Mating System

sSpecies-specific pattern of association betw

een males, fem

ales and offspring

April 14: M

ating Systems: D

efinitions

What defines a m

ating system•

Whether each individual of a given sex m

ates with one or

many of the opposite sex

•W

ho cares for the offspring

Nam

eM

ale-female association

Sex providing careE

xamples

Monogam

y1 fem

ale : 1 male

Both

Many birds

Polygyny>1 fem

ale : 1 male

Females

Many m

amm

als;som

e birds

Polyandry1 fem

ale : >1 male

Males (som

etimes both)

Some birds;

many insects

Polygynandry>1 fem

ale : >1 male

Both (or only m

ales)Som

e birds;som

e fish

Promiscuity

>1 female : >1 m

aleN

either (or only females)

Many fish; chim

panzees

Cares

Cares

Deserts

Male

Fem

aleO

utcome m

atrix

Why D

o Mating System

s Vary A

mong Species?

Mating system

results from conflict of interest betw

een males

and females (and is outcom

e of evolutionary game)

•For both sexes, R

S would be m

aximized by having m

ultiple mates,

and having each mate care for offspring produced

•W

hether either sex can achieve this outcome depends upon sex-

specific ecological factors

April 14: M

ating Systems: W

hy variation?Deserts

Monogam

y(&

polygynandry)

Promiscuity

Polyandry

Polygyny

•O

ffspring require high parentalinvestm

ent, and/or•

Fem

ales widely dispersed (hence

not defendable by males)

•O

ffspring can be reared byone parent

•F

emale not w

idely dispersed(hence defendable)

“Monogam

y” in Birds

April 14: M

ating Systems: B

ird monogam

y

More than 90%

of bird species are socially monogam

ous•

Males and fem

ales form stable pair for duration of breeding season

(sometim

es longer)•

Both parents help rear offspring

But…

...•

DN

A fingerprinting studies find high levels of

Extra-Pair C

opulation (EPC

)•

Both m

ales and females m

ust be fooling around•

Raises functional questions

“Monogam

y” in Birds

April 14: M

ating Systems: B

ird monogam

y

Why should m

ales and females seek E

PC

s?•

Males: R

S maxim

ized by finding additional mating

opportunities (but there may be costs)

•Fem

ales: RS not lim

ited by mating opps, so w

hyseek additional ones?•

Benefits of genetic diversity

•G

ood genes (better than mate’s)?

Why should E

PC

s be furtive?•

Males: fem

ale may leave altogether (find m

ale that will invest m

ore)•

Females: m

ale incentive to invest in brood is diluted by loss of paternity

Fig. 13.7 A

lcock:B

lue tit males that attract

more E

PCs have m

orefaithful m

ates

Female visits per hour

Ow

n mate’s

absencesper hour

In other species, tendency to attract extra-pair females is

correlated with the sam

e sort of traits (song, plumm

age) thataffect ability to attract first m

ate

Genetic diversity and m

ate choice in mam

mals

April 14: M

ating Systems: B

ird monogam

y

Fem

ales mam

mals benefit by having offspring that are heterozygous at

MH

C* locus (allow

s greater diversity of antibody production)

•W

ith respect to MH

C, the ideal m

ate is one different from you

•H

uman m

ated pairs more likely to differ at M

HC

than youw

ould expect for random m

ating (I.e., mating is assortative)

•W

omen prefer the body odor of m

en who are different at M

HC

to that of men or are sim

ilar at MH

C (Sw

eaty T-shirt test)

MH

C*: M

ajor Histocom

patibility Com

plex: a highly polymorphic

(many allele) gene com

plex involved in imm

une response

Suggests advantage of extra-pair copulations by female: don’t put all

your eggs in one MH

C basket

Polygyny IA

pril 14: Mating System

s: Polygyny

I. Polygyny based on resource defense

•M

ales that offer better resources (e.g. territory) may attract additional m

ates

Major functional question in resource defense polygyny: w

hy shouldfem

ale settle with already m

ated male?

•She m

ust share resources with fem

ale already there (CO

ST)

•She can’t count on as m

uch help from m

ale (CO

ST)

Male red-w

inged blackbirdfem

ale

Resource D

efense Polygyny (cont’d)A

pril 14: Mating System

s: Polygyny

H1: P

erhaps quality of territory more than com

pensates for costs•

Typically it doesn’t--secondary fem

ales would probably produce m

oreoffspring if they found unm

ated male

Major functional question in resource defense polygyny: w

hy shouldfem

ale settle with already m

ated male?

H2: P

erhaps there are good-genes benefits (Sexy Son hypothesis)•

This w

orks only if male m

ating success is highly heritable•

Male m

ating success IS heritable, but heritable enough for females to have

a chance of recouping costs by producing sexy sons

H3: F

emale search costs/m

ale deception

Fem

ale group sizeA

lcock Fig. 13.14

Polygyny--cont’dA

pril 14: Mating System

s: Bird m

onogamy

II. Polygyny based on fem

ale defense•

If females are aggregated spatially, then

one male can exclude other m

ales

Conflicts of interest in m

armots

•M

armot fem

ales don’t disperse, so there may be

several (up to 6) breeding females in group

•M

ales that gain access have high RS

•Fem

ales do better in small groups

Female RS

Male RS

Why should fem

ales ever live in large groups?SEV

ERA

L HY

POTH

ESES: SEE BOO

K

Polygyny -cont’dA

pril 14: Mating System

s: Bird m

onogamy

III. Polygyny based on fem

ale choice of male traits (L

ek polygyny)•

Lek: A

ggregation of males from

which fem

ales choose mates

•N

O resource defense; N

O fem

ale defense; (may be m

ale dominance hierarchy)

http://ndwild.psych.und.nodak.edu/tvpages/sage1.htm

l

Sage Grouse

Stalk-eyed flies

Polygyny -cont’dA

pril 14: Mating System

s: Bird m

onogamy

III. What favors evolution of leks?

•H

1: Hotspot hypothesis (som

e feature of environment

attracts females, w

hich attracts males)

•H

2: Hotshot hypothesis (m

ales recognize attractivem

ales, and gather near him)

•H

3: Female discrim

inate against males in sm

all groups

TH

ER

E IS SO

ME

EV

IDE

NC

E FO

R E

AC

H O

F TH

ESE

--SE

E B

OO

K!

http://ndwild.psych.und.nodak.edu/tvpages/sage1.htm

l

PolygynyA

pril 14: Mating System

s: Bird m

onogamy

Why D

o Mating System

s Vary A

mong Species?

Mating system

evolution as a game

•Part of w

hat determines optim

al decision for each sex are payoffs thatdepend on w

hat partner does

April 14: M

ating Systems: W

hy variation?

Deserts

B

Cares

Cares

Deserts

Male

Fem

alePayoff m

atrix(m

ale payoffs) b + P x b

b 0

B, b =

number of offspring

reared successfullyP

=probability of rem

atingafter desertion

(Monogam

y)

(Promiscuity)

(Polygyny)

(Polyandry)

If probability of remating (P

) is very high, then monogam

y is notstable com

pared with polygyny and polyandry

Why D

o Mating System

s Vary A

mong Species?

Mating system

evolution as a game

•Part of w

hat determines optim

al decision for each sex are payoffs thatdepend on w

hat partner does

April 14: M

ating Systems: W

hy variation?

Deserts

B

Cares

Cares

Deserts

Male

Fem

alePayoff m

atrix(m

ale+fem

ale) b + P x b

b 0

Bb + P

x b

0b

B, b =

number of offspring

reared successfullyP

= probability of rem

atingafter desertion

If probability of remating (P

) is very high, then monogam

y is notstable com

pared with polygyny and polyandry

Why D

o Mating System

s Vary A

mong Species?

Mating system

evolution as a game

•Part of w

hat determines optim

al decision for each sex are payoffs thatdepend on w

hat partner does

April 14: M

ating Systems: W

hy variation?

Deserts

B

Cares

Cares

Deserts

Male

Fem

alePayoff m

atrix(m

ale+fem

ale) b + Pxb

b 0

Bb +

Pxb-C

0b

B, b =

number of offspring

reared successfullyP

= probability of rem

atingafter desertionC

= additional cost

experienced byfem

ales•

If cost of desertion is often higher for females than for m

ales, thenpolygyny is only stable outcom

e•

If, males also incur costs for desertion, then m

onogamy m

ay be stable