EEB W7: Monogamy, Polygamy, and Sexual Selection

polygamy

When an individual of either sex has 1+ mate; characterized by uniparental care (polygyny, polyandry, promiscuity)

polygyny

Genetic mating system of 1 male: many females;

may have diff social structures;

common in mammals with female parental care

(ex: plains zebras, birds of paradise, black bears, kangaroos)

Plains zebras

Male zebra stallion begins in bachelor group, then competes for, takes care of a harem of females, polygyny; can also have polyandry (2+M, 1F) and monogamy, satellite harem males

kangaroo

Embryo pulls itself from birth canal to pouch for secondary gestation in female, example of polygyny with marsupial

polyandry

Genetic mating system of 1 female: many males;

male parental care;

more rare but seen in egg-laying species (fish/birds);

often characterized by sexual dimorphism and female competition

(ex: wattled jacana larger female with multiple clutches per male, pipefish with male gestation, social insects like honeybees)

Sexual dimorphism (aka sex role reversal, reversed sexual dimorphism)

Difference in phenotype between sexes of same species; when females are larger and more colorful, called reversed sexual dimorphism/sex role reversal

monogamy

Genetic mating system of 1 male: 1 female;

common in many birds;

typically biparental care and a stable pare bond (serial monogamy);

little to no sexual dimorphism but may still have ornaments/courtship

(ex: emperor penguin, siamang primate, seahorse, grebes)

Serial monogamy

Sequential 1:1 pair bond with different mates across lifetime

promiscuity

Genetic mating system of many males: many females;

often egg-laying species;

no parental care;

commonly low investment in zygote, large clutches, external fertilization

(ex: green anoles, frogs, snakes, insects)

Analyzing mating systems

Consider that system evolved in response to offspring needs;

examine parental investment/care thru reproductive physiology, ecology (resources);

examine availability of mates thru spatial distribution, density of conspecifics

Reproductive physiology

Anisogamy leads to idea that females have more investment in offspring, especially if gestation or internal development (female care → monogamy or polygyny)

Trivers' parental investment theory

Null hypothesis that because of anisogamy, female should benefit more from investment in offspring thus should be choosier and have fewer partners, invest more in current than future offspring

Bateman's Rule/Law

Males benefit more than females do from multiple matings and thus have higher reproductive variance than females, tested in fruit flies in flawed experiment;

true for polygynous mating systems but not true for polyandrous, promiscuous systems

fish

Generally low parental investment, cheap eggs and produce many, likely promiscuous

birds

Medium parental investment, egg is costly but incubation shared, likely monogamous

mammals

High parental investment especially from female, internal gestation, live birth, lactation, likely polygamous and commonly polygynous

Natural selection and mating systems

Favors maximal production of surviving offspring, not just fertilizations;

need to balance parental care vs. more copulations;

monogamy adaptive when one parent cannot raise young alone

(european starling incubation, song sparrow nestlings, emperor penguin, california mouse)

Ecology (with mating systems)

Degree of difficulty to feed young, contributes to mating system; scarce resources and low asymmetry = monogamy; abundant resources and high asymmetry = polygamy (ex: across reed warblers, increase food supply cor. decreasing parental care) (ex: red-winged blackbird females choose males with better resource territory in polygynous system)

Polygyny threshold model

Takes female perspective: maps territory quality against female fitness; monogamous male with low quality territory may = polygynous male with high quality territory

Availability of mates

If clumped mates, easier to defend/monopolize ~ female defense polygyny (plains zebra);

if clumped resources, easier to defend ~ resource defense polygyny (Grevy zebra, red-winged blackbird);

if low-density and not clumped, harder to find new mate ~ monogamy (rock-hunting possum, clown shrimp)

anglerfish

Low density, deep sea species characterized by monogamous males and potentially polyandrous females, males fuse to females and become sexual symbionts, leading to sexual maturity for both and search time 0; mate scarcity favoring monogamy

Female defense polygyny

Polygynous system formed due to female aggregation or female sociality, allowing for a male to guard and mate with the group (plains zebra, muskoxen, lionesses, Montezuma oropendolas)

Resource defense polygyny

Polygynous system formed when resources are clumped, easier to defend, allowing for male to defend resource and mate with females that visit it

Lek polygyny

Polygynous system in which males concentrate at a display site (lek) and females visit and mate with 1+ males,

often results in reproductive skew;

leks may be meaningless territories or represent resources;

often on grassland areas

(ex: ruffs, topis, red-winged blackbirds, sage grouse, lance-tailed manakins)

Topis

Display on grassland leks; males in the center of the lek are more popular with females (possibly bc higher competition, higher traffic)

Sage grouse

Lek polygynous birds with highly energetic displays to females, often females pick the same male

Lek polygyny hypotheses

Hot spot, hot shot, female preference

Hot spot hypothesis

Theory that males congregate in leks based on where females tend to travel (female presence attracts males);

not supported by little bustard decoys

Hot shot hypothesis

Theory that males congregate in leks near highly attractive dominant males; not supported by little bustard decoys;

supported by great snipes, removing dominant male made subordinates leave

Female preference hypothesis

Theory that males congregate in leks so that females can view them all and compare them more easily (groups of males attract females); supported by little bustard decoys; supported by ruffs; supported by male Uganda kobs

Malte Andersson's model for polyandry

1) Primarily male parental care of eggs leads to males investing more in care

2) Females can lay more eggs than the first male can care for

3) Females compete to solicit additional mates

Male parental care in fish

Often have external fertilization, low initial parental investment, egg-guarding by males; ex: Male Sergeant major (guards eggs), mouth-brooding jawfish

Polyandrous fish

Feature male parental care, limited clutch sizes (ex: pipefish, mouth-brooding jawfish)

Polyandrous birds

Feature male-only care,

common when nest predation is high,

re-nesting is common,

females can lay more eggs than they can care for,

precocial chicks,

abundant resources during breeding season,

female-biased operational sex ratio;

ex: shorebirds like wattled jacana, spotted sandpipers

Classic sexual selection

Following Trivers' theory, females should be choosy, males promiscuous; Darwin's "ornaments and armaments" as males compete for females

Reproductive variance

Reproductive skew; as this increases, sexual increases...high sexual dimorphism increases sexual selection and increases this skew

Nuance in mating systems

Males can be choosy, females in "monogamous" relationships may have EPCs, females in polygynous systems might mate multiple times (lek polygyny maybe?)

Reasons for female monogamous EPCs

Good genes (find higher fitness male) (fairy wren), genetic diversity (higher genetic diversity in single brood) (social insects)

Honeybee polyandry

Queen bee goes on a nuptial flight, increases genetic diversity of colony and increases survival rate through winter