Sexual Selection and Reproductive Behaviour

Sex vs. Reproduction Sex - process by which a genome is changed by rearrangement and combining of genes Reproduction - process by which individuals are added to a population Sex -changes in kinds of individuals in populations Reproduction -changes in the number of individuals in populations

Reproductive Behaviour 1. What is ‘sex’? 2. What are some of the costs and benefits of sex? 3. Why are the sexes generally in a 1:1 ratio?

Reproductive Behaviour Anisogamy Frequency of producers Gamete size

Reproductive Behaviour Anisogamy - lots are produced by not enough mass for cell division - fewer produced but are competent - very few produced but are very competent

Reproductive Behaviour Anisogamy + - lots of contact but won’t divide properly + - will divide but fewer contacts + - will divide but very few contacts + greatest success best combination of numbers and competence

Reproductive Behaviour Costs of sex 1. Cost of meiosis must combine genes with another organism lose 50% every generation relative to asexuals -often referred to as cost of males - females can always do better asexually

Reproductive Behaviour Costs of sex 2. Cost of recombination take a functioning genome, split it and combine it with another one -chance of deleterious combinations

Reproductive Behaviour Benefits of sex 1. Produce new combinations Unique offspring can cope with different niches x

Reproductive Behaviour Benefits of sex 2. Muller’s ratchet - in asexual species, deleterious mutations accumulate Frequency of mutant gene Mutation A Mutation B Mutation C Time

Reproductive Behaviour Benefits of sex 2. Muller’s ratchet - in sexual species, such mutations tend to be eliminated Frequency of mutant gene Mutation A Mutation B Mutation C Time

Reproductive Behaviour Sex ratio -why 1:1? Excess males -some males won’t mate -females that produce more females are favoured 1:1 sex ratio -some females won’t mate -females that produce more males are favoured Excess females Time

Darwin - Theory of sexual selection Natural selection - Observations and deductions Observation - all species can increase exponentially but don’t - more are produced than can survive Deduction - there is a struggle for existence Observation - there is variation in all organisms that is heritable Deduction - some are better equipped to survive than others - advantage is passed to offspring Differential survival = Natural selection

Darwin - Theory of sexual selection Two parts 1. Intrasexual selection (Male-male competition) All those structures and behaviour patterns employed by males to fight other males for the chance to mate with females - claws, antlers etc.

Darwin - Theory of sexual selection Two parts 2. Intersexual selection (Female choice) All those structures and behaviour patterns employed by males to attract females - plumage, song.

Reproductive Behaviour Males and females have different reproductive interests What does each sex have to provide to produce offspring? Females Males Manufacture eggs (expensive) Manufacture sperm (cheap) 2. Incubate eggs or fetus (expensive) 3. Post partum care (expensive) 4. Loss of mating opportunity when pregnant (expensive) Males invest less in any offspring

Reproductive Behaviour In more general terms Females Difference in reproductive investment Males higher investment lower investment lower reproductive rate higher potential reproductive rate lower levels of mating activity higher levels of mating activity Biased operational sex ratio Selection among potential mates Competition for mates Best mate = best fitness benefit Achieve greatest number of matings

Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio The ratio of the number of males to the number of females in the population Operational Sex Ratio The ratio of the number of males to the number of females in the population who are available for reproduction

Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females

Reproductive Behaviour Operational Sex Ratio Males Females Remove males for: Remove females for: 1. Pre-puberty 2. Sterile ( Post-menopause) 3. Age 4. Pregnant 5. Lactating Operational sex ratio

Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females Operational Sex Ratio (very different from 1:1) Competition in more abundant sex Selectivity in less abundant sex

Reproductive Behaviour Evidence?? Bateman (1948) -Drosophila melanogaster - nearly all females mated but not all males -variance in male reproductive success - higher Males Bateman gradient # of offspring Females # of mates

Reproductive Behaviour Evidence?? - Lions Females % of breeders 10 # of surviving offspring

Reproductive Behaviour Evidence?? - Lions Females Males % of breeders 10 # of surviving offspring

Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits -females choose males that give them a concrete resource e.g. Nuptial gifts

Intersexual Selection Size of prey is critical 20 min Duration of copulation # sperm transferred 8 25 16 mm Size of prey Time in copulation

Intersexual Selection 2. Good genes models Female should look for best genetic complement in male Traits should reflect genetic quality

Intersexual Selection 2. Good genes models Fluctuating Asymmetry Horns in oribi - Ourebia ourebia Arese. 1994. Anim.Beh. 48:1485

Intersexual Selection 2. Good genes models 3 2 1 Fluctuating Asymmetry Harem size Symmetric Asymmetric Damaged

Intersexual Selection 2. Good genes models Good genes and parasites Number of matings Clean Infected No choice

Intersexual Selection 3. Runaway Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers Gene B - In females - preference for trait ‘X’ A + B Expressed in males as the trait A + B Expressed in females as a preference for the trait

Intersexual Selection 3. Runaway Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers Gene B - In females - preference for trait ‘X’ Frequency in males Frequency in females

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Variation in males Female

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 1. Standard disruptive selection every generation - allow shortest and longest stalked males to mate with randomly selected females Short stalked Long stalked 13 generations Stalk length Stalk length

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 2. Female choice Female offspring prefer short stalked males Female offspring prefer long stalked males Stalk length

Intersexual Selection Other factors affecting mate choice - Learning Sexual imprinting -already discussed this 2. Mate choice copying - chances of a male being preferred as a mate at time 2 increases as a result of being preferred at time 1. Male 1 Male 2 Model female Test female

Intersexual Selection Other factors affecting mate choice - Learning Sexual imprinting -already discussed this 2. Mate choice copying Frequency Near ‘chosen’ male Near ‘non-chosen’ male Time spent by test fish

Intrasexual Selection - Male/Male Competition Competition before mating Fiddler Crab (Uca) 1. Direct Fighting Female digs burrow Male Male enters burrow and mates Female Male defends burrow and female No evidence of female choice Male leaves when female ovulates

Intrasexual Selection - Male/Male Competition Competition before mating 2. Interference -male tries to interfere with copulating pair

Intrasexual Selection - Male/Male Competition Competition before mating 2. Interference Elephant seals (Mirounga) -females incite males to fight via interference Males fight for dominance -8.3% of males mate

Intrasexual Selection - Male/Male Competition Competition before mating 2. Interference Logic - if subordinate male tries to mate - female protests - dominant interferes -female gets ‘best’ mate % of matings protested by female Alpha Adult male Subadult male Rank of male

Intrasexual Selection - Male/Male Competition Competition before mating 3. Cuckoldry Bluegill sunfish Males set up territories where females lay eggs

Intrasexual Selection - Male/Male Competition Competition before mating 3. Cuckoldry Three kinds of males 1. Parental - larger, aggressive territory holders 2. Sneaker - smaller 3. Satellite - look like females spawning Male digs nest Female lays eggs Male fertilizes eggs

Intrasexual Selection - Male/Male Competition Male digs nest Female lays eggs Male fertilizes eggs Parental male Parental - larger, aggressive territory holders Female Sneaker - smaller - rush in before parental male and fertilize eggs Satellite Satellite - look like females - spawn with pair

Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition Females of many species can Store sperm Mate with several males before fertilization Raises probability that sperm will compete for fertilizations Males can a) reduce the chances that a second male’s sperm is used (first male adaptations) b) reduce the chances that a first male’s sperm is used (second male adaptations)

Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘First male adaptations’ a. Postcopulatory mate guarding

Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘First male adaptations’ a. Postcopulatory mate guarding Stick insects

Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘First male adaptations’ Postcopulatory mate guarding Antiaphrodisiacs Mating plugs

Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘Second male adaptations’ a. Sperm removal Dunnock Damselfly Penes

Intrasexual Selection - Male/Male Competition Competition after mating 2. Bruce effect - Mice - strange male (or male odour) -females abort fetuses and become receptive

Intrasexual Selection - Male/Male Competition Competition after mating 3. Infanticide

Summary - Sexual Selection Intersexual Selection 1. Direct Benefits Intrasexual Selection Premating 2. Good Genes a. Male fighting 3. Runaway Selection b. Cuckoldry c. Interference 2. Postmating a. Sperm Competition b. Bruce Effect c. Infanticide