pedometer, during the high-fertility phase of their cycle. however, other research has found no difference in locomotion patterns across the ovulatory cycle, and many studies on activity across the cycle have small sample sizes and substantially differing methodologies, making it difficult to draw definitive conclusions. despite a possible increase in activity, many studies have found that women consume fewer calories during their fertile phase. some researchers have suggested that these changes in activity and food consumption may indicate that during estrus, women are motivated to focus more of their energy on mating-related behaviors like going out to meet new potential mates, instead of survival-related behaviors like seeking food. competitiveness with other women parental investment theory posits the idea that natural selection designed each sex to have different mating strategies based on how much investment the sex is required to devote to offspring for their survival. the sex that invests more in offspring should be more intersexually selective, or picky when choosing a sexual partner, because they have more time and resources to lose if they make a poor choice. the other sex should be more intrasexually competitive, or competitive with members of their same sex, in order to access and attract the more selective sex. in humans, as in all mammals, females are the sex that invests more in parenting, simply through the lengthy and taxing process of pregnancy and lactation, whereas males need only to contribute one act of sexual intercourse to pass on their genes. thus, females are expected to be the more selective sex, and males are expected to be more competitive. however, unlike many species where males do not contribute to parenting at all, humans have highly dependent offspring, and a complex social structure that allows males to make significant and important investments in parenting effort. according to parental investment theory, this indicates that natural selection may have designed women to be somewhat competitive with other women for access to the best mates and potential fathers for their offspring. some studies have indicated that women engage in more competitive behaviors with other women when they are at high fertility. during the fertile window, women not using hormonal contraceptives self-report increased feelings of intrasexual competitiveness, describe other women as less attractive, and use more dehumanizing terms when talking about women, but not men. women's choices to purchase more attractive or revealing clothing at high fertility are also increased when they are first shown a photograph of an attractive woman, but not photographs of men or unattractive women, suggesting clothing may not be chosen to attract men, but rather as a competitive display for other women. additionally, some studies have used economic games to show that women are less likely to share resources or engage in cooperative bargaining with other women during the fertile window. some researchers have noted that the reason why women should be more competitive during the fertile window is unclear. the ovulatory shift hypothesis proposes that women should be seeking short-term sexual partners at peak fertility, but men can effectively have multiple sexual partners, so competition over one high-quality man should not be necessary. if women were competing for a long-term partner, there is no reason why they should be more competitive during the fertile window than any other time in their cycle. ovulatory cycle shifts hassleton and gildersleeve (2011) wrote that both men and women can subconsciously detect cues to women's fertility that change across the ovulatory cycle. some researchers have suggested that natural selection designed women to signal their fertility in order to attract a mate. other researchers have proposed that women evolved to have concealed ovulation but they still "leak" subtle cues of their fertility, and men have evolved to detect these cues. body odor during estrus, many species produce pheromones, or body odors that indicate to potential mates that one is in the fertile phase. while no specific human pheromones have been identified, humans may exhibit similar scent changes at high fertility. body odors of high-fertility women not using hormonal contraceptives are rated in some studies as more attractive by both men and women. vaginal odors from high-fertility women are also rated as more attractive than odors from the same women at low-fertility. some studies have shown that men exposed to high-fertility body odors of women exhibit increases in testosterone, a feature associated with mating motivation and behavior, although other studies have failed to replicate this effect. physical attractiveness studies using facial photographs found that both men and women rate physical features of women at high fertility more attractive than when they are at low fertility and that facial attractiveness increases in fertile phase women. it has been hypothesized that this shift may be due to subtle changes in soft tissue symmetry that increase during high-fertility. vocal pitch studies have found that fertile phase women speak with a slightly higher vocal pitch. one study reported that recordings of women's voices in the fertile phase are rated, by both men and women, as more attractive than recordings by the same women during low fertility. however, these effect sizes are relatively small compared to other cues of ovulation. partner jealousy several studies have found that men in a relationship tend to be more protective and possessive of their partner when she is at peak fertility, as well as more jealous of any advances their partner might make on other men. one study found that after interacting with their partner during the fertile phase, men shown a photograph of an attractive man exhibit increased testosterone, which may be a competitive response. effects of hormonal contraception since it has been proposed that changes in hormone levels across the ovulatory cycle are the primary mechanisms that causes cycle shifts, some studies have explored the effects of hormonal contraception, like the pill, on both women's cycle shifts and other people's ability to detect cycle shifts. studies have reported that hormonal contraceptives weaken or eliminate cycle shifts entirely. it has been proposed that the synthetic hormones present in hormonal contraception that suppress ovulation also suppress the subsequent cognitive and behavioral changes found in naturally-cycling women. other studies have stated that changes in synthetic hormones produce cycle shifts similar to effects produced by the real hormonal changes in naturally-cycling women. alternative hypotheses within-cycle vs. between-cycle shifts while the ovulatory shift hypothesis proposes that adaptive changes in mating-related cognition and behavior occur within each ovulatory cycle, some researchers have posited a between-cycle shift theory. many women experience regular anovulatory cycles, or non-fertile cycles where ovulation does not occur, therefore hormonal changes between ovulatory cycles may be a more reliable indicator of true fertility, as higher levels of estradiol are more likely to produce a fertile ovulatory cycle. thus, some researchers have proposed that hormonal changes between cycles, primarily in elevated estradiol levels, are responsible for changes in mating-related cognition and behavior. within-cycle shifts may be simply a byproduct of between-cycle shifts caused by elevated estradiol. meta-analyses and reviews one meta-analysis and a review of the literature have been conducted on both published and unpublished data that support the claim of the ovulatory shift hypothesis that women experience changes in attraction preferences at high fertility. however, another meta-analysis and subsequent commentary concluded that the effect is not actually significant and may be a result of some studies using imprecise measurements of when women are in the fertile window, as well as publication bias. a review subsequently published also does not show women changing the type of men they desire at different times in their fertility cycle. another study found no correlation between current fertility status and sociosexual attitudes and desires.