Fertility and Population growth

Lecturer: Deborah Judge, Biological Anthropologist, School of Human Sciences.
Today’s focus:
• Evolutionary perspective on fertility and its role in human life-history.
• How fertility interacts with population dynamics.
• Preview: next lecture will cover menopause (a rare mammalian trait).
Pedagogical note: Lecturer invites feedback because recorded format lacks real-time visual cues.

Three population components: 1) fertility, 2) mortality, 3) migration (latter ignored for global view).
Fertility is under natural selection; our modern reproductive physiology was shaped under Pleistocene foraging conditions:
• Scarcer foods, higher mobility, unpredictable resources.
Humans are phenotypically plastic: traits can adjust within a lifetime in response to environment.
• Plastic traits: age at menarche, age at first reproduction, inter-birth interval (IBI).

Onset: menarche (first menstruation) → often followed by a short sub-fertile period.
Culturally/physiologically delayed age at first reproduction.
Age at last birth usually precedes menopause.
Human “stopping behaviour”: parity-specific stopping – couples cease child-bearing after reaching preferred family size.

Survival to reproductive age – prerequisite for any fertility.
Adult survival – determines number of reproductive events.
Timing variables:
• Age at onset.
• Birth spacing (IBI).
• Infant/child survival rates.

Graph (log–log): female body mass vs age at sexual maturity.
• Humans have similar body size to chimps/orangutans but much later maturity.
Later maturity signals a slow life-history (long juvenile phase, lower annual fertility, slower population growth).

Resources accelerate maturation; scarcity delays it.
• Example: Kikuyu girls
– Rural Kenya: 15.9\,\text{years} avg. menarche.
– London diaspora: 13.5\,\text{years}.
Historical trend: ~4\,\text{months} earlier menarche per decade across \approx 130 years.
Contemporary population means range 12.3 \rightarrow 18.6\,\text{years}.

IBI = pregnancy duration + lactational amenorrhoea + waiting cycles.
Pregnancy duration: species-constant.
Lactational amenorrhoea: highly variable; shortened by provisioning, plentiful food, lower nursing frequency.
• Provisioned vs non-provisioned primates: 6!\text{–}!12-month difference.
Birth spacing benefits:
• Higher infant & maternal survival.
• Prevents maternal energetic depletion.
Spacing maintained through: energy balance, physiology, lactational contraception, postpartum sexual taboos, modern contraceptives.
Fetal or infant loss resets clock early → shorter IBIs (no full lactation phase).

Graph: female body mass vs (gestation + lactation length).
• Humans align with ape trend: 266\,\text{days} gestation + \approx 4\,\text{years} lactation in traditional societies.
Post-agriculture ((~10!–!12\,000) years ago): lactation falls to \approx 2.2\,\text{years} – makes humans short-spaced breeders relative to apes.

Population	Ecological mode	Avg. IBI (months)	Total Fertility Rate (TFR)
!Kung San (desert)	Foraging	48!–!55	4.7
Ache (forest, mobile)	Foraging	37.6	?? (baseline)
Ache (settled)	Semi-sedentary	31.5	+0.5 (≈5.2)
Yanomamö (horticulture)	Semi-sedentary	34	6.9

Explanation:

Settlement & food production shorten IBI and raise TFR.
Environmental richness and workload differences explain outliers (e.g., high Ache fertility even in forest).

Age-specific survivorship: by age 4, 25\% dead; by 15, 50\% dead.
High childhood mortality ⇒ prehistoric human populations grew slowly unless territories expanded.

With farming (~10!–!12\,000 BP):
• Higher fertility (shorter IBIs, earlier menarche).
• Higher mortality, mainly infants.
• Fertility increase exceeded mortality increase ⇒ net growth.
Ache case-study: settled groups show earlier reproduction & higher age-specific fertility but infant mortality spike in first 2 yrs.

Madagascar (agrarian): broad base, steep taper → high fertility & high child mortality.
England 1967 (post-transition): narrow base, rectangular middle → low fertility & low mortality.
Age structure depends jointly on fertility & mortality patterns.

Absolute change: N{t+1}=Nt + B - D.
Rates (per capita):
• Birth rate b = \dfrac{B}{N}.
• Death rate d = \dfrac{D}{N}.
• Net rate r = b - d = \dfrac{B-D}{N}.
Projection: Nt = N0\,(1+r)^t (e.g., r=0.013 ⇒ 1.3\% annual growth).

K = population size the environment can sustainably support.
Species- & habitat- specific: desert might support 1 human/30\,\text{km}^2 vs rainforest 30 humans/1\,\text{km}^2.

Pre-potato (~1650): population ≈3\,000,000.
Post-potato adoption (~1800): rises to 8\,000,000.
1840s blight:
• \approx 1\,000,000 deaths.
• Mass emigration to N. America & Australia.
• Marriage postponed (mean age at first birth rose 20 \rightarrow 30).
Post-famine population falls back to \approx 3\,000,000.

Long-term downward trend (fewer births per capita).
Notable fluctuations:
• 1890s Depression (sharp drop).
• Early 1900s uptick.
• 1930s Depression (drop).
• Post-WWII baby boom (spike).
• 1960s contraception era (decline).
Population still rose due to momentum: many adults applying even a lower per-capita rate.

First transition (foraging → farming):
• Earlier maturity, shorter IBIs, population growth.
Second (~European 1700–1800s):
• Mortality decline first, fertility decline follows (parity-specific stopping, longer IBIs).
• Requires confidence in child survival.
Third (recent developing nations):
• Death rate drops rapidly (public health), birth rate lags ⇒ prolonged high growth.
• Eventually fertility also falls.

Human fertility is environmentally responsive, yet evolutionarily shaped for energy-limited foraging contexts.
Birth spacing (IBI) is central to individual fecundity; lactational biology and cultural practices modulate it.
Population growth depends on the balance between fertility and mortality; resource shocks can reverse or accelerate trends.
Demographic transition unfolds differently across historical eras and regions, but ultimately links health improvements to voluntary fertility control.

Feedback/questions: email the lecturer (address provided during lecture).