Evolutionary Psychology
Lecture 4: Reproductive Strategies
“What do you mean no!”
Learning Outcomes.
 At the end of this session you should be able to:
 1. Explain the concept of sexual selection.
 2. Explain sex difference in reproductive strategies in light
of differential parental investment.
 3. Describe factors involved in animal mate choice.
Sexual Selection.
 Darwin distinguished between two types of sexual selection:
 1. Intrasexual selection: Individuals of one sex (typically
males) evolve traits that enable them to compete with other
individuals of the same sex and win mating opportunities
(large antlers, etc).
 2. Intersexual selection: Individuals of one sex evolve traits
(physical and behavioural) that are preferred by members of
the opposite sex (typically females). This is referred to as
'mate choice'.
 Sexual selection thus leads to sexual dimorphism in physical
and behavioural traits between males and females.
Sexual Dimorphism.
 Amongst vertebrates, the clearest dimorphism is between
gamete (sex cell) size. This single physical difference
explains why behavioural sex differences exist.
 Females gametes: large, nutrient-filled, expensive to
produce, limited in number, and produced infrequently. If
fertilised this will lead to high costs to the female.
 Male gametes: small, have no nutrients, cheap to produce,
constantly made throughout life.
 Reproductive Capability: females are thus classed as the
‘slow sex’ and males the ‘fast sex’.
Female Reproductive Strategy.
 Females have much to lose if they mate with the wrong
male, they are thus selective about who they mate with.
 They look for certain criteria:
 Physical Features: size and strength which confer
dominance and so preferential access to resources.
 Behavioural Features: may indicate willingness to invest or
good parenting skills.
 Females will compete with other females for the right to
choose the most desirable (alpha) males.
 They gain little from multiple matings and seek quality not
quantity.
 Almost every reproductively capable female will be able to
find a mate of some sort.
Male Reproductive Strategy
 Males are far less choosy as they as they little to lose and
everything to gain if they can have as many mating
opportunities as possible.
 Males are not tied to rearing offspring and so seek quantity.
 While they would prefer a superior female, they are less
choosy.
 If presented with a sexual opportunity they will take it.
 Males compete vigorously with other males for access to
fertile females.
 Male reproductive success is however very variable, a small
number of males will achieve many matings, while many
males may never mate.
Differences in Average
Reproductive Success.
From Low (2000), p 55.
Bateman’s Gradient.
 Bateman (1948) observed
that the number of offspring
fathered by male fruit flies
increased in proportion to
the number of females with
which the male had mated.
 Female reproductive success
did not increase as her
number of partners
increased.
 This is 'Bateman's gradient'
- the steeper the gradient
the stronger is sexual
selection.
From Anderson & Iwasa (1996) p 54.
males
females
No. of mates
Parental Investment (PI).
 Trivers (1972) argued that a driving force behind sexual
selection is the degree of parental investment each sex
devotes to their offspring.
 Parental investment (PI) is defined as:
 “any investment by the parent in an individual offspring
that increases the offspring’s chances of surviving (and
hence reproducing) at the cost of the parent’s ability to
invest in other offspring”.
 In most species, females invest heavily in their offspring
while males do not.
Predictions Arising from PI Theory.
 1. The sex that invests most heavily in an offspring will be
more choosy about who they mate with (i.e females).
 2. The sex that invests least in an offspring should compete
more vigorously for access to the higher-investing sex (i.e
males).
 3. Male competition leads to physical dimorphism, and will
be most marked in animals where there is little male
parental investment (e.g. elephant seals).
 4. Where male investment approaches that of females,
competition will be reduced as will sexual dimorphism (e.g.
swans).
 5. Where male parental investment is higher, females will
possess typically male characteristics (e.g. seahorses, the
wattled jacana).
The Female’s Weapon!
 PI theory gives the impression that females are exploited
by males, but females have a powerful means of redress.
 While a female is always 100% certain that an offspring is
hers, the male lacks paternity certainty (due to concealed
ovulation, lack of permanent mate-guarding).
 “Mother’s baby - fathers maybe”.
 To avoid ‘cuckoldry’ (unknowingly rearing another males’
offspring) the male must continually provide resources and
protection, and will generally devote an almost equal
amount of effort to child-rearing.
 Around 10% of human children are reared by a father who
is unaware that they are not in fact his biological product
(Baker, 1996).
Female Mate Choice.
 Female animals consistently select males who are more
colourful, more conspicuous, louder, more daring and
aggressive (Dugatkin & Godin, 1998).
 Across the animal kingdom, successful males are bigger,
louder, more colourful, engage in extravagant displays and
rituals, and are more aggressive.
 Traits that females prefer (e.g. brightly coloured feathers,
singing, mating dances) have been selected for and can be
very impressive (e.g. the peacocks tail).
 Such traits can even impair the males survival chances:
Choose Me!
Courtship Attraction.
 Fisher et al., (2002) pointed out the evolution of such traits
must have been accompanied by corresponding neural
mechanisms in the female to enable her to become
'attracted' to some traits and reject others.
 They called this hypothetical neural mechanism 'courtship
attraction' and hypothesised that it is associated with a
more widespread neural 'attraction' system that combines
perceptual, sexual, and motivational mechanisms.
 In humans, these neural mechanisms may form the
physiological basis of romantic love.
Problems in Choosing a Mate.
 Miller & Todd (1998) argued that choosing a mate is
perhaps the most important long-term decision-making
task facing an animal.
 Through good mate choice an individual can positively
influence the quality of their offspring. In choosing a mate
there are three problems to solve:
 1. Perception: Perceiving certain sexual cues that provide
reliable information about a potential mate.
 In humans, factors such as height, intelligence, sense of
humour, facial symmetry, waist-to-hip ratio, social status,
and wealth, have all been shown to be important (lectures
6 & 7).
2. Integration.
 Information from various cues are integrated to gain
information about health, fertility, parenting skills, and
social status.
 What do the cues mean?
 In many species the male has to carry out an intensive and
time-consuming task before the female will consent to
mate.
 Where the male does not provide an initial investment, the
female has to choose a mate on the basis of how good his
genes are.
 How do females know which males possess the best genes?
 Why don’t males cheat by faking the traits associated with
having good genes?
 There are several inter-related mechanisms:
a) The ‘Handicap Principle’.
 Zahavi (1975) argued that females only assess those traits
that are ‘honest’ indicators of male fitness and are ‘costly’
to produce and maintain (e.g. the peacocks tail feathers).
 Such traits would only be associated with the most healthy
males who could afford to produce and maintain such
features.
 E.g. Dugatkin & Godin (1998) showed that male guppies do
not normally approach a predator.
 If females were present, then males would approach the
predator.
 Such behaviour is ‘honest’ as slower males get eaten.
 They manipulated predator approaches, and evaluated
subsequent reactions by females.
 Females did indeed prefer the most adventurous males.
b) Parasitism.
 Hamilton & Zuk (1982) proposed that secondary male
sexual characteristics (plumage colour, tail feather length,
antler size etc) facilitate female appraisal of a males ability
to withstand the detrimental effect of parasitism.
 This theory is a modification of the ‘handicap principle’ as
the development and maintenance of secondary sexual
characters may be a considerable handicap.
The Role of Testosterone.
 Testosterone produces aggressive, sexual, and competitive
behaviours at puberty. Higher levels confer more impressive
features.
 However, high levels of testosterone impair the functioning
of the immune system making the individual more prone to
disease and parasitism.
 Folstad & Karter (1992): Males with a superior immune
system can produce and maintain secondary characteristics
but are not prone to disease and parasitism. Those with
inferior immune systems may develop the characteristics,
but cannot maintain them.
 Møller et al., (1998) showed that males with brighter
plumage were consistently preferred by females.These males
had low levels of immune activity and more obvious signs of
good health.
c) Fluctuating Asymmetry (FA).
 FA is defined as: “random deviation from perfect bilateral
symmetry in a morphological trait for which differences
between right and left sides have a mean of zero and are
normally distributed” (Watson & Thornhill, 1994).
 FA therefore provides a measures of the ability of the
individual to undergo identical development in bilaterally
symmetrical characters in the face of genetic and
environmental stressors.
 There are large individual differences in FA particularly in
secondary sexual characters (e.g. antlers, plumage) as
these show greater proneness to FA.
 The ability to resist parasites may be signalled by FA in
certain body ornaments.
Choose Me!
Evidence:
 Lageson & Folstad (1998): Found that male reindeer with
more symmetrical antlers had a more efficient immune
system and were better able to cope with parasitism.
 Møller (1992): Manipulated tail length and asymmetry in
swallows. Females preferred males who possessed large
symmetrical tail feathers.
 Such males mated earlier and enjoyed larger annual
reproductive success than males with shorter, more
asymmetrical tails.
However..
 Not all studies find a link between FA and sexual selection
and Swaddle (1999) has criticised the degree to which FA
acts a sexual signal.
 The degree of FA in a secondary ornament is typically only
1-2%, and it seems surprising that such tiny differences
can be detected.
 In his study, male and female starlings were trained to
discriminate between symmetric and asymmetric images,
and then presented with pairs of test stimuli that varied in
their degree of symmetry.
 When the differences ranged between 5-10% the birds had
no problem.
 However at 1.25% (approximately the same as in nature)
they were unable to tell the difference.
d) Major Histocompatibility Complex
(MHC).
 The MHC is a large chromosomal region containing genes
that play a role in immunological self/non-self recognition.
 Female rodents learn the MHC identity of their relatives
during development via pheromones, and prefer to mate
with males who carry dissimilar genes.
 Wedekind et al., (1997): humans prefer the body odour of
MHC-dissimilar individuals.
 Ober et al., (1997): human couples were less likely to share
MHC-haplotypes than chance; in couples who had similar
MHC genes they had more fertility problems (unconscious
avoidance of inbreeding?)
 Milinksi & Wedekind (2001): People seem to select
perfumes and deodorants that enhance their own body
odours to reveal their immunogenetics.
3. Decision.
 Searching through potential partners and deciding which
ones to accept or reject involves shopping around.
 In some species males gather at display grounds (leks)
where they perform mating rituals.
 Superior males receive most of the female attention and
perform most of the matings.
 Peer pressure is important as females observe and copy
other (generally older and more experienced) females.
 On crowded grouse leks for example, Hoglund et al., (1995)
surrounded non-dominant males with stuffed dummies
representing interested females, and young females began
to choose the previously uninteresting male as their mate.
Peer Pressure or Instinct?
 Dugatkin & Godin (1998) compared peer pressure with
instinctive bias in female guppies.
 Females prefer a male with more orange colour than
another male.
 They staged a copying opportunity in which a test female
observed another female apparently choosing the less
orange of a pair of males.
 When males differed by only small amounts of colour, a
female would be influenced by peer pressure, but when the
males differed by a large amount of colour then the genetic
biases held firm.
 In guppies there is thus a colour threshold, below which
social cues steer mate choice, and above which genetic
factors are more important.
Remembering and Comparing.
 If males do not gather at one convenient spot, females may
have to remember the characteristics of the males so that
she can compare them before choosing.
 Females can rank the characteristics of sequentially
presented males.
 E.g female three-spined sticklebacks are more likely to
show interest in a male if his red colouring is brighter than
a previously seen male, and more likely to reject a male if
his colouring is duller than one previously seen.
 Similarly, female green tree frogs prefer males who call the
loudest and most frequently.
 Female deer also prefer stags who have the loudest call
(Dugatkin & Godin, 1998).