Mate choice and Life History

Mate choice and Life History Ch. 7.3-7.6, Bush

Outline • Mating systems and Mate choice • Territoriality • Sociality and altruism • Life History and reproduction

Mating systems and mate choice • Asymmetries in the game of sex begin with gametes • Anisogamy • “not same-size gametes” • The sex with the big gamete is female - by definition

Investment in offspring • The sex with low investment per offspring • selection for mating effort • less choosy about mating • Females begin with bigger investment per gamete. • Often (esp. in mammals) females continue with greater parental investment per offspring. • selection for parental effort • choosy about mating

Mating systems • Polygyny • Males mate with several females • If sex ratio is 50:50, some males never get to mate • Common among mammals, 8% of bird species • Polyandry • Where a single female mates with a number of males • Common among insects, some species of snakes, 2% of bird species • Monogamy • Males and females mate only with one individual • Most common mating system among birds (90%)

Mating systems and mate choice • In polyandrous systems, general promiscuity reigns and very little mate choice occurs • In polygynous systems, females are choosy with which males they mate • In very few systems where male parental care occurs, males may be choosy

Polygyny and Sexual selection • Sexual selection • “…depends on the success of certain individuals over others of the same sex, in relation to propagation of the species…” - Charles Darwin, 1871

Mechanisms of sexual selection • Intrasexual selection • Male-male competition • Intersexual selection • Female mate choice

Inciting male competition • Squirrel mating chase • Female leads group of males on marathon chase • the winner among pack of males gets to mate • Benefits of mate choice are generally clear • Females mate with male that have superior genes which get passed onto offspring

Female mate choice and Male ornaments

Carotenoid pigmentation and mate choice • Carotenoid pigmentation seen in many birds and fish come from diet • Carotenoids increase resistance to parasites– indicates that coloration may provide an “honest signal” of mate quality • Frugivorous birds are more often sexually dimorphic than granivorous birds

Polyandry and mate choice in insects • Females have not evolved ornaments but are larger • Some evidence that males choose bigger females • No parental care

Sperm storage in female insects and mate choice Many female insects have the ability to store sperm from many males, only choosing the best to fertilize her eggs when the reproductive season is over

Male-male competition in Drosophila • Drosophila flies have sperm cells that are up to 6 centimetres long • Their testes take up 11% of their body mass Male Drosophila bifurca

Deserting and mating systems • In cases of external fertilization (like in the stickleback), the female deposits eggs first and can then flee the scene • male is stuck with the responsibility of parental care

Ornaments and parental care • Pipefish – • male parental care • polyandrous • females are the more ornamented sex • Seahorse: • monogamous • Both males and females look similar

Territoriality • Types of territories • Territory, sexual dimorphism and mating systems • Human mating systems

Territoriality • Territory: • An area that an individual defends and from which other members of the same species are excluded • Home range: an undefended area used by an individual

Types of territories • All-purpose – • are used for all the activities of the individual (mating, foraging, rearing young, etc.) • Breeding – • are used for mating and rearing young, and foraging occurs elsewhere • Lek: a place where males display in groups and females choose a mate • Foraging – • Used for foraging but breeding occurs elsewhere

Territoriality is not always fixed • Iwi bird of Hawaii is territorial only when food supply is low

Territoriality and male size • Keeping a territory takes energy • Often territorial animals are ones where the males are rather large

Size dimorphism and polygyny

Sexual dimorphism • Pinnipeds (e.g., sea lions, walruses) exhibit high levels of sexual dimorphism • Male pinnipeds keep very large harems of females • A few males get lots of mates whereas most males get none

Patterns in Sexual dimorphism and mating system • In species without polygyny, it is often the females that are larger • E.g., the butterfly species, Eupterote harmani

Territories, fitness, and polygyny

Human mating system • Average N. Amer. Female height is 162 cm, average male height 175 cm • Does this mild sexual dimorphism translate into mild polygyny?

Size dimorphism and polygyny

Territoriality in humans • Because most humans do not “live off the land”, we don’t have typically territories • Analogous to territories, however, is wealth

Human polygyny • Wealthiest 5% of males in the U.S. have more extramarital offspring than do other men • Sex is what is called a zero-sum game, caused by the fact that every child has one father and one mother • if some males are having more offspring, then other males are having fewer

Extramarital matings by females • Based on A, B, O blood types, an estimated 10% of children born in North American hospitals could not possibly be the genetic offspring of the putative fathers • Cuckolded males waste valuable resources and get no evolutionary fitness • Females may seek extramarital copulations as a way to gain “good genes” for their offspring

Is monogamy a myth? • Socially monogamous birds are often not sexually monogamous • The Dunnock has an extremely varied mating system with polygyny, polyandry, and monogamy • In polyandrous trios, the dominant male tries to prevent the subordinate male from mating with the female, while the female tries to copulate with him so that he contributes parental care to offspring

Who wins the war between the sexes? • From a fitness point of view, nobody • Because every product of a sexual union has one mother and one father, each sex has the same fitness • If ever one sex is at a serious disadvantage, their offspring suffer and selection will act upon the system to increase the other sexes investment in offspring

Social Mating Systems • Mating systems are ultimately determined by the fitness realized by individual males and females under different behavioural schemes • Some mating systems are puzzling in that individuals appear to sacrifice their own fitness for the good of others (altruism)

Types of altruism • There are two main types of altruistic behaviour schemes: • Eusociality • Cooperative breeding

Eusociality • Eusociality occurs mostly in 3 orders: Hymenoptera (all ants, some bees, wasps), Isoptera (termites) and Homoptera (aphids) • Eusocial insects are characterized by 3 traits: (1) cooperative care of young by more individuals than just the mother (2) sterile castes (3) overlap of generations so that older sterile offspring aid their mother in raising younger siblings.

Eusociality in mammals • The naked mole rate represents the only known case of eusociality in mammals • One queen mates with 1-3 males in the colony • Non-breeding workers number between 70-295

Co-operative breeding in higher vertebrates • additional adults play a role in raising young Female lionesses often suckle one another’s young • Exists among rodents, mammalian carnivores, & more than 300 species of birds • E.g., female lionesses often suckle one another’s young • Occurs mostly in species where a lot of parental care is required to rear young

Altruistic behaviour • Ground squirrels give warning calls when a predator comes near • Protects others but increases risk to the caller

Explaining altruism • Kin Selection: • a process that favors evolution of traits that enhance the reproductive success of related individuals (genetically ‘profitable’ altruism)

Inclusive fitness • a measure of an individual’s total genetic contribution to subsequent generations • directly through production of viable offspring • indirectly through effects on the ability of relatives to produce viable offspring

Evidence for kin selection • Ground squirrels are much more likely to give warning calls when they are in the presence of kin members than when they are not

Reciprocal altruism • exchange of altruistic acts between two or more individuals • acts can be separated considerably in time • only found in social mammals and birds • E.g, vampire bats in Costa Rica

Explaining altruistic behaviour between non-kin • Reciprocal altruism is a strategy than wins over all other strategies • Analogy is the “Prisoner’s Dilemma”: • Separate two criminals and interrogate each alone • If either one incriminates the other, one is imprisoned. If they both incriminate the other then both are imprisoned • If neither turns the other one in, both go free • When both prisoners do not rat their buddy out, the pair has a higher “fitness” overall even if individual’s that “cheat” might win in the short term

What is meant by “Life History”? • life history ('strategies') • history of the life of an individual • species-specific pattern of development, reproduction, and mortality • life-history characteristics • size, longevity/survival • age of first reproduction, number of reproductive events in a lifetime • degree of investment per offspring • dispersal abilities, competitive abilities, responses to disturbance

Resource allocation • key activities: • survival-related activities (e.g. movement, defense, baseline metabolism) • growth • reproduction: acquisition of mates, production of gametes, parental care

Mate choice and Life History