Ecology Lecture 11 Life History Patterns 2
Overview • A mating system includes • how members of a particular species (or population) choose and bond with mates • how many mates per individual • how parental care (if it occurs) takes place. • Types of mating systems • Monogamy: One male mates with one female • Polygyny: One male mates with several females • Polyandry: One female mates with several males • “Social” vs. “genetic” monogamy
Key principles • The system that evolves depends upon the individual interests of each gender • Male and female interests are often in conflict. Why? • Differences in gamete investment and/or total parental investment • Male “default” = ____________. Why? • Is there a female default? • Interests/behavior of one gender serve to constrain options available to the other gender.
Polygyny • Resource defense polygyny • Example: African cichlid fish, Lamprologus callipterus • Defended resource = shells in which females lay eggs
Polygyny • Female defense polygyny • Example: Elephant seals (females aggregate) Photo: www.driftersister.com
Polygyny • Female defense polygyny • Example: Elephant seals (males compete for beachmaster status) Photo: www.wetasschronicles.com
Male-male competition and sexual dimorphism (seals)NOTE: Each point represents a species
Polygyny • Lek polygyny • Males clump, but not due to another resource • Males become the clumped resource! • Example 1: satin bowerbirds
Satin Bowerbirds: multiple signals of health and fitness (and good genes?)
Polygyny • Lek polygyny • Example 2: bullfrogs • Females choose males with longest, loudest and deepest calls • But don’t forget the sneaky f--kers www.tc.umn.edu
Polygyny: benefits/costs • Male • number of offspring likely to correlate with number of mates (+) • Female: • gets a high-quality male (+) • gets less of the male’s time and attention for • raising young • being defended against predators
Monogamy • Common or rare? • In which group of animals is it most common? www.magicmud.com
Monogamy: alternate hypotheses • Mate assistance: it takes two parents to raise the offspring • Example: Adelie penguins • Both parents needed for chick survival
Monogamy: alternate hypotheses • Mate guarding: guarding assures paternity; not guarding jeopardizes it • Especially critical if females are rare or receptive for a limited time • Example: many crab species (see sexual selection lecture)
Monogamy: alternate hypotheses • Female-enforced monogamy • Similar to mate-guarding, but done by female. • Example: Burying beetles • A female would lose resources, and possibly her offspring if she allows her male to mate again. www.royalbertmuseum.ca
Monogamy: alternate hypotheses • Danger “theory” • Leaving increases chance of dying if predation rates are high. • Example: The mantis shrimp Lysiosquilla sulcata Lysiosquilla sp. Opencage.info
Monogamy: alternate hypotheses • Pop ‘em out “theory” • Highly fertile mate • Not worth time/energy to seek another. • Example: Djungarian hamsters bbs.petsky.com.cn
Social Monogamy and extra-pair copulations • Extra-pair copulations can increase fitness of participants • Males: More mates more offspring possible. • Females: • Historical (not current) ideas: no advantage for females • Observational/experimental evidence: clear fitness benefits documented for some species • Example: Yellow-toothed cavy
Yellow-toothed cavy: Offspring survival as a function of multiple mates for females
Social Monogamy and extra-pair copulations • Direct fitness benefits: genetically based • Good genes • What does this mean? • Genetic compatibility • What does this mean? • Genetic variability among offspring • Why important?
Social Monogamy and extra-pair copulations • Other benefits that may improve fitness for females • More resources hypothesis • Example: Orange-rumped honeyguides swap food for sex. • Better protection/care hypothesis • Example: Dunnocks (European song bird) • Mate with two males both care for young • Infanticide reduction hypothesis • Example: chimpanzees (who’s dad?)
Polyandry (w/o polygyny) • Spotted sandpipers: near-complete sex-role reversal • Females arrive on breeding grounds; compete with other females for territories. • Initial male arrives, mates, cares for her first clutch. • Second male arrives later, mates, and cares for her second clutch.
What circumstances promote polyandry? • Female: only lays 4 eggs at once • Add eggs (experimentally) decrease the total young successfully raised • Related to incubation effort and protection • Female can reproductive success by laying a second brood • Needs second mate • Reproductive success limited by mates rather than gametes in this case
What circumstances promote polyandry? • Why would males “comply?” • Operational sex ratio biased toward males (related to absolute ratio for this species) • She abandons • He stays offspring survive • He leaves offspring die • Male 1: Certain of paternity for clutch 1; possibility of paternity for clutch 2 • How is this possible? • Male 2: Later arrivals less dominant, but still have a chance of paternity if they stay.
What circumstances promote polyandry? • Food fluctuation hypothesis • In food-poor years, females put all energy into eggs and have no energy left for care of eggs/young. • Mate assistance (by male) essential monogamy • In food-rich years (i.e. many mayflies), the female “recovers” her body mass and can lay another batch • Monogamy Polyandry
What circumstances promote polyandry? • Heavy predation pressure on nests • Multiple nests assure that at least some young will survive. • Male is needed to prevent predation • Young will all be lost if he doesn’t stay.
Patterns of reproductive effortVariations • Numbers of young produced at a time • More young = less parental investment/individual high mortality among young • Care of eggs/larvae • Variability in parental investment • Type of young produced • Precocial vs. altricial offspring (What is the difference?)
Patterns of reproductive effortVariations (cont.) • Number of reproductive events in a lifetime • Semelparous: one big reproductive event in lifetime/many offspring • Many are relatively short-lived (squid, annual plants) • But some are long-lived (periodical cicadas) • Itoparous: many reproductive events in lifetime/ fewer offspring per event. • Common especially birds and mammals • Timing is an issue: • begin early materials/energy into reproduction, • Begin later materials/energy into survival and growth