Lecture 4: Sexual Selection & Female Choice. Differential Investment. Differential Investment in Mammals. Triver s Hypothesis.

Transcription

1 Lecture 4: Sexual Selection & Female Choice Differential investment Classic views of male-female behavioral differences Natural Selection Sexual Selection Inter-sexual choice Intra-sexual competition Inter-sexual coercion Differential Investment Behavioral Biology of Women 2007 Differential Investment in Mammals Females Large gametes Internal gestation Lactation Lengthy parental investment Males Small gametes Paternal Investment variable Triver s Hypothesis The sex investing most becomes a limiting resource for the sex investing least ORANGUTANS: Females most investing sex YELLOW SEA HORSE Males most investing sex Classical view of malefemale Behavioral Differences Natural Selection Males Compete for females Aggressive Sexual Females Choosy Passive Non-sexual Especially true when: No investment in offspring by males No cost of reproduction for males There must be variation in a trait The trait must be heritable There must be differential reproductive success 1

2 The Enigma of the Peacock Reproductive Success (RS) Successfully leaving more copies of your genes in subsequent generations than do other individuals. The sight of the peacock s tail makes me sick Darwin - letter to a friend SOLUTION: Sexual Selection Sexual Selection Inter-Sexual Choice (Female Choice) Intra-Sexual Competition (Male-Male Competition) Sexual Selection Rejected by Victorians as ludicrous Didn t receive much attention until 1970 s with Trivers Co-discoverer of evolution through natural selection Features such as the peacock s tail due to superabundant energy of males Abraham Lincoln February 12, April 15, 1865 Charles Darwin February 12, April 19,

3 Inter-Sexual Choice (classically called Female Choice) Birds of Paradise One sex choosing to mate with a member(s) of the opposite sex Bower Birds Why be Choosy? Two types of explanation: Possession of good genes Investment in offspring Why by Choosy? Why be choosy? Possession of good genes Sexy Son Hypothesis Possession of good genes Handicap principle 3

4 Why be choosy? Possession of good genes Low pathogen load Indicated by lower Fluctuating Asymmetry Why be choosy? Possession of good genes Sperm Quality > Lower Fluctuating Asymmetry = better sperm quality Why be choosy? Ability to invest in offspring Reproductive ability Why be choosy? Ability to invest in offspring Resources/territory Ache Kipsigis Female Choice: Sperm Donor, Mate, EPC Character Kind Understanding Dependable Considerate Honest Self Confident Health Family health History Family longetivity Record Physical Abilities Hair color Handsome Eye color Height Abilities Creative Musical talent Intelligent Health Background Scheib, 1997 Hypotheses Women value those attributes they believe would affect a child (i.e. can be passed on) such as health and physical attributes OR Women value the same attributes they do in a long term mate. Scheib,

5 Mean Importance Rating Female Choice: Sperm Donor vs Mate Sperm donor Mate Character Health Physical Attributes Abilities Heritability Scheib, 1997 Mean Importance Rating Heritability Female Choice: Sperm Donor vs Mate Sperm donor Mate Extra Pair Copulation Character Health Physical Abilities Resources Attributes Scheib, 1997 When not to be choosy? When not to be choosy? Males: where there is no limit on insemination and low investment in offspring can afford not to be choosy Females: where advantageous to confuse paternity Intra-Sexual Competition = (classically called Female Choice) Competition among members of one sex for members (or a member) of the opposite sex. 5

6 Is Male-Male Competition a Constraint on Female Choice? Can female only mate with the victors? Is female choice negated by outcome of male-male competition? Sexual Dimorphism Female choice constrained by male-male competition only if: Dominant males are able to keep all other males away from females, so female has no choice. Female choice constrained by male-male competition only if: Dominant males are able to keep all other males away from females, so female has no choice. If winners can t keep other males away, then gain a reproductive advantage only if: Coerce females into mating Are freely chosen by females Why Compete? Genetic quality of mate Resources of mate Parenting ability Reproductive Ability Sperm Competition Fecundity Mates are limited Energetic/time Constraints 6

7 Direct Competition How to compete? How to compete? Reproductive Competition in Baboons Females in first 25 part of the 20 menstrual cycle 15 and pregnant 10 females received 5 0 the most -5 aggression from -10 other females. Observed-expected attacks E1 E2 E3 P1 P2 P3 L1 L2 L3 Cycling Pregnant Lactating (Wasser, 1983) Recipient of estrous female aggression Mangabeys Frequency per female-hour * * 0 Estrous Post-estrous Pregnant Nursing Reproductive State (focal subject) Why are women often the Showier Sex? Compete for high status males where: Male parental investment important Differential male status Long term pair bonds (Emery, unpublished) Women of Surma 7

8 Compete to be chosen How to compete? How to compete? Compete to be chosen Sperm Competition Sperm Competition Highest rank female gets to mate first Sperm Competition Sexual Selection Inter-Sexual Choice Intra-Sexual Competition Inter-Sexual Coercion Martin Muller Barbara Smuts 8

9 Inter-Sexual Coercion Inter-Sexual Coercion The use of force or threat of force by members of one sex to coerce mating from a member(s) of the opposite sex. Can override the results of inter-sexual choice and intra-sexual competition. Imposes a cost on the sex that is coerced. Flanged Males Unflanged Males Forced vs. Cooperative Copulations Flanged Males Forced vs. Cooperative Copulations Unflanged Males Suaq Balimbing Ketambe SUMATRA Suaq Balimbing Ketambe SUMATRA Study site Tanjung Puting Study site Tanjung Puting Kutai BORNEO Kutai BORNEO Gunung Palung Gunung Palung % of total copulations % Forced % Cooperative % of total copulations % Forced % Cooperative 9

10 Chimpanzee Sexual Coercion Chimpanzee Sexual Coercion & Cortisol Males attack parous females more often during swelling stages than during nonswelling stages Hits/Hour Nulliparous non-swelling Parous Swelling Cortisol (an indicator of metabolic or psychological stress) levels are associated with rate of aggression received by females from males. Pmol Cortisol/mg Creatinine Nulliparous Non-Swelling Swelling Parous (Kahlenberg & Wrangham, unpublished data) (Muller, unpublished data) Female Strategies to Combat Sexual Coercion Escape from males Aggressive towards males Coalitions amongst females Association with a particular male Mate Choice Criteria Healthy Kind and understanding Exciting personality Wants children College graduate Creative Easygoing Intelligent Physically attractive Good earning capacity Sex Differences in Mate Choice Sex Differences in Mate Choice FEMALE PREFERENCE Kind and understanding Exciting personality Intelligent MALE PREFERENCE Kind and understanding Exciting personality Intelligent FEMALE PREFERENCE Kind and understanding Exciting personality Intelligent Easygoing Creative Healthy Physically attractive College graduate Good earning capacity Wants children MALE PREFERENCE Kind and understanding Exciting personality Intelligent Physically attractive Healthy Easygoing Creative Wants children College graduate Good earning capacity 10

11 Mate Preference Cross-Culturally PREDICTION Females value mates with a high earning potential more than do males. Females value mates with high ambition and industriousness more than do males. Both males and females prefer that the husband be older than the wife. Males value mates who are physically attractive more than do females. Males value mates with no prior sexual experience more than do females. Significant Difference? Yes No The Menstrual Cycle Changes in Cognition with Cycle Phase Next Time... 11