Key Terms in Evolutionary Biology of Sex, Gender, and Sexuality: Estrus

Definition
Estrus is a period during the estrus cycle when a female will allow mating usually because she is able to become pregnant. When applied to an animal this is the time in an animals cycle is known colloquially as "in heat". Most mamilian females have a reoccurring estrous cycle. Generally during estrous she is receptive to sexual activity and when she is not in estrus, she is not receptive to sexual activity. Typical behaviors of a female in estrus are willingness to seek out a mate and willingness to mate. Hormones secreted during the estrus phase are different than in any other phase because they support this mating behavior and the release of an egg.

According to Diamond, the word Estrus orignated from the greek word meaning gadfly. Gadflies would drive cattle wild.

Example(s) of use in context
If you have seen an animal in heat, you have seen behaviors that accompany estrus. Rats for example will assume the position called lordosis. This position is assumed when a male rat touches the female rat's flanks. If she is estrus, she will become completely still, curve her spine and raise her tail in order to make her vaginal opening accessible to the male rat. She allows the male rat to mate with her. If she did not assume this position she it would be impossible for the rat to mate with her. If she were not in estrus, she would not go into the lordosis postion, and the male rat would be unable to mate with her. Randy J. Nelson provides us with three experiments that can serves as one collective example of a rat's behavior while in estrus. One was done in 1927 by Warner, the second in 1961 by Berman and the last by Mathews et al., in 1997. These studies showed that a rat in estrous would cross an electric floor, despite the pain it caused, to get to the male rat in order to mate. They also created separate compartment for a female rat and a male rat. The female rat was able to stick her nose through an opening to smell the male rat in the second compartment only after pressing a lever continously. The rats that were not in estrus would not cross the electrified field or press the lever with as much enthusiasm to get to the male rats.

Scientific background
Estrus is often split up into three components, attractivity, proceptivity and receptivity. Attractivity is how attractive she is to a male. Proceptivity is the degree to which she seeks to mate. Receptivity is how able she is to get a male to ejaculate inside of her.

Estrus has several phases, begining in the Vaginal Estrus phase. Ovulation occurs as follicles have just ruptured while new follicles begin to develop. The next phase is Diestrus, which lasts for 48 hours and is often characterized in two phases within itself. Those being Diestrus I and Diestrus II. Diestrus is marked by a reduced number of white blood cells, cornified cells and an increased number of epithhel cells. The last phase of estrus, is proestrus. Proestrus is the phase during which females exhibit mating behavior typical of Estrus. This description was modeled after a rats Estrus phase.

History
Until the 1900s there was much more research done involving male sexual behavior than female sexual behavior. Nelson speculates that the lack of studies done on female sexuality might have been because of male dominated scientific field that assumed female complacency in the matter of reproduction. Or, he argues, it could have been simply because an ovariectomy is more difficult to perform than a gonadectomy. To study male sexual behavior, scientists typically removed the gonads, and then observed sexual behavior. In the 1900's an ovariectomy was performed and it was found that similar to males, female sexual behavior halted after the procedure.

Debates
The idea that Estrus is present in humans is some what controversial. Ivan Ward and Oscar Zarate writes

"Human sexuality is quite peculiar and unique. it is not like a duck's, or a rabbit's, or a chimpanzee's.  The distinguishing feature is the loss of oestrus.  the periodic character of sexual excitation disappears, and the relation between female menstruation and male excitation is completed transformed.  In order words, 'male excitation' is no longer depended on a female sexual cycle.  Sexuality has become unhinged form reproduction." Yet, many scientists have reported that women behave differently based on where they are in their estrus cycle. Thornhill and Gangestad write in their article Coevolutionary Processes on page 300 write “women reported that their partners engaged in both propriety and attentive tactics more frequently when they were fertile than during the infertile luteal phase (Gangestad et al., 2002) This extra mating was found during the “fertile” phase because “Men can potentially detect female fertility status through scent, appearance, or behavior cues”. Some women even claim to know when they are ovulating. "In a polet study of two hundred college, women, I found that 67" percent siad they thought they knew when they ovulated (Small ms)" Some scientists, like Thornhill and Gangestad, claim that humans too have this state of estrus that drives them to behave differently if they are ovulating.  It is a bit controversial to clai that humans have a state of estrus that can be compared to animal in estrus.  It is must be remebmered that most research done on estrus cycles have been performed on rats, not humans.  Nelson writes “...virtually all that we know about the regulation of female sexual behavior has been derived from studying cycling female rodents. The development of oral contraceptives for human use was accomplished by understanding the endocrinology of cycling rodents and nonhuman primates” . So in effect, the ways men respond to women are based on how rodents respond to each other.

Author
Sarah Adelaida McIntire

Contributors
http://en.wikipedia.org/wiki/Human_sexuality