Sibling competition lengthens while hazardous environments shorten optimal human birth spacing

Matthew G Thomas†1, Daryl P Shanley†2,Thomas B L Kirkwood2 and Ruth Mace1

1 Human Evolutionary Ecology Group, University College London, UK2 Institute for Ageing and Health, Newcastle University, UK

Birth intervals increase with age…

… and range 3-5 years in natural fertility populations

Younger children can increase their siblings’ risk of dying

Question: How do birth intervals adapt to ecology and sibling competition?

We developed a state-dependent optimality model

Optimises reproductive decisions,given female’s state

Females have a state:Her age and the ages of her children

How do birth intervals adapt to ecology?

0 10 20 30 40 50 60 70 80 900

0.2

0.4

0.6

0.8

1Probability of death

Age (years)

High mortality environment(Tsimane foragers)

Low mortality environment(1960s Sweden)

Sibling competition increases mortality

0 2 4 6 8 10 12 14

-0.2

0

0.2

0.4

0.6

0.8

1

Low intensitycompetition

Medium intensitycompetition

High intensitycompetition

Child age (years)

Weight on siblings’ mortality

No competition

Results

High mortality environments shorten birth spacing

15 30 45 60 75 900

0.5

1

1.5

2

2.5

3

3.5

Age (years)

Birth interval(years) Low mortality environment

(1960s Sweden)

High mortality environment(Tsimane)

Sibling competition lengthens birth intervals

15 40 65 900

1

2

3

4

5

15 40 65 90

Age (years)

Birth interval(years)

Low mortality environment(Modern Sweden)

High mortality environment(Tsimane)

High competition

Medium competition

No competition

Low competition

We have shown plausible mechanisms for how reproductive schedules adapt to mortality risks

But no menopause

Our state-dependent framework can also- examine juvenile help- test ‘mother effect’

Collaborators:Daryl Shanley (joint first author)Tom Kirkwood; Ruth Mace

Thanks:Human Evolutionary Ecology Group (UCL)