Wenda R. Trevathan

Birth, obstetrics and human evolution.

There are several characteristics that set our species apart from other mammals. We are the only living or extant mammal that habitually walks on two legs. For our body size, we have the largest and the most complex brains of all animals. Humans depend on material culture or ‘tools’ for their survival. Humans universally communicate with each other through abstract symbols known as language. Finally, human females routinely seek assistance when they give birth. In fact, many have argued that midwifery or obstetrics, not prostitution, is the ‘oldest profession’. Fossil evidence shows that although two-legged walking, or bipedalism, traces its origin to the very beginning of human ancestry, tools and language appeared much more recently in human evolution. We will argue in this paper that, along with bipedalism, some aspects of the human pattern of birth trace their origin to the very beginning of human evolution. Based on fossil evidence from Africa, human paleontologists agree that the mammalian family to which humans belong, Hominidae (‘hominids’), originated approximately 5 million years ago. The crucial hallmarks of our earliest hominid ancestors are skeletal indicators of bipedalism. These markers appear clearly first in Australopithecus anamensis (ca. 4 million years ago) or possibly earlier. In spite of a human pattern of locomotion, these early members of the genus Australopithecus had brains that were smaller than modern humans—in proportion to their bodies—very similar to those of modern chimpanzees. Significant brain expansion did not begin until the origin of our genus, Homo, about 2.5 million years ago, when we also have evidence of the first stone tools. Evidence of language is not directly preserved in the fossil record but several investigators suggest that our ancestors were capable of the complex speech patterns that are part of all spoken languages today by at least 200,000 years ago. As noted above, we argue that assisted birth may be associated with bipedalism, and may thus be as old as the hominid family itself. However, assistance at birth is not the only significant birthrelated difference between humans and our close relatives in the primate order. The unusual way in which modern humans give birth is the result of a set of constraints imposed by bipedalism, a large brain, and ‘secondary altriciality’, or the delivery of the infant in a relatively helpless state. Birth in non-human primates

No evidence for menstrual synchrony in lesbian couples.

Menstrual synchrony was investigated in a sample of 29 cohabiting lesbian couples, ranging in age from 22 to 48 years. One or both partners kept prospective daily records of variables including menses onset dates, intimate contact, and sexual activity. All women reported daily intimate interaction with their partners; none reported intimate interaction with men. Despite these potentially optimal conditions for the manifestation of synchrony, the differences between dyad members in menses onset dates were distributed randomly, and there was no evidence of convergence. In fact, most dyads exhibited divergence of onset dates. Reasons for lack of synchrony in this sample are discussed; one conclusion is that there is no solid evidence that menstrual synchrony is a stable attribute of past or contemporary human populations.

Primate pelvic anatomy and implications for birth

The pelvis performs two major functions for terrestrial mammals. It provides somewhat rigid support for muscles engaged in locomotion and, for females, it serves as the birth canal. The result for many species, and especially for encephalized primates, is an ‘obstetric dilemma’ whereby the neonate often has to negotiate a tight squeeze in order to be born. On top of what was probably a baseline of challenging birth, locomotor changes in the evolution of bipedalism in the human lineage resulted in an even more complex birth process. Negotiation of the bipedal pelvis requires a series of rotations, the end of which has the infant emerging from the birth canal facing the opposite direction from the mother. This pattern, strikingly different from what is typically seen in monkeys and apes, places a premium on having assistance at delivery. Recently reported observations of births in monkeys and apes are used to compare the process in human and non-human primates, highlighting similarities and differences. These include presentation (face, occiput anterior or posterior), internal and external rotation, use of the hands by mothers and infants, reliance on assistance, and the developmental state of the neonate.

Sexual behavior in lesbian and heterosexual women: relations with menstrual cycle phase and partner availability

Using a prospective design over three complete menstrual cycles, 147 heterosexual and 89 lesbian women made daily recordings of their basal body temperature (BBT), cervical mucus status, menses, and completed a daily checklist of various sexual behaviors (including sexual self-stimulation and sexual activity with a partner). They also gave their age, height, weight, age at menarche, number of pregnancies, duration of sleep, tobacco, caffeine, and alcohol use, and whether they had a live-in sexual partner. Using BBT, cervical mucus status, and menses information, cycle days were grouped into five discrete phases: menses, follicular, ovulatory, early luteal, and premenstrual. Daily frequencies of sexual behavior with a partner and autosexual behavior were computed for each phase. Mixed ANOVAs on the resultant proportional data revealed similar patterns for autosexual behavior across the phases for both heterosexuals and lesbians who did not have a live-in partner, in which autosexual behavior was highest during the follicular and ovulatory phases. For those with live-in partners, autosexual behavior did not vary across the phases. Lesbians engaged in more autosexual behavior overall. Allosexual behavior peaked during the follicular phase for both heterosexuals and lesbians, and the phasic pattern was unrelated to live-in partner status. Additional analyses suggest that the observed patterns were unrelated to anticipated changes in sexual activity due to menses. Results are discussed in terms of social variables and hormonal fluctuations associated with the menstrual cycle.

Gender, anatomical knowledge, and pottery production: Implications of an anatomically unusual birth depicted on mimbres pottery from southwestern New Mexico

The anatomical details of a birth scene depicted on Classic Mimbres (A.D. 1000-1150) bowls from southwestern New Mexico can provide clues to gender relations in Classic Mimbres society. The scenes show an infant emerging facing forward (unusual in human birth) with its arms up (virtually unknown in human birth). These details suggest that the scene was painted by someone unfamiliar with the birthing process. Ethnographically, men rarely see human births. Thus, it is likely that the birth scene, and perhaps other Mimbres pottery designs, were painted by men.

Knowledge, Perception of Risk for HIV, and Condom Use: A Comparison of Registered and Freelance Female Sex Workers in Cebu City, Philippines

The HIV infection rate is lower in the Philippines than would be expected based on the numbers of people believed to engage in high-risk behaviors. A national behavioral surveillance system has been implemented to monitor selected behaviors among vulnerable populations. This paper reports on an interview survey of 360 registered and 360 freelance sex workers in Cebu City, the Philippines, from 1997 to 1999. All of the women had engaged in sexual activity in exchange for money during the week preceding the interview. The freelance workers reported more sexual partners than the registered sex workers (p < .00001), assessed themselves as more at risk for HIV (p < .00001), and were less likely to always use condoms during sex (p < .00001). The groups were comparable on a measure of knowledge about HIV transmission. As these women continue to engage in risky behaviors, some form of intervention will be necessary to prevent the escalating HIV/AIDS rates seen in other Southeast Asian countries.

Knowledge and perception of risk for HIV and condom use among male injecting drug users in Cebu City, Philippines.

Injecting drug users (IDU) represent a small fraction of the HIV and AIDS cases in the Philippines. To determine if these people are engaging in behaviors that put them at risk for HIV, interviews were conducted with 360 male IDUs in Cebu City, Philippines, from 1997 to 1999, as part of a national surveillance system. The interviews assessed knowledge about HIV transmission, sources of information about HIV/AIDS, perceived risks of contracting HIV, needle-sharing practices, condom use, self-reported signs and symptoms of STDs and number of sex partners. Although most of the men were able to recognize behaviors accurately that put them at risk for HIV, more than two-thirds claimed that they shared needles and almost two-thirds of those who were sexually active claimed that they never used condoms. Intervention strategies must be developed for this population if the nation is to avoid the dramatic increase in HIV infection among IDUs that has been witnessed in neighboring Southeast Asian nations such as Malaysia and Vietnam.

Heterosexual activity and cycle length variability: Effect of gynecological maturity ☆ ☆☆

Previous studies linking heterosexual activity to womens menstrual cycle variability have failed to take into account the effects of gynecological maturity. One hundred thirty-two women, all at least seven years postmenarche and not using birth control pills, completed daily records of their cycles and their heterosexual behavior. Data from women classified as sexually celibate or as regularly sexually active (having sex at least once per week in every nonmenstruating week) replicated previous findings while controlling for gynecological maturity: Women classified as celibate had more variable cycles than women who engaged regularly in heterosexual activity. An interaction between gynecological maturity and sexual status was also found, precluding a comparison involving women who were sexually active on an irregular basis. The interaction revealed that increased gynecological maturity is associated with less variable cycles in the sexually sporadic women, but is not associated with cycle variability in either celibate or sexually regular women. Possible biological mechanisms for these findings and their implications are discussed.

HETEROSEXUAL ACTIVITY: RELATIONSHIP WITH OVARIAN FUNCTION

Previous research demonstrated a relationship between the temporal pattern of heterosexual activity and an index of ovarian functioning. In the current study, this relationship was investigated in 147 menstruating heterosexual women (aged 19-53). They kept prospective daily records of menses, basal body temperature, sexual activity, and other behaviors for three consecutive menstrual cycles. In contrast to previous findings, women with intermediate levels of sexual activity displayed more frequent optimal menstrual cycles. Pheromones, semen absorption, and orgasm-related changes were tested as mediators for a causal influence of sexual activity on ovarian functioning; none was supported. Exploratory analyses tested the hypothesis that anovulatory cycles (with presumably lower progesterone) would display more sexual activity than ovulatory cycles. This hypothesis was supported, and the difference in sexual activity was limited to the second half of the cycle, after ovulation would have occurred. Thus, the findings incorporate temporal precedence of ovulation to support the idea that physiological processes influence the level of sexual activity in heterosexual women.

Neonatal Shoulder Width Suggests a Semirotational, Oblique Birth Mechanism in Australopithecus afarensis

Birth mechanics in early hominins are often reconstructed based on cephalopelvic proportions, with little attention paid to neonatal shoulders. Here, we find that neonatal biacromial breadth can be estimated from adult clavicular length (R2 = 0.80) in primates. Using this relationship and clavicular length from adult Australopithecus afarensis, we estimate biacromial breadth in neonatal australopiths. Combined with neonatal head dimensions, we reconstruct birth in A. afarensis (A.L. 288‐1 or Lucy) and find that the most likely mechanism of birth in this early hominin was a semi‐rotational oblique birth in which the head engaged and passed through the inlet transversely, but then rotated so that the head and shoulders remained perpendicular and progressed through the midplane and outlet oblique to the main axis of the female pelvis. Any other mechanism of birth, including asynclitic birth, would have resulted in either the head or the shoulders orthogonal to the short anteroposterior dimension of the A.L. 288‐1 pelvis, making birth untenable. There is a tight fit between the infant and all planes of the birth canal, perhaps suggesting a difficult labor in australopiths. However, the rotational birth mechanism of large‐brained humans today was likely not characteristic of A. afarensis. Thus, the evolution of rotational birth, usually associated with encephalization, may have occurred in two stages: the first appeared with the origin of the australopiths with their platypelloid pelves adapted for bipedalism and their broad‐shouldered neonates; the second which resulted in the modern mechanism of rotational birth may be associated with increasing brain size in the genus Homo. Anat Rec, 300:890–899, 2017.