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Sexual Selection Sexual Dimorphism Reproductive Systems Sexual selection Selection that "depends on the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction" – Charles Darwin sexual selection arises through variation in mating success Fitness – determined by the number of genes in future generations Factors involved in fitness: • number of matings • survival of offspring • fertility of offspring • the environment Number of matings is only one part of fitness The survival and fertility of offspring is a major factor driving sexual selection • Fundamental Question: – What is male and female ? • Females produce fewer, larger, more energetically costly gametes. • Males produce more, smaller, less energetically costly gametes. Sexual Dimorphism • The occurrence of two distinct forms among sexes within a species Gamete Dimorphism is Huge • Male reproduction success is limited by mate access. Mean # offspring produced • Female reproduction success is thought to be limited by resource access. 140 120 100 80 Males Females 60 40 20 0 0 1 2 Number of mates 3 Sexual dimorphism - Birds Sexual dimorphism - Reptiles Sexual dimorphism - Fish Sexual dimorphism – Weapons? Sexual dimorphism Weapons female male male Sexual dimorphism Ornaments Sexual dimorphism Sexual size dimorphism Extreme Dimorphism The peacock’s tail Osedax (bone-eating worms) – a polychaet worm that lives on whale falls (whale carcass at the bottom of the ocean) O. frankpressi on whale ribs Scavengers feeding on whale fall a typical female Osedax roseus Female Males Males remain in a larval-like stage Live inside the females Essentially a sac of sperm, used to fertilize the female’s eggs Green spoonworm, Bonellia viridus Lives on the sea-floor (10-100 m) Lives in burrows/crevasses in rock and gravel Females are 15-cm long body with a 1.5-m long extendable feeding tube Male spoonworms • 1-3 mm (mostly reproductive organs) • live inside the genital sac of females Sexual or Asexual Reproduction • Asexual reproduction – – – – Rapid If fitness is high, matches organism to environment If fitness is low, possible extinction Stress can result in use of sexual cycle to give new gene combinations • Sexual Reproduction – Greater energy commitment • Specific organelles • Mating rituals • Feeding offspring – Cost not equal between sexes – Can produce new gene combinations able to cope with a changing environment – Greater loss due to selection against divergent genotypes in a static environment Mate Choice and Sexual Selection in Guppies • Given a choice, female guppies will mate with brightly colored males. – Brightly colored males attract predators. – Color characteristics that have been shown to confer mating advantage: • • • • Brightness Number of spots Total pigmented area Frogs Mate Choice Among Scorpionflies • Adult scorpionflies (genus Panorpa) feed on dead arthropods in the forest understory. – Dead arthropod supplies are often limited. • Thornhill found that male Panorpa will take possession of, and guard, dead arthropods as nuptial offering to female. – Larger males are more successful. – Small males offer spit. Mating systems Monogamy - One male mates exclusively with one female Polygamy - Individuals mate with more than one partner Polygyny - Some males mate with more than one females Polyandry - Some females mate with more than one males Promiscuity - Males mate with more than 1 female and vice versa Monogamy Polygyny Promiscuity Mating Strategies • Different mating stategies have different advantages and disadvantages – Monogamy • Seasonal or permanent – – – – Allows sharing of cost of raising offspring Increases survival chances of offspring Many bird species and some mammalian species Cheating does occur and has specific advantages to fitness – Polygamy • More than one mate of one sex for a single individual of the other sex – Free individual to compete for resources and protect territory – Better food etc for mates – Some protection of offspring from competition – Promiscuity • Greatest number of offspring • Large amount of competition • Female only responsible for offspring in terms of resources – Poorer survival change for offspring Mating Success Monogamy Promiscuity All males: 1 mating All females: 1 mating Many males: 0 matings Some males: +++ matings All females: 1 mating Mating Success Polygyny Some males: 0 matings Some males: 1+ matings All females: 1 mating Polyandry Most males: 1 mating All females: 1+ matings Monogamy • paring of a single male and single female • can be for a short time or for life • very rare among mammals (<10% of species) • usually associated with male parental care Monogamy • 92% of the 9,700 species of birds are monogamous • often, the male is needed to help incubate the eggs, provide food for the young, and protect the nest from predators Emperor penguins, Aptenodytes forsteri Why might it be adaptive for a male to mate with only one female? 1. Mate assistance hypothesis 2. Mate guarding hypothesis 3. Female-enforced monogamy (monogamy is not adaptive for males) Mate assistance hypothesis: Monogamy is adaptive because male parental care greatly increases the offspring’s survival Effect of male parental care on survival of the California mouse Male short-head seahorse, Hippocampus breviceps, giving birth. Males take care of the young, ensuring their fitness Mate guarding hypothesis: Monogamy is adaptive because it ensures that the female will not mate with other males; the male remains with the female while she’s receptive Male clown shrimp, Hymenocera picta, remains with a female for a long time because other females are very scarce Male bank swallow, Riparia riparia, must stay with the female to prevent her from copulating with other males Female enforced monogamy hypothesis: Monogamy is not adaptive for males, but the female forces the male to stick around and help raise the offspring • A male and female burying beetle will bury a carcass on which the female will later lay her eggs • Once the carcass is buried, the male will try to attract another female partner by releasing pheromones • The female will push and bite the male to get him to stop releasing the pheromone Polygyny • paring of a single male and more than one female • very common among mammals • two types: • territorial defense • dominant/fittest male Territorial defense polygyny Territory An area that is defended for exclusive use of the defender against rivals What is being defended? • Resources (e.g., food, dens, hiding places) • Females “choose” the male based on the resources in his territory Territorial defense polygyny Male African cichlid fish, Lamprologus callipterus, collect empty snail shells that are used by females to deposit their eggs. Territorial defense polygyny Advantage for males with good territories multiple matings many healthy offspring Advantage for males with poor territories – none Advantage for females – abundant resources, protection from predators, fittest males yellow-headed blackbird, Xanthocepalus xanthocephalus Dominant/fittest male polygyny The fittest (i.e., strongest) male mates with many females, while keeping other males away • harems • leks • scramble competition plains zebra male with a harem of females Dominant/fittest male polygyny Advantage for dominant males multiple matings many healthy offspring Advantage for other males - none Advantage for females – the most fit male will father offspring Harems Elephant seals - breeding females cluster together on beaches; males defend the harem from other males Bull male elephant seals engage in violent, bloody fights over females – large size confers an advantage in combat • male reproductive success is highly variable • 8 individual males mated with 348 females in one study Leks Lekking - males aggregate in particular areas called leks; display for females Involves male competition • males may fight for position in center of the lek Involves female choice • females choose a mate…often the dominant male Lekking in Black Grouse and Fallow Deer Scramble Competition Males can’t defend territories (e.g., females and resources widely dispersed) Males still try to mate with as many females as possible The fittest males are the fastest, most persistent, and most aggressive Horseshoe crabs mating Polyandry • paring of a single female and more than one male • very rare Polyandry Jacanas • Males maintain small territories • Males perform all parental care • Females mate with multiple males and then leave eggs with males Sexual dimorphism • Females larger than males by 60% in mass (unusual for birds) • Females aggressively fight other females and also kill their chicks (infanticide) Polyandry When is polyandry favored? • When males become a limiting resource for reproduction (females fight over the few males) • When offspring survival requires male parental care Saddleback Tamarins, Spotted Sandpipers, and Red-necked Phalaropes are examples of polyandrous species Advantage for female multiple matings to ensure fertilization of eggs many healthy offspring Advantage for males only way to ensure fitness; male parental care needed for offspring to survive Yellow-toothed Cavy, Galea musteloides • Females who mate with more than one male will have more surviving offspring 3.0 # offspring 2.5 2.0 1.5 1.0 0.5 0.0 1 male 4 males Number of mates Inbreeding • If a population is small then inbreeding occurs – Self-fertilization • Reassortment of genes but no new genes – A population in terms of genetics always has some degree of inbreeding • Proportional to size of population • Small population – Large inbreeding index • Inbreeding depression – Rare recessive alleles become homozygous and are deleterious – Only when outbreeding between populations occurs is this avoided • More new gene combinations • Outbreeding depression – Maladaption to environment such as by colour Hermaphrodites – Exhibit both male and female function. • Simultaneous • Sequential – Charnov et.al. identified (3) conditions favoring hermaphroditic populations: • Low mobility limiting male:male competition. • Low overlap in resource demands by male and female structures. • Sharing of costs for male and female function. Hermaphroditic Reef Fish Simultaneous Protogyny =♀→♂ Protogyny =♀→♂ Protogyny =♀→♂ = ♂→♀ Protandry END Food supply is important to reproductive success • Birds in temperate regions have a larger clutch size than tropical birds • Animals in temperate regions have a larger litter size than those in tropics – Food supply with longer day length? – Adaption to large climate variation to allow rapid regrowth after a poor season – Greater mortality in winter results in more food next spring Nonrandom Mating Among Wild Radish • Wild radish flowers have both male (stamens) and female (pistils) parts, but cannot self-pollinate (self-incompatible) • Marshall found non-random mating in wild radish populations. – Interference Competition - Aggressive or inhibitory interaction between individuals. – Some characteristic of the pollen from certain individuals increases pollination success seed plants. Experiment: Comparing the effect of a “tethered” female (i.e., one that can’t move) on the duration of pheromones released by males to attract other females Is sexual selection different from natural selection? • Darwin saw them as distinct - only sexual selection could produce traits that compromise survival • The basic principles are identical – selection favors whatever gets more genes into the next generation • In sexual selection, fitness is measured relative to members of the same sex