Lecture 12- Human life histories 4: Adaptive sex ratio adjustment in humans

Question 1

What is the outline of the lecture?

Answer 1

-

Question 2

What do you have to think of when thinking about sex allocation?

Answer 2

• Two levels of analysis
• Population: expect 50:50
• Individual: possible advantages to sex bias
• producing rarer sex is beneficial for the mother, it increases the sex ratio but eventually this leads to approaching 50:50 either way
• at population level expect 50:50 but expect variation at individual level

Question 3

What are the population sex ratios at birth?

Answer 3

• ## the sex ratio will so adjust itself, under the influence of natural selection, such that the total parental expenditure incurred in respect of children of each sex, shall be equal (Fisher 1930)

Question 4

What is the sex ratio like in India?

Answer 4

-sons more valuable than daughters in society
-becomes a problem
-A declining sex ratio in some states are changing the rules of engagement (so daughters suddenly much more valuable)!
-in some state now have a shortage of females, less than 841 females for 1000 males
-

Question 5

What were the sex-ratios in pre-industrial humans?

Question 6

What was the case in Croatia post-war?

Answer 6

-Wars claim more men than women, biasing OSR towards females
-Croatia: 1991-1995 war
-

Question 7

What is the case with latitude and sex ratio bias?

Answer 7

-Humans at tropical latitudes typically produce more females
-Strong pattern despite continental variation in lifestyle, wealth
-

Question 8

Why are birth sex ratios male-biased?

Answer 8

-In most global populations, more sons than daughters are born (51.2 - 51.4%) – 106 males for 100 females
-males survive less well than females from birth to age 5
-

Question 9

What is the human sex ratio from conception to birth?

Answer 9

-Analysed data from: 3 day to 6 day old embryos, induced abortions, chorionic villus sampling, amniocentesis and foetal deaths and live births
-SR at conception is 0.5 so exactly 50:50 (parity)
-

Question 10

Are sons or daughters more costly to produce?

Answer 10

-Sons are physiologically more demanding to produce than daughters (faster intrauterine growth, heavier birth weight, longer IBI)

Question 11

What happens to females with a male co-twins?

Answer 11

-Human females with a male co-twin have reduced fitness:
- reduced probability of marriage
-

Question 12

What is the reproductive rate of success of sons?

Answer 12

-their reproductive rate may be higher
- Males have higher reproductive potential, but also greater variance in reproductive success
-

Question 13

What is the Trivers-Willard hypotehsis?

Answer 13

If,
a)a mother’s investment influences the physical conditionof her young into adulthood and
b) reproductive success is influenced by condition
Then,
c) mothers are predicted to invest in offspring according to their own condition
-Mothers in good condition are usually expected to favour sons, because body size in males is more important for competitive ability.

Question 14

What happened in Ethiopia? (the effect of nutrition on sex ratios)

Answer 14

-Oromo agro-pastoralist community in southern Ethiopia

-

Question 15

What is the case of social ranking and sex ration in Rwanda?

Answer 15

-Rwandan society allows polygynous marriage

-

Question 16

What is the biased maternal investment?

Answer 16

Three possible stages:

a) Conception b) Before birth c) After birth

Question 17

What are the sex ratios in gypsies vs hungarians?

Question 18

How can mothers influence these sex ratios in the gypsy and hungarian example?

Question 19

What should the benefits from investment be like for gypsies and Hungarians?

Answer 19

-

Question 20

What are the mechanisms of sex ratio adjustment?

Answer 20

• Primary sex ratio could be adjusted by two means:
  1. Differential production of X- and Y-bearing sperm
  2. Differential fertilisation success of X- and Y-bearing sperm

Question 21

How does Differential production of X- and Y-bearing sperm work?

Answer 21

1. Differential production of X- and Y-bearing sperm

Question 22

How does 2. Differential fertilisation success of X versus Y sperm work?

Answer 22

-

Question 23

Increased copulation frequency should lead to insemination earlier in the cycle and therefore higher sex ratio?

Question 24

Summary?

Answer 24

1. Population-levels biases in offspring sex ratios are uncommon, because of the evolutionary benefits to production of the rarer sex
2. Adaptive benefits to biased production at an individual level will depend on relative costs of production and fitness returns (eg Trivers-Willard)
3. Mechanisms of sex ratio adjustment remain poorly understood