Evolutionary Psychology Lecture 4: Reproductive Strategies

1. Evolutionary PsychologyLecture 4:Reproductive Strategies “What do you mean no!”

2. 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.

3. 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.

4. 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’.

5. 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.

6. 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.

7. Differences in Average Reproductive Success. From Low (2000), p 55.

8. 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. males No. of offspring females No. of mates From Anderson & Iwasa (1996) p 54.

9. 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.

10. 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).

11. 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).

12. 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:

13. Choose Me!

14. 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.

15. 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).

16. 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:

17. 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.

18. 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.

19. 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.

20. 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.

21. Choose Me!

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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).