Lecture 3

Sex Differences II: Investigating Human Sex Differences

Parental Investment & Sexual Selection

the different mating/reproductive strategies of the sexes are based on the relative amount of investment a parent makes in an offspring

Which idea is describe in the following: Any investment by the parent in an individual offspring that increases the offspring’s chance of survival at the cost of the parent’s ability to invest in other offspring.

parental investment

3 components of parental investment theory?

1. there is an investment made by the parent in the offspring

2. the parent has a limited budget

3. parental investment is the relative parental investment of the sexes in their young

In mammals, _____ investment is generally higher.

female

In mammals, female investment is generally higher than males in which 3 ways?

1. anisogamy

2. pregnancy

3. post-birth care

Anisogamy

reproduction by the union of 2 different gametes; ova & sperm

Females higher minimum _____ _____ than males.

physiological obligations

In _____ & _____, paternal investment is found in 30-40& of genera.

1. carnivores

2. primates

In general, females are the _____-investing sex & males are the _____-investing sex

1. females

2. males

Reproductive Consequences for Females

on average, over the course of her lifetime, an individual female will produce less offspring than an individual male in a population

In mammals, _____ _____ creates an important sex difference that biases individual males in terms of rate of reproduction.

internal gestation

Describe the variance in reproductive success between individual females.

predicted to be low; close to the mean for most females

The limiting factor for females in reproduction is _____; the limiting factor for males is _____.

1. resources

2. females

Describe the variance in reproductive success between individual males.

can be very high; more deviation from the mean

Bateman’s Principle

describes the sex difference in variance in reproductive success; higher variance among males

Does Bateman’s principle occur in humans?

yes

Together, _____ _____ _____ and _____ _____ help us to understand why we would see different reproductive strategies between males & females in a species.

1. parental investment theory

2. Bateman’s principle

4 qualities of higher investors?

1. start with a higher parental investment

2. potentially lower reproductive rate/number of offspring

3. more selective about mates

4. less likely to compete for a mate

4 qualities of lower investors?

1. initially low parental investment

2. potentially higher reproductive rate

3. not selective with mates

4. intrasexual competition

Are females always the choosier, higher investors?

no, some species have males that are higher investors & are therefore choosier

Which sex typically invests more in each offspring?

females

Over the course of a lifetime, _____ are predicted to have a lower number of offspring compared to _____.

1. females

2. males

_____ would be predicted to have a greater variance/skew in their reproductive success when compared to _____.

1. males

2. females

Does parental investment theory apply to humans? Are there any issues with this?

1. yes

2. yes; males can invest heavily in offspring

David Puts presents the idea that sexual selection is occurring in _____.

humans

Puts states that sexual selection theory helps us understand what kinds of _____, _____, & _____ sex differences we might find in humans.

1. psychological

2. behavioural

3. morphological

Sex Difference

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

Behavioural differences between the sexes are _____ (not absolute) and are not _____.

1. relative

2. hierarchical

Is there evidence for male-male competition among our human ancestors? Does it remain true today?

1. yes, looking at the Hominin Fossil Record, there is size dimorphism

2. yes, though it has decreased

Competition

rivalry between 2 or more individuals for a resource that is perceived to be insufficient in quality, or limited in quantity

Does physical aggression contribute to evidence of male-male competition?

yes, among humans, same-sex physical aggression is a large and reliable male-female behavioural difference; this is an expression of male-male competition

How do same-sex homicide rates contribute to ideas of male-male competition in humans?

97% of perpetrators are males; not all cases will be competitive conflict, but they may be representative of this idea

When looking at sex differences in violent crime, it is important to examine this _____.

relatively

Regardless of cultural differences, we expect sex differences in violent crime to _____.

remain

What are the 3 male body traits that have been shaped by male-male competition? How are they evidence of this?

1. facial hair (beards)

2. deep voices

3. robust facial shape

4. they appear to be more effective at intimidating other men than they are at attracting women

How is male-male competition costly to males? What does this high cost suggest?

1. homicide and violent aggression rates lead to male harm

2. that male-male aggression would not have arisen via natural selection

Reduced _____ _____ in humans suggests that other factors may be more important than _____ aggression for winning conflicts/besting rivals.

1. size dimorphism

2. physical

What aspect of sexual selection hasn’t changed in humans?

Bateman’s principle; men still have greater variance in RS

Rather than physical aggression, male-male competition is now better reflected through _____ _____.

social status

Social Status

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

Non-Agonistic

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

High Status

entails greater access to desirable things; access that is typically not actively resisted by subordinates (deters rivals)

Early research focused on the idea that men may use luxury cars to impress women (_____ _____). Newer studies suggest that this conspicuous consumption is a _____ _____ among males as a way to deter same-sex competitors.

1. female choice

2. costly signal

What is a luxury car a vehicle for in terms of sexual selection?

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

In a nutshell, a luxury car can represent an indirect _____ in male-male competition or as an _____ in female choice.

1. weapon

2. ornament

Describe 2 aspects of reproductive success of higher status males in Tsimane people in Bolivia.

1. higher intra-marital fertility

2. lower offspring mortality

Although Tsimane males had more affairs, their reproductive success gains are mainly from their _____.

marriages

2 possible proximate pathways for increased reproductive success among higher status Tsimane males?

1. having more money to pay for food, etc.

2. more allies to help acquire more goods

2 proxies for status used for human males in contemporary industrialized society?

1. high positions in the hierarchy of organizations

2. high income

Describe the results of Hopcroft’s study on social status & reproductive success?

positive relationship between income & biological children

What proxy for social status did Hopcroft use?

income

Does Hopcroft’s study give us proximate pathways for male reproductive success & social status?

no

Caveats about male social status & reproductive success?

1. social status is not permanent

2. female choice must be considered

3. high status has some costs (high cortisol levels)

High status can increase males’ _____ of mating when compared to his rivals.

probability

Do we see proxies for social status in females as improving reproductive success? Why?

1. no; some proxies show a negative relationship with fertility

2. delaying first birth pushes back subsequent births

In terms of social status & female RS, there appears to be a _____ trade-off on top of the already existing _____ trade-off. Essentially, we add another _____ _____.

1. social

2. biological

3. limiting factor

3 benefits for females achieving high social status?

1. proximate level benefits

2. investment in existing offspring

3. widening mate choice options

Why do females typically compete with other females in the animal world?

resources; in species with female dominance hierarchies, higher dominance rank increases access to resources

Does female-female competition in the animal world ever occur over males? Why?

1. yes

2. compete for male “friends” which protect females & their infants from aggression

3 reasons males will be “friends” with females in the animal world?

1. preferential grooming

2. preferential proximity

3. female may mate with her friend

2 situations where female-female competition is predicted to occur? How does this relate to the red neck phalarope?

1. when males provide access to resources for females and/or their offspring (paternal care/territorial resources)

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

3. females compete over sexual access to males because males invest heavily in offspring

Direct Competition

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

Indirect Competition

instigator manipulate others to attack the victim or makes use of the social structure in order to harm the target