Puberty and menstruation

What is the adolescent period and is it unique to humans?

● Adolescence = period from puberty to ~20 years (after puberty but before full adult maturation)

● Humans have a longer pre-adult period than other primates, but adolescence itself is not unique to humans — other primates also have puberty and a juvenile-adolescent phase.

● What is unique in humans is how long it is.

What is menstruation? In what animals do we see it?

Menstruation = shedding of the thickened uterine lining (endometrium). It is rare — found mainly in: ● Humans ● Some primates ● Few mammals (certain bats, elephant shrews)

Differences between menstruation in industrialized vs natural fertility populations

● Industrialized societies: ○ Earlier puberty/menarche due to higher caloric availability (e.g., Europe: 17 → 13 years)

● Natural fertility populations:

○ Later menarche

○ Often longer interbirth intervals ○ Less frequent cycles for much of reproductive life (pregnancy, breastfeeding)

Most likely explanation for why menstruation is so severe in humans

● Human embryos are extremely invasive → the uterus evolved a very thick, highly vascular endometrium as protection.

● It is less costly to shed that thick lining each cycle than to maintain it.

● Menstruation may also help discard abnormal embryos (which are common in humans).

What is spontaneous decidualization? What is the alternative?

● Spontaneous decidualization: ○ The uterus prepares (“decidualizes”) every cycle, before an embryo implants. ○ Controlled by the female’s hormones alone (progesterone). ○ Characteristics of humans and other menstruating species. ● Alternative (non-spontaneous decidualization): ○ Uterus changes only in response to an implanting embryo (triggered by the embryo). ○ Most mammals have this and do NOT menstruate.

Possible explanations for sexual swellings & what mating system they occur in

Sexual swellings occur in multi-male/multi-female (polygynandrous) primate groups (chimpanzees, bonobos). Explanations include: ● Best male hypothesis: signal ovulation so males compete; female mates with the winner. ● Many-male (paternity confusion) hypothesis: reduce infanticide by confusing paternity. ● Reliable indicator hypothesis: swelling size reflects female fertility. ● Graded signal hypothesis: allows both attracting top males and confusing paternity.

Possible benefits of concealed ovulation

Concealed ovulation likely evolved to: ● Confuse paternity → reduce infanticide ● Encourage pair bonding → males must stay close because they can’t detect fertile days ● Enable long-term mate guarding It is strongly associated with monogamy.

Possible signs of ovulation in humans

Even though ovulation is “concealed,” studies show subtle cues: ● Changes in attractiveness ratings ● Behavioral changes (more assertive, more expressive, more likely to initiate sex, more shopping) ● Hormonal effects on voice, scent, and clothing choices