Mating Systems

Mating systems

Can be defined in terms of the number of sexual partners an individual acquires during a breeding season

• Male:

– Monogamous

– Polygynous

• Females:

– Monogamous

– Polyandrous

mating systems and sexual dimorphism

The mating relationship between males and females varies greatly from species to species.

• Promiscuous: no strong pair-bonds/lasting relationships

• Monogamous: one male mates with one female

• Males and females with monogamous mating systems have similar external morphologies (monomorphy)

• In promiscuous mating: sexual dimorphism

In polygamous relationships, an individual of one sex mates with several individuals of the other sex

– Polygyny/Polygynous: one male mates with many females; the males are usually more showy and larger than the females

– Polyandry/Polyandrous: one females mates with many males; the females are often more showy than the males

sexual selection

Sexual selection: a "special case" of natural selection

• It acts on an organism's ability to obtain (often by any means necessary) or successfully copulate with a mate

• Sexual selection is often powerful enough to produce features that are harmful to the individual's survival: extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex

• Peacocks (male), peahens (female)

mating systems influence sexual selection

strength of sexual selection and variance in mating success

• Monogamy: all males have the same mating success = 1 mate

• Moderate polygyny: some males = 0 mates, some male = 1 mate, some mates = 2 mates

• Strong polygyny: most male = 0 mates, one male = 8 mates (max)

• Elephant seals: harem polygyny; territorial defense polygyny

male competition

Males compete for access to females, the amount of time spent mating with females, and even whose sperm gets to fertilize her eggs. Male damselflies (∼dragonflies) scrub rival sperm out of the female reproductive tract when mating

female choice

Females choose which male to mate with, how long to mate, and even whose sperm will fertilize her eggs. Some females can eject sperm from an undesirable mate

lekking polygyny

Combination of:

• Male competition: male may fight for position in center of the lek

• Female choice: female choose a mate, often dominant or in the center

Remember: reproductive success is more variable in males than in females

sex roles

• Angus John Bateman (1919 -1996)

• Bateman’s principle: the sex which invests the most in offspring will become a limiting resource over which the other sex competes ( = sexual selection); Angus John Bateman (1919 –1996)

• It holds that males are under sexual selection to optimized offspring quantity (number of mating opportunities), whereas females optimize quality of sperm (genetic contribution to the offspring)

• Bateman’s principle thus views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully

The Reproductive Success (RS) of individuals in a population is the production of independent offspring per breeding attempt or lifetime.

• Mating opportunity limits male reproductive success

• Fecundity limits female reproductive success

Anisogamy: difference in the size of male and female gametes

Gametic investment: sperm are “cheaper” than eggs; female: a greater level of parental investment; male can easily produce enough sperm to fertilize all of a female’s eggs

male sex roles

• Limitless sperm

• Fertile throughout life

• Number of potential offspring determined by number of potential female partners

• Required level of parental investment= low

many female partners to increase inclusive fitness

female sex roles

• Egg is more than 100x larger than sperm

• Few are released

• Limited fertility

• Gestation periods

• Nutrients for fetus from mother’s own supplies

• Required level of parental investment= very high

ensure survival of few precious offspring

polyandry in jacanas

• Harem polyandry and “sex role reversal”

• Males: maintain small territories, perform all parental care.

• Females: mate with multiples males and then leaves eggs with males.

• The number of males a female mates determines her reproductive success (she does not care for eggs)

• 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 species

Saddleback Tamarins,

Spotted Sandpipers,

Red-necked Phalaropes

when is polyandry favored?

– When males become limiting resources for reproduction

– This often occurs when low offspring survival requires male parental care, so that males have the greater reproductive effort and lower reproductive rate.

• What is the consequence?

– Sexual selection is stronger on female than males

wild saddle-back tamarins

In southeastern Peru, this population contain both monogamous and polyandrous groups simultaneously

• Adults mate monogamously only if they have non-reproductive helpers (older offspring) to help rear infants

• Without helpers, the reproductive success of both males and females is higher, on average, if they mate polyandrous than if they mate monogamously (cooperatively polyandrous: groups in which two or more males copulate with a single female during a single breeding season and then cooperate to raise the female’s subsequent offspring).

sexual dimorphism & sexual selection

Sexual dimorphism: the two sexes of a species differ in external appearance

Although some sexual dimorphism is the result of natural selection, most is thought to have evolved through sexual selection.

• Sexual dimorphism in body size: common yet highly variable among marine mammals. In some species, males are much larger than females (e.g., sperm whales, elephant seals) but females are larger than males in others (e.g., baleen whales)

• Fecundity selection

– Selection on female fecundity = number of offspring produced

– Fecundity is different than mating success = number of mates

• In many species where females are larger than males, dimorphism is thought to reflect fecundity selection favoring large female size, since larger female can produce more eggs

mate choice by females

• Female choice is a type of intersexual competition: members of one sex choose mates on the basis of certain traits.

• Females can drive sexual selection by choosing males with specific behaviors or features of anatomy.

• “Ornaments” often correlate with health and vitality

the walrus

• Walrus tusks: found on both the female and the male; they grow continually throughout their life (male’s tusks much longer and wider in proportion to their body than those of the female).

• Masculinity, to fight, to help in walking on land (“the toothwalkers”), minor role in feeding.

• Pharyngeal pouches: after diving, walruses can inflate throat pouches with air → they can rest at the surface; males use them to make a loud bell-like sound during breeding season

• Mature males claim an aquatic territory (harem) near a group of resting female: the strongest male gets the best territories

• The female come to the male, one by one, and join him in the water where courting occurs.

• Each male may mate with several females each season, but the female only choose one male.

• Lowest reproductive rate of all the pinnipeds: only 1 calf every 2-3 years; babies stay with mother for over 2 years.

• males mature at ∼10 years of age, females ∼ 8 years; males most do not mate until they are at least 15 and females usually do not give birth until ∼ 10 years old

mate choice by females: zebra finches

Female chicks who imprint on ornamented fathers are more likely to select ornamented mates

• Experiments suggest that mate choice by female zebra finches has played a key role in the evolution of ornamentation in male zebra finches

male ornaments: extravagant characters due to sexual selection

• Long-tailed widowbird: male with unnaturally long tails attract females away from the nests of “normal” male or male with shortened tail.

• Barn swallow with elongated tail: greater reproductive success but experience a cost in tail size the following year (flight deficiencies from feather breakage)

male ornaments: sexual selection

• Fisher’s “runaway” hypothesis: mate choice originally evolved to facilitate adaptive choice for traits conferring a survival advantage; once females preference evolved, any genes that conferred survival advantage, but compromised attractiveness would not be passed on because surviving male would fail to mate.

• Sons inherit trait that makes them sexually attractive; daughter inherit the majority mate preference

• Sir Ronald Fisher (1890-1962): Fisher’s statistical techniques, ANOVA, P-value

runaway selection hypothesis

Some traits (prominent plumage, elaborate courtship behaviors, extreme body ornamentation) so strongly preferred by female of certain species that they will mate only with those males having the strongest expression of the trait.

• In subsequent generations, male offspring are more likely to possess that physical trait, whereas female offspring are more likely to possess a preference for that trait in male; over time → extreme sexual dimorphism.

1. Female’s choice cause deleterious characteristic development in males.

2. Change in male characteristic does not equal to higher quality

male ornaments

• Zahavi’s “handicap” hypothesis: extravagant males traits are costly to develop and maintain; choosing a mate with “good genes” requires an honest signal of genetic quality → only males in good condition (those with good genes) will be able to fully develop and maintain an ornament.

• Males evolve costly signals to indicate their underlying genetic quality for survivorship.

• Amotz Zahavi

Zahavi’s handicap hypothesis

• Females prefer males with handicaps (mating characters that reduce survival chances) because handicaps are indicators of heritable viability (best genotypes of the male population).

• Traits that encumber the owner are physiologically costly (exertion in flight) and more expensive to develop.

• Asymmetry: developmental instability, “bad genes”; symmetry is chosen in some species

• Bright color honestly signals immunocompetence, dominance, parasite/disease resistance

mating systems and parental care

Needs of the young are an important factor constraining evolution of mating systems.

• Asymmetry in parental investment between males and females:

– Male: small gametes, large number, energetically cheap.

– Female: large gametes, small numbers, energetically costly.

• Bird species where chicks need a continuous supply of food/protection (e.g., altricial)

– a male maximizes his reproductive success by staying with his mate and caring for his chicks (monogamy)

• Bird species where chicks are soon able to feed and care for themselves (e.g., precocial)

– a male maximizes his reproductive success by seeking additional mates (polygyny)

certainty of paternity influences parental care and mating behavior

• Females can be certain that eggs laid or young born contain her genes, conversely, paternal certainty depends on mating behavior.

• Paternal certainty is relatively low in species with internal fertilization because mating and birth are separated over time (high in external fertilization!).

• Infanticide

• Heteropaternal superfecundation: dogs, cats (same mother different fathers)

mating system and parental care in mammals

– Internal gestation, lactation

– Over 90% of species are polygynous

– Male parental care is rare

mating system and parental care in birds

– Eggs must be incubated, chicks must be fed (in most species)

– Over 90% of species are monogamous

– Male parental care is often essential