Fertility and Population growth

Lecture Context & Aims

• 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.

Why Fertility Matters in Evolution & Population

• 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).

Anatomy of the Female Fertile Lifespan

• 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.

Fertility Components & Their Population Effects

1. Survival to reproductive age – prerequisite for any fertility.

2. Adult survival – determines number of reproductive events.

3. Timing variables:
   • Age at onset.
   • Birth spacing (IBI).
   • Infant/child survival rates.

Comparative Life-History: Humans vs Other Apes

• 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).

Plasticity in Maturation

• 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 $12.3 \rightarrow 18.6\,\text{years}$.

Inter-Birth Interval (IBI) Mechanics

• 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).

Parental Investment: Gestation + Lactation

• 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.

Field Data on Traditional Populations

Explanation:

• Settlement & food production shorten IBI and raise TFR.

• Environmental richness and workload differences explain outliers (e.g., high Ache fertility even in forest).

Survival Curves: The Hiwi Example

• Age-specific survivorship: by age $4$, $25\%$ dead; by $15$, $50\%$ dead.

• High childhood mortality ⇒ prehistoric human populations grew slowly unless territories expanded.

Agricultural Revolution Effects

• 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.

Population-Age Structures (Pyramids)

• 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.

Basic Population Mathematics

• Absolute change: $N<em>{t+1}=N</em>t + B - D$.

• 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: $N<em>t = N</em>0\,(1+r)^t$ (e.g., $r=0.013$ ⇒ $1.3\%$ annual growth).

Carrying Capacity (K) & Resource Limits

• K = population size the environment can sustainably support.

• Species- & habitat- specific: desert might support $1$ human/$30\,\text{km}^2$ vs rainforest $30$ humans/$1\,\text{km}^2$.

Case Study: Irish Potato Famine

• 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 $\approx 3\,000,000$.

Case Study: Australia Birth-Rate History

• Long-term downward trend (fewer births per capita).

• Notable fluctuations:
  • $1890$s Depression (sharp drop).
  • Early $1900$s uptick.
  • $1930$s Depression (drop).
  • Post-WWII baby boom (spike).
  • $1960$s contraception era (decline).

• Population still rose due to momentum: many adults applying even a lower per-capita rate.

Demographic Transitions Summarised

1. First transition (foraging → farming):
   • Earlier maturity, shorter IBIs, population growth.

2. Second (~European 1700–1800s):
   • Mortality decline first, fertility decline follows (parity-specific stopping, longer IBIs).
   • Requires confidence in child survival.

3. Third (recent developing nations):
   • Death rate drops rapidly (public health), birth rate lags ⇒ prolonged high growth.
   • Eventually fertility also falls.

Key Take-Home Concepts

• 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.

Looking Ahead

• Next lecture: Human Menopause – evolutionary rarity and implications.

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