing physical world (ie., to provide temporal order). Indeed, an entrained rhythm of a more repetitious harmony has just been described - the daily bimodal fluctuation in secretion of melatonin. Further diverse examples are plentiful. Rats maintained on a routine lighting schedule (14 hours of light and 10 hours of dark/day) exhibit a preovulatory surge of gonadotropins in a synchronous manner on the afternoon of proestrus; if switched into continuous light they will divert to a persistent state of estrus (and if mated, reflex ovulate). Circulatory concentrations of testosterone in young men are elevated at night. The majority of mammalian births occur in the early morning. Menses tend to be attuned with the onset of a full moon.
Time-keeping of some reproductive processes is a function mainly of endogenous, genetically-regulated rhythms. Certain ground squirrels exhibit a strict circannual rhythm of testicular function that is maintained under controlled laboratory conditions of light and temperature. In contrast, seeming beneficial standards of seasonal receptivity can be transcended by genetic controls - some white-faced breeds of female sheep (Dorset, Merino, Rambouillet) are fertile throughout the year regardless of environmental constraints.