Lecture 3: Sex Differences 2: Investigating Human Sex Differences

Who created Parental Investment Theory?

Robert Trivers

What is Parental Investment?

Any investment by the parent in an individual offspring that increases the offspring’s chance of survival (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring

• Higher vs. Lower investors

High-Female Investment

In mammals (including humans), female investment is generally higher than males in terms of

• Anisogamy: reproduction by the union of two different gametes (the ova and the sperm)

• pregnancy

• post-birth care

The minimum physiological obligations are higher for females vs males 

Female vs male investment

• Among carnivores and primates (including humans) parental investment is found in 30-40% of genera

• compared to most other mammals, human males invest heavily in offspring

• in SOME species, males are the higher investing sex (red-necked phalarope)

Reproductive Consequences

• on average, an individual female will produce less offspring than an individual male in a population (in their lifetime)

• Female record: 69 — Male record: 893

T/F?: The Variance in Reproductive Success between individual females in a population is predicted to be LOW

True

T/F?: The Variance in Reproductive Success between individual males in a population can be HIGH

True

Bateman’s Principle

The sex difference in Variance in Reproductive Success (males = higher variance, females = lower variance)

Summary: Higher Investors

• Higher (female) parental investment (due to minimum physiological obligations being higher)

• higher investor has a potentially lower reproductive rate/# of offspring, vs. a lower investor (male) in a species

• Higher-investors are predicted to be more selected about mated than lower investors; and less likely to need to compete to mate

• Led to the process of Intersexual Selection (female choice)

Summary: Lower Investors

• Lower (male) parental investment

• Lower investors have a potentially higher reproductive rate than higher investors

• Lower investors are predicted to seek out to mate as much as possible/to best their rivals to try to improve their reproductive success

• Lower investors are predicted to be less choosy than higher investors

• Led to process of Intrasexual competition (male-male competition)

• SOMETIMES male investment is higher than female investment (red-necked phalarope)

Parental Investment Theory (+ human parental investment)

• applies to humans (with the caveat that males can invest heavily in offspring)

• We will likely find sex differences in some traits in humans

How do you determine a sex difference in behaviour?

• there is no clear biological divide about how to quantify/identify a sex differences in behaviour

• “one that exists along a continuum in which males or females can fall at any point but the average differs between the sexes

THINGS TO KEEP IN MIND

• Behavioural differences between the sexes are relative

• behavioral differences between the sexes are not hierarchical 

• biology is not destiny. It does, however, affect probability

Male-Male competition in humans

1. Sexual Dimorphism in size

• Size Dimorphism (males larger than females):sexual selection theory predicts that larger male body size is the outcome of male-male competition

• Today: size dimorphism between male/female is LOW (15%)

2. Sex Differences in Physical Aggression

• physical aggression towards other males

• can be viewed as an expression of competition

• one of the largest and most reliable male-female behavioural differences

• same sex homicide, data for 14,000 same sex homicides nearly 97% were male 

3. Male body traits that may have been shaped by male-male competition

• Facial hair

• vocal characteristics: deep voices

• Facial shape

• more effective at intimidating other men than they are at attracting women

Social Status

Your relative access to contested resources within a social group by non-agonistic means

non-agonistic

an individual doesn’t use force or the threat of force to get what they want

Why buy a Porsche?

• dual utility/dual function. As an indirect armament (e.g. social status) in male-male competition (deterring rivals, rather than males engaging in direct competition) and/or as an ‘ornament’ potentially swaying female choice

• tldr: is a signifier of social status which functions as a way to deter rivals without having to get into a direct conflict

Tsimane people (high and low status males)

Higher status males had:

1. Higher intra-marital fertility 

• the high status males had more affairs, but they mainly had children within their marriages. Tend to invest in a single wife their entire reproductive careers

2. Lower offspring mortality

Do human high status males in a contemporary industrialized society have more children than low status males?

High status can increase SOME males’ probability of mating vs his rivals

• just because a male attaints high status doesn’t mean he gets to mate or that his reproductive success will automatically be high

• need to factor in female choice of other desirable traits 

Do high status human females in a contemporary industrialized society have more children than low status females?

No, they do not tend to have more children than low status females

• some studies have shown some forms on social status (ex. educational achievement) are inversely/negatively related to fertility

Why?: Delaying or postponing first birth

In the animal world, what is Female-Female competition over?

• More frequently over access to resources (food, nests), rather than sexual access to mates

• Among primates, food is generally the main limiting resource for females

• sometimes they will compete over a Male Friend (as a resource)

F-F competition over the attention of males is predicted to occur when:

1. When males provide access to resources (such as paternal care or territorial resources for females and their offspring

2. If males vary in the quantity or quality of the resource they provide (offspring care)

F-F competition Male Friend

• Protects female ‘friend’ and their infants from aggression

• male gets: preferential grooming, preferential proximity, female MAY mate with her friend

F-F competition (indirect vs direct competition)

Direct: Force or threats of force are used to exclude same-sex rivals from resources

Indirect: Instigator manipulates others to attack the victim, or, by other means makes use of the social structure in order to harm the target