Chapter 11 The evolution of mating systems

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Chapter 11 The evolution of mating systems

• Monogamy one male one female
• Polygamy
• Polygyny one male, multiple females.
• Polyandry one female, multiple males.
• Polygynandry multiple males, multiple
  females.

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Monogamy Prolonged, essentially exclusive
bond maintained with one member of opposite
sex. Generally a rare system. Rare in mammals
(except for some rodents, primates and
dogs). However, is commonest avian mating system.
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Monogamy armed compromise rather than happy
collaboration. Males would generally like to
seek extra mates. Why dont they?
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Several hypotheses
Mate-guarding hypothesis Mate assistance
hypothesis Female enforced monogamy
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Mate guarding hypothesis (MGH)
Monogamy may be best choice if female would mate
again if male deserted her and if 2nd male would
fertilize eggs.
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• Mate guarding should pay off when females
• Scarce and hard to find.
• 2. Remain receptive after mating.

Example Clown shrimp.
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Mate assistance hypothesis (MAH)
Male stays with partner because male assistance
increases youngs survival. Increased survival
of young outweighs extra young gained by seeking
extra mate
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In seahorses, males carry brood in pouch during
3 week pregnancy. Pair stays together for
series of matings.
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Male can hold only one clutch, so no benefit in
courting extra females. Females choose monogamy
because males are scarce and because females are
poor swimmers and thus vulnerable to predators.
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Female enforced monogamy hypothesis (FEMH)
In some species females actively prevent males
obtaining extra mates.
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A female burying beetles will attack her mate
if he tries to release pheromones to attract
other females to a carcass the pair have buried.
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In experiment, males whose female had
been tethered so she could not attack
him released pheromones for longer period
than males whose mate wasnt tethered.
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Monogamy in birds
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Male birds can feed young as well as females
(unlike most mammals). Male assistance
essential to rearing young. Probably explains
why gt 90 of birds are monogamous.
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Male assistance in Snow Buntings essential to
rearing young. Females whose males were removed
reared fewer than 3 young. Those with males
reared 4 or more.
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In many birds raising young so hard, it takes a
pair to rear even one young (e.g. albatrosses).
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In Tree Swallows polygynous males father fewer
surviving young (0.8 fledglings) than monogamous
males (3.0 fledglings).
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More offspring of polygynous males die because
male cant help both females (MAH). Females also
mate with other males because male cannot guard
two females effectively (MGH). Monogamy best for
both male and female Tree Swallows.
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Extra-pair copulations (EPCs) in birds. Even
though monogamous males assist one primary
female, males also seek EPCs. DNA
fingerprinting has shown EPCs very common.
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DNA fingerprints
Right Beta () male unique band occurs in
offspring D, E, and F. Alpha male () and
offspring G share unique band.




Above indicates unique alpha bands Alpha
fathered all young.
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Male benefits of EPCs are obvious (increased
offspring at low cost). Why would females seek
EPCs?
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• Female may gain by
• Increasing chances of her eggs being
• fertilized.

Female red-winged blackbirds who mate
with multiple males have higher egg hatching
rates.
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Similarly, female adders who mate with multiple
males have fewer stillborn young.
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2. Obtaining better genes for her offspring
Many female Blue Tits mate with neighboring
males whose mates dont seek out other males.
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These males survive better and produce more
young than males with unfaithful mates.
Suggests they have better genes.
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3. Obtaining resources from male
Female red-winged blackbirds that copulate with
neighboring males are allowed to forage on the
males territory. Males RWBs also assist in
attacking predators in vicinity of those females
nests.
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Polygamy Any mating system involving
mating with and, in many cases, forming pair
bonds with multiple members of the opposite sex.
Three kinds Polygyny Polyandry Polygynand
ry
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Polygyny One male mates with two or more
females. Examples Birds Lark Bunting,
Red-winged Blackbird, Dunnock, Marsh Wren.
Marsh Wren
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Mammals Lions, Gorillas, Bats. Also found in
many fish, insects, etc.
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Three basic types of Polygyny
1. Resource defense Polygyny 2. Female defense
Polygyny 3. Lek Polygyny
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1. Resource defense polygyny. Male defends
resources that females need to produce young
(food, nesting sites).
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Resource defense polygyny in an African cichlid
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African cichlid fish Lamprologus
callipterus exhibits extreme sexual dimorphism
(males 13X times larger than females).
Females lay eggs inside empty snail shells
and remain inside shell until eggs hatch.
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Males collect suitable shells and steal them
from other males.
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Males gather shells into large collections and
defend them from rival males.
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Up to 86 shells have been recorded in
one collection and up to 14 females at
once. Males with good territories obtain high
reproductive success. Extremely large male body
size has been selected for because it enables
males to collect shells and to defend their
territories.
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Male Red-winged Blackbirds hold territory on
marshes. Males with the best territories attract
harems of up to 15 females. Females choose males
on the basis of territory quality.
Males red epaulettes are essential in
male-male competition.
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Polygyny threshold model
Some females choose to mate with already mated
males who will not help them feed their chicks
even though unmated males with territories are
available. Why would a female do this?
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Polygyny threshold model predicts that female
will accept role of 2nd mate (polygyny) when
superior resources on males territory mean that
female would do better there than as 1st mate on
a poor territory
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Curves represent payoffs to female. Female can
choose between males A and B. A has a mate, B is
unmated.
Polygyny threshold model
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Example of polygyny threshold.

• Male Lark Buntings establish territories in
• grassy, open habitats. Mate with gt 1 female
• but assist only first female to settle on their
• territory.

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Some female LBs accept secondary female role on
good territory to obtain a high quality
nesting site. In bad nest sites young die from
exposure to the sun.
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Some male Pied Flycatchers establish two
territories. Sing to attract a female.
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Males provide little help to female on 2nd
territory, so female has low reproductive
success.
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Each female mated to a polygynous male has lower
reproductive success than a monogamous female.
However, males r.s. is higher than that of a
monogamous male
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Male Pied Flycatchers clearly try deceive
females into polygyny. Not clear yet if females
really fooled or have no better alternative.
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2. Female defense Polygyny.
Common when females cluster in groups that are
defensible. Males then defend clusters against
other males.
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E.g. Elephant seals, lion prides, elk and
deer herds.
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In some marine siphonoecetine amphipods, which
build protective cases out of gravel and shells,
males collect females and glue their houses to
their own.
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In general, female defense polygyny possible
because females cluster for their own
reasons and males exploit this. E.g. Lionesses
cluster to defend feeding territories. Deer
gather for protection. Elephant seals gather
on the few suitable nursery beaches.
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3. Lek Polygyny
Males do not help in raising the young. Variance
in male mating success is greatest in this
system.
Examples Grouse, Ruffs, manakins. Cock-of-the-ro
ck.
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In lekking species males display for females at a
predictable location (a lek) and females come to
the site to choose mates. Males provide no
resources except sperm.
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Males display for females. Females choose males
on basis of appearance and displays (sexual
selection).
Sage Grouse displaying on a lek.
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Highly skewed mating success is normal in lekking
systems. A few males obtain most of the
matings. By mating with best possible male,
females obtain the best available genes for
their offspring.
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In well-studied Black Grouse and Sage Grouse
lekking systems lt 10 of males obtain 70-80 of
the copulations.
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Why do males gather in leks?
Gathering in leks may reduce predation risk.
Open country birds display in groups whereas
forest species usually display
solitarily. Birds-of-paradise that display in
leks are edge or second-growth species (where
predation risk is high) whereas primary forest
species display solitarily.
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Three most favored hypotheses for evolution of
lekking are 1.Hot-spots hypothesis 2.
Hot-shots hypothesis 3. Female preference
hypothesis.
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Hot-spots Hypothesis males gather at sites
where they are likely to encounter females.
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Lekking bees, wasps and other insects often use
same locations for leks. Territories of lekking
flycatchers, manakins and hummingbirds also
often overlap. Gather along streams or
ridgelines that act as highways for female
movement. Convergence of different species on
same location supports hot-spot hypothesis.
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Hot-shots Hypothesis subordinate males cluster
around most attractive males -- hot-shots --
in order to be seen by or to intercept females
attracted to these males.
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In Great Snipe (a bird) removing central
dominant bird caused neighbors to leave
territories. Removal of subordinates resulted
in their territories being refilled.
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In Black Grouse on long-lasting leks location of
most popular territory shifts from year to year.
Suggests male quality more important than
location in lek.
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Female Preference Hypothesis females prefer to
choose from groups of males because comparisons
are easier to make.
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The mating behavior of the Ruff appears
consistent with all three hypotheses. Male
ruffs are named for their well-developed ruffs,
which they use to display to females (reeves).
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Ruffs are polymorphic with ruffs occurring in a
variety of colors.
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Male ruffs use a variety of mating strategies. Th
ey pursue females (followers), wait for them at
rich feeding ground (interceptors) or wait at
classic leks (lekkers).
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White-ruffed males appear to have evolved as
specialist Followers skilled at tracking the
movements of females between neighboring leks.
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Male ruffs may switch tactics but committed
lekkers have the highest mating success.
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Dark morph ruff displaying to a female.
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Controlled experiments suggest that female ruffs
prefer larger leks. This preference increases
the mating success of males at large leks and
favors that breeding strategy.
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Female ruffs prefer groups of at least five males
and visit such groups more often.
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Leks with gt5 males do not attract more females,
thus satellite males reduce success of dominant
males by intercepting some of the females.
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Hot-spots and hot-shots hypotheses also relevant
to ruff mating system. Leks tend to be located
by ponds where females come to feed
(Hot-spots). Satellite males gather around most
successful males (Hot-shots).
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The clustering of males on leks may in part
be due to a tendency of young or
inexperienced males to gather near older or
successful males. Such satellite males may get
occasional matings and perhaps gradually improve
their status. Such associations are most
extremely developed in the Central and South
American manakins.
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Cooperative leks displays. In many manakins
males perform cooperative displays. Three or
four males may cooperate to display but usually
only the alpha male gets to mate.
Round-tailed Manakin
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Cooperative display of Swallow-tailed Manakins.
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In Long-tailed Mankin males may take 8 years to
move up to alpha position. Four year study in
117 observed copulations only 8 of 85 males
copulated. 90 of copulations by 4 males and 67
by one alpha male.
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Long-tailed Manakin
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Manakin mating system works because birds are
long-lived and females tend to return to where
they mated before. As a result, the beta and
lower-ranking males can expect to inherit a
high-quality display ground and can afford to
delay mating.
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Polyandry One female forms pair bonds with two
or more males. Female reproductive success is
more variable than male reproductive success in
polyandrous mating systems. There are two forms
classic polyandry and cooperative polyandry.
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Classic Polyandry Females lay clutches for
multiple males and compete for males. Examples
Jacanas, Phalaropes, Spotted Sandpiper.

Cooperative Polyandry Two or more males
cooperate to assist a female at one
nest. Examples Acorn Woodpeckers, Dunnock.
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In classic polyandry females brightly colored and
compete for territories and males. Males
incubate eggs and care for young.
Female Red Phalarope.
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Male Jacanas (lilytrotters) defend small
territories against other males. Females defend
larger territories that include several male
territories.
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Female jacana lays clutch of eggs for each male
in her territory. Male alone incubates eggs and
cares for the young. Losses of eggs and chicks
to predators and nest flooding may be high.
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Clutch of jacana eggs.
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Male Pheasant-tailed Jacana incubating.
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If female jacana loses her territory or dies and
another female takes over the territory, the new
female destroys the eggs and kills young of any
male on territory. This behavior frees the male
to incubate a replacement clutch, which new
female provides.
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Female Spotted Sandpipers are 25 larger than
males. Female will lay clutches for a primary
male and from 1-3 secondary males.
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Only the last secondary male is assisted by the
female in caring for the young. However, later
males are likely to have lower reproductive
success because sperm from earlier males may
fertilize some of eggs.
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Not clear how classic polyandry has evolved. May
be a result of heavy losses of eggs which favor
females maximizing egg output (Jacanas).
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Alternatively, in Spotted Sandpipers cause may
be phylogenetic constraints that limit females
to four egg clutches.
Females can produce more eggs because food
sources are rich, but must lay more clutches
not bigger ones. Hence, need males to incubate.
Rare case of females being limited by access
to mates rather than by gamete production.
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Cooperative Polyandry also occurs in which more
than one male assists a female. Appears to be
result of shortage of breeding opportunities
because there are few territories available.
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Groups of Acorn Woodpeckers compete for
territories that contain granary trees.
Cooperative Polyandry may arise when a
multiple male coalition controls a
territory with only one breeding female.
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In Dunnocks cooperative polyandry occurs when two
males partition a females territory.
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Polygynandry Two or more females form pair
bonds with two or more males. Examples
Ratites (i.e. Rheas and Ostriches), Dunnocks,
Acorn Woodpeckers. Arises in similar
circumstances to those just described for
Cooperative Polyandry.
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The mating system of the Dunnock
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Unobtrusive, quiet and retiring, without being
shy, humble and homely in its deportment and
habits, sober and unpretending in its dress,
while still neat and graceful, the dunnock
exhibits a pattern which many of a higher grade
might imitate, with advantage to themselves and
benefit to others through an improved example.
The Reverend F.O. Morris (1856) encouraging
his parishioners to emulate the humble life of
the Dunnock. Little did he know!
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The Dunnock is an unobtrusive brown and gray bird
that is common in woodlands, hedgerows, and
urban areas in Europe. The Dunnocks breeding in
the Cambridge University Botanical Gardens have
been the subject of long-term research by Nick
Davies and colleagues. These studies have
revealed a mating system that the Reverend Morris
might have hesitated to recommend to his
parishioners.
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Dunnocks feed on small arthropods and establish
and defend exclusive territories, which they
retain year-round. Subject of long-term studies
by Nick Davies and colleagues in Cambridge
University Botanic Gardens
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Females establish and defend their territories
against other females. Females choose their
nesting territories independently of males and
compete for space with other females.
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Territory size is a function of food
availability. The more food is available the
smaller territories are.
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Dunnock 1990 female territories
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Male Dunnocks also defend territories but as is
the case for females, only against members of
their own sex.
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Males impose themselves on the female
distribution and attempt to monopolize access
to the females territories.
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Dunnock 1990 male territories
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Female territories 1990
Male territories 1990
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Monogamy arises when a male can control all of
a females territory. In spring, males pursue
females around their territories thus learning
the territorys boundaries and singing to stake
their claim. A male who can control a single
territory is monogamous. A male who can control
more than one territory is polygynous.
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Polyandry usually arises when a females
territory lies between the territories of two
males. Each male attempts to pursue the
female into the other males territory.
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At first each male is dominant in its own
territory, but eventually one male (the
alpha) establishes dominance and the two
males (alpha and beta) defend the females
territory together. Polyandry may also arise
when a young male persistently intrudes onto an
older males territory until he eventually is
accepted as a beta male.
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Polygynandry may occur when in adjacent
monogamous pairs one male invades the adjacent
territory and eventually becomes the alpha male
in a two female territory.
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Female territory size is crucial in determining
the mating system. The larger the female
territories are the harder it is for any mating
system other than monogamy or polyandry to arise
because males cannot defend very large
territories.
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Territory size is a function of food
availability. The more food available the
smaller female territories are. If food is
added, female territory size is reduced and this
facilitates polygyny and polygynandry
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Males are larger than females and dominant over
them in aggressive interactions. When food is
scarce, females lose out to males.
female
?
?
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As a result of male dominance, female mortality
is higher in severe winters.
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As a consequence of differential mortality the
population sex-ratio is male-biased after severe
winters.
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A shortage of females leads to an increase in
polyandry.
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Males and females both try to maximize
their reproductive output. Male and female
payoffs differ in different mating systems.
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Payoffs
For male For female
Polyandry Share one Sole access female to
multiple males Monogamy Sole access to Sole
access to one female one male Polygynandry S
hare several Share several females males Poly
gyny Sole access to Share one male several
females
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When more than one male feeds a brood more young
are fledged and they are bigger. The larger
young are at fledging the better their chance of
surviving to independence.
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A males payoff is highest when he can mate with
multiple females. A females reproductive
success is highest when she can obtain the
assistance of more than one male to care for her
brood. These aims are in opposition in
polyandrous and polygynandrous mating systems.
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In a polyandrous or polygynandrous mating system
a female dunnock tries to encourage both males to
feed the young. She does this by mating with
both males and giving them paternity in the
brood. The alpha male, however, wishes to
control mating access to the female because
although more young are reared when both males
feed the brood, the number of young he fathers
is reduced.
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Male Dunnocks try to maximize their reproductive
success by engaging in sperm competition and
mate guarding.
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Sperm Competition In Dunnocks sperm
competition is intense. Male Dunnocks produce gt
1000 times the amount of sperm that the
comparably sized Zebra Finch does. Females store
sperm in special sperm storage glands.
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Sperm Competition Both males mate with the
female as often as they can to maximize the
amount of their sperm in the females sperm
storage glands. Both males also engage in
cloacal pecking. In this behavior the male
before mating pecks the females cloaca and she
ejects stored sperm from her sperm storage glands.
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Mate Guarding
The alpha male guards the female and tries to
prevent her from mating with the beta male. If
the beta male attempts to mate, the alpha
male intervenes and drives him away from the
female. The female encourages the beta male
to mate and attempts to escape from the alpha
male and mate with the beta male.
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Mate Guarding
?
?
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The alpha male intervenes in copulation
attempts made by the beta male.
?
?
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Both males frequently inspect the nest to see if
eggs have been laid and value copulations during
the egg-laying period particularly highly as
these are most likely to produce offspring.
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After the eggs have been hatched beta males base
their decision on whether to feed the brood or
not on the amount of mating access they have had
to the female.
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The greater the mating access a beta male has had
the more often he feeds the young.
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DNA fingerprinting results have been very useful
in teasing apart the mating success of males and
females in these complex mating systems.
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Right Beta () male unique band occurs in
offspring D, E, and F. Alpha male () and
offspring G share unique band.




Above indicates unique alpha bands Alpha
fathered all young.
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The Dunnocks mating system results from the
interaction of multiple factors both ecological
and behavioral. Food supplies and winter
conditions affect female territory size which
determines how many female territories a male
can defend. This in turn leads to complex
behavioral maneuvering as both sexes attempt to
maximize their reproductive success. Hardly, the
picture of domestic tranquility that the
Reverend Morris had in mind!