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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