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

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 offspringselection for mating effortless 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

Outline

Mating systems and Mate choice

Territoriality

Sociality and altruism

Life History and reproduction

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

Outline

Mating systems and Mate choice

Territoriality

Sociality and altruism

Life History and reproduction

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

Outline

Mating systems and Mate choice

Territoriality

Sociality and altruism

Life History and reproduction

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

Principles of allocation

allocation of resources to one kind of tissue/activity leads to reduced allocation to other activities– finite supply of resources– all activities require resources and have

costs trade-offs between reproduction and all

other activities

Extreme Energy allocation

Divert all energy to reproduction and as little as possible to growth– Opportunist Species

Divert all energy to growth and little per year to reproduction– Competitor Species

Reproduction and survival

Fitness = Reproductive output X Probability of Survival until next year

If you have a chance to reproduce again, then the benefit of saving your offspring is not as great as the benefit of saving yourself– E.g, when food becomes

scarce, the Galapagos penguin will abandon its chicks and try again next year

Opportunist versus competitor species

Opportunist species are often very small and devote all their resources to reproduction in one year (Semelparous)

Competitive species are often larger and often survive for a number of years and can reproduce each year (Iteroparous)

Semelparous and opportunist

After hatching and feeding for a few weeks, the mayfly becomes a sexually mature adult

Shortly thereafter both sexes flies over the water and mate.

Then, the female lays her eggs on the surface of the water – both sexes then die.

Iteroparous and competitive

Provide a lot of resources for their cubs and defend them

Bears have 2 cubs 10 times in their lifetime of ~20 years

Semelparous and competitive

A bamboo plant reproduces asexually for 100 years. Along with other individuals, it forms dense stands of plants

Then in one season, all the individuals in the population flower simultaneously, reproduce sexually and die

One hundred years later the

process is repeated

Number vs. survivorship of offspring

Type I species are those such as humans with highest mortality among elderly

Type II experience steady mortality throughout life (some birds and invertebrates)

Type III experience mortality peak at young ages (salmon)

Number of offspring and parental care

Human babies need to be taken care of for up to 20 years

Hence, we do not have too many babies during our lifetime

As plants give minimum parental care, they often have very large number of offspring

Selection on clutch size

Experiments have shown that increasing the number of young a bird must raise in one season has resulted in a decrease in the fitness of all the young

This has resulted in some organisms being genetically programmed to have the same number of offspring every reproductive season

Animals with pre-programmed clutch size

Galapagos penguin has 2 eggs every season

Humans (and a large number of other mammals) usually only have one young at a time (there is a higher mortality of twins)

Life history and applied ecology

invasive & threatened species do not possess a random collection of life history characteristics

invasive/non-native species– rapid growth rates– well-developed dispersal abilities– disturbed environments– opportunists

endangered species– large size, low density/low population size– poor dispersers– stable environments– competitive

Summary

The sex that is most choosy in picking mates is the sex that has the most investment per offspring

Mating systems are constantly in flux as the war rages between the sexes, with different points of balance between investment per offspring and number of offspring

Opportunist, competitor, iteroparous and semelparous species are all extreme strategies in a continuum of strategies for optimum fitness