1. No category

Evolution of mating systems and
parental investment
Parental investment into
– gametes
– embryo
– offspring (and grandchildren)
• Protection (predators, weather etc.)
• Feeding
• Teaching
Investment carries a cost, investment into present offspring
reduces the number of offspring in the future
Evolution of mating systems and
parental investment
Investment carries a cost, investment into present offspring
reduces the number of offspring in the future
If true:
–
–
–
Which sex would invest (conflict between the
sexes)
How much a parent would invest (conflict
between the parent and offspring
How much investment one offspring should force
(conflict among offspring)
Evolution of mating systems and
parental investment
Equal large investment needed
→ monogamy
Small investment:
→ promiscuity
The sex investing less can have more offspring
But if sex ratio is 1:1 → conflict within the sex
If males invest less
→ polygyny
If females invest less
→ polyandry
Evolution of mating systems and
parental investment
But if sex ratio is 1:1 → conflict within the sex
Mechanism of conflict in polygamy: mating systems
– Resource defence
– Female (male) defence
– Hierarchy (e.g. lek polygyny)
– Scramble competition
Whom to mate with (conflict between the sexes)
– The sex investing more → has less offspring →
more choosy
What to choose: whatever increases fitness
Evolution of mating systems and
parental investment
Factors in choice: whatever increases fitness
– Good genes→ healthy offspring
– Sexy partner → sexy sons
– Provisioning → fit offspring
– Good match → healthy offspring
– Protection
→ live offspring
Conflict between factors: e.g. what if sexy partner seeks
extra-pair copulation instead of provisioning? → parent
with provider socially, seek EPC for good or sexy genes
Evolution of mating systems and
parental investment
Complications
Social and genetic mating system can be different
Internal fertilisation: fathers can never be certain → males
are less likely to invest into offspring
Not real polyandry.
Extra Pair Fertilisation (EPF)
independent of social mating system
Evolution of mating systems and
parental investment
Complications
Social and genetic mating system can be different
Internal fertilisation: fathers can never be certain → males
are less likely to invest into offspring
Ecological constraints may influence mating system
• e.g. sparse distribution of food → sparse distribution
females → monogamy (male provisioning may be
secondary)
• Group structure may influence mating system
• e.g. if cooperation among males is vital → sperm
competition
Parental investment into gametes
How did anysogamy
evolve?
Parental investment into gametes
How did anysogamy evolve?
Parental investment into gametes
How did anysogamy evolve?
Small + small → low fitness
Small + large → medium fitness
Large + large → high fitness
Selection for large gametes?
Simplistic approach:
smaller gametes → more can be produced and higher
motility
If small gametes can avoid to unite with small ones,
selection would favor small and large gametes.
Parental investment into embryo
Investment asymmetry between sexes:
Internal fertilisation: females invest more (mammals, birds,
some fishes, amphibians, insects)
External fertilisation: both sexes can invest equally
Genetic asymmetry between sexes:
Internal fertilisation does not guaranty genetic fatherhood
In which case do you expect paternal care?
Parental investment into embryo
External fertilisation: more species with paternal
care.
Stickleback: territorial
Reversed sex roles
Male care is frequent in fishes, rare in other taxa
Some exeptions (waterbugs: Abedus,
Belostoma)
• Female deposits large eggs on male’s back
• Eggs are attractive for other females
• Male can carry more batches
Tasks for the male
• Aeration
• Humidity
• Male should balance
between the two
• Why paternal care?
Why paternal care?
• Predators -> large body size -> large eggs
• Producing large eggs is costly for females
• Carrying eggs would significantly reduce
female’s fitness (and that of males)
• Male can easily carry several batches
Large water bugs: parental care. Small bugs: no care
IN THE NEWS
Peru poison frog reveals
secret of monogamy
Genetic tests have revealed
that male and females of one
species of Peruvian poison
frog remain utterly faithful.
After mating, a female frog
lays her eggs on the surface
of leaves
The male frog then takes away the tadpoles that hatch, carrying
them one by one on his back to pools of water which collect in
bromeliad leaves high up in the branches of trees.
If the pools were bigger, the frogs wouldn't have to remain
faithful…
Monogamy:mate assistance hypothesis
When partners staying together can have higher fitness
than searching for new mate
• Example: hippocampus. Male carries brood in pouch
for 20 days
• Daily greeting ritual: female checks the condition of
brood
• Male carries the brood until new eggs are produced
by female
Monogamy: the female
egg laying rate is limiting.
Seeking new partner
would be costly.
Females are not choosy
Sex role reversal in pipefish
Males care for brood in pouch
During the time of male pregnancy
females of some species can
produce enough eggs to fill 2 male
pouches.
Male pouch space is in short supply
Polyandry: males in these species tend
to be choosy (they select females
which provide the most eggs)
Parental investment into embryo
Asymmetry between sexes: females invest more into
embryos in species with internal fertilisation (mammals,
birds, some fishes)
Difference between birds and mammals
Birds: once the eggs are layed both sexes could incubate
them
– cca. 90 % of bird species are socially monogamous
–
cca. 5% in mammals
Monogamy
Factors ensuring monogamy:
Mate guarding: if distribution of females is sparse it is
impossible for males to monopolise more than one female
Mate assistance: if investment of both parents is possible and
needed for offspring survival
(monogamy widespread in precocial or in altricial birds?)
Female-enforced monogamy: female prevents polygyny to keep
male’s help
These 3 hypotheses are not mutually exclusive
Female enforced monogamy
• Female needs help of male
• Prevents male to provision to more females
• Example: Yellow-breasted chat (Icteria virens)
11.4 Dual11.4
mate-enforced
monogamymonogamy
11.4Dual
Dual
mate-enforced
monogamy
mate-enforced
Agression toward modell birds
(A) Males
(B) Females
Mate assistance in birds
90% of birds species are socially monogamous
Extra pair copulation (EPC) in 90% of
monogamous species
Mate assistance in mammals
Example: California mouse (Peromyscus californicus)
• Monogamous.
• Some of the males were removed in experiment.
• In presence of males 3 times more offspring
• („Males exhibit apparently more sexual fidelity than females.”
Gubernick and Nordby)
Interesting
California mouse: postpartum estrus
The male should defend territory, copulate and
take care of offspring
Aggression and copulation: high level of
testosterone
Testosteron normally inhibits paternal care
Testosterone is converted into estradiol in some
brain areas by aromatase !!!
Trainor and Marler (2001)
Mechanism of sexual fidelity
Montane vole (Microtus montanus) polygynyous
Prairie vole (Microtus ochrogaster) monogamous
Social attachment: „love”
Experiment
Copulation then test. Female in mid chamber. Which
females are in love?
Prairie vole females spend more time with partner
Mechanism of sexual fidelity
No difference in circulating oxytocin level
But: more receptors in brain
N. accumbens, prelimbic cortex
Mechanism of sexual fidelity
Experimental manipulation of OXI receptors
Fidelity developes following first copulation
Blocking OXI receptors would inhibit development of
fidelity?
In which brain areas?
OXI antagonist in different brain areas + control
Copulation
Test of fidelity
Mechanism of sexual fidelity
N. accumbens, prelimbic
cortex, caudate putamen
Green columns: time spent with partner
Yellow columns: time spent with stranger
Love is in the
Nucleus accumbens and prelimbic cortex
Mechanism of sexual fidelity
Males: Vazopressin receptors
Prairie vole males express
more V1aR (vazopressin 1a
receptor) in
ventral pallidum (VP)
Mechanism of sexual fidelity
Prairie vole males
without mating or
before copulation
received
arginine vazopressin
(AVP), cereb. spine
fluid (CSF) or V1
receptor antagonist
AVP alone without copulation increased pair fidelity
Antagonist prevented fidelity even in mated males
Paternel help in apes
How to explain monogamy?
• Assistance against infanticide?
– Females are dominant in most monogamous sp.
• Do males care for the offspring?
– Males may invest in polygynous sp.
• Help in carrying the young?
– Independent of the number of offspring
Mate assistance:
offers protection
against infanticide
Purple: females carry young
male-female pair
Yellow:young stays in nest
males do not follow
Two exceptions:
Varecia variegata:
antipredatory?
Pongo pygmaeus:
male-female
Mate assistance:
offers protection against infanticide
a) Long lactation period, no
post-partum estrus, female
exclusively carries the
young
infanticide: 49%
b) male and female
cooperate in carrying the
young
c) young stays in nest
Evolution of mating systems
Monogamy: one male – one female
Polygyny: one male – more females
Polyandry: one female – more males
Polygynandry (Promiscuity):
more males – more females
Wilson's Phalarope
(Steganopus tricolor)
polyandry
Poliandry
Rare
• Example: spotted sandpiper
(Actitis macularia)
• Females are bigger and brighter
• Arrive earlier, occupy territories
• Competition among females
• Males settle in females' territories, mating
• Female lays 4-4 eggs to nests of several males
• Males incubate the eggs, protect chicks
Factors favouring polyandry
•
•
•
•
•
Abundance of food at hatching
Precocial chicks need not much investment
A single parent is not worse than biparental care
Sex ratio is male biased
Sandpipers can incubate no more than 4 eggs.
Origin of polyandry
Not in the textbooks:
Oring et al. BEHECO. Observation for 17 years
26% of females are monogamous (especially young)
Type of polyandry depends on age
Yearlings: sequential resource defense
Two year old: simultanious polyandry (equivalent
proportions of resource-defense and mate-access
polyandry.)
Older females: primarily simultaneously resourcedefense polyandrous (this is in the textbooks:-)
Polygyny
•
•
•
•
•
One male mates with more females
Paternal investment is small
Common in mammals
Competition among males
Mechanism
–
–
–
–
–
Scrambling
Exhibiting quality
Defence of resource
Defence of females
Establishing hierarchy
not mutually
exclusive
Scramble Competition Polygyny
Distribution of females is sparce because of
ecological constraints
Males cannot monopolise more females
(one possible factor in monogamy)
Males can visit many females.
Males reaching more females in estrous have
higher fitness
Thirteen-lined Ground Squirrels
Males “make the rounds” visiting female burrow sites
Male remembers which females are about to enter estrus
Experiment: Remove near estrus female or control
female, not near estrus
Resource Defence Polygyny
• Male occupies and defends resource important
for females (for offspring)
• Mates with females on resource
• Bigger, better quality resource – higher fitness
Northern elephant seal
(Mirounga angustirostris)
Males: 2300 kg. Females: 640 kg.
Great sexual size dimorphism (SSD)
Bulls arrive at
terrestrial breeding
ground and fight
Northern elephant seal
Arrival mass correlates with rank
Rank correlates with
copulatory success
Selection for size (Haley et al. 1994)
Variance of success:
25-40% large size
+ age, experience
Southern elephant seal
(Mirounga leonina)
males: 3000 - 5000 kg; 4 - 6 m (length)
females: 400 - 900 kg; 2.8 m (length)
Larger than Northern species, higher SSD
(male 8 times bigger than female)
Fierce competiton
Harem size: 2-125 females
Southern elephant seal
Fabiani et al. (2004)
Harem size: 2 - 125 females (median 37)
Male mating success: huge variance (0 → 80)
Southern elephant seal
More than one male around females
Alpha male, beta male, peripheral males
Alternative male strategies:
Mating with females at periphery,
Raping females entering the sea
http://www.eleseal.org/sli/sli_socio.html
Rensch rule
High SSD – high degree of polygyny
Large body mass: higher SSD and higher polygyny
(Rensch rule)
Degree of SSD
correlates with
harem size in seals
Female defence polygyny – Red deer
Mating: September – (October)
Rut: dominant stags visit traditional rutting place
if successfull they monopolise groups of (related) females
Mating: September – (October)
Rut: stag copulates with females in estrous (vomeronasal organ)
Red deer
Male- male competition:
Deep voice – large body in most species
Trick in red and fallow deer: lowered larynx
Fitch and Reby: The descended larynx is not uniquely human
As in humans...
Costs? More men than women die chawking in restaurants…
Red deer
Male- male competition:
Roaring contest
Clutton-Brock: replay experiment
Frequent roaring deters weaker
stags.
Frequent roaring takes energy,
indicates strength
Red deer
Male-male competition:
Parallel walk
Antler fight
Red deer
Male-male competition:
Parallel walk
Antler fight
Decision points during contest
Red deer
Male-male competition: why so fierce?
(Mortality can be up to 10%)
Most fighting around
conception date.
Female-defence
polygyny: harem
Fights and injuries:
what is to be gained?
Red deer
Advantages of
fighting for
females
Hinds:
0-14 calves
Stags:
0-24 calves !!!
No choice for
males
but to fight for
females
Males are not so
picky...
polygynous males
Low threshold for
mating
Red deer
Reproduction succes depends on age: 7-11 year old stags
Red deer
Limits of dominance
Large body
Large antler
Sexual dimorphism: females develop earlier, males keep growing..
Red deer
Mortality of stags and hinds
Red deer
Femal strategies
Fertility depends on condition
Red deer
Femal strategies
Cost of breeding
Carcass weight,
kidney fat,
rump fat.
Condition of hinds
with calves
is poor.
Calving: every second
year on average in
poor
habitats
Red deer
Trade-offs
You can’t have everything
Natural selection: favors high reproductive success.
Among males:
competition (ritual and direct fight)
high mortality
males in poor condition do not breed.
Among females: less competition
higher mortality with calves
females in poor condition do not breed.
Would you have a boy, or a girl?
Hinds in good condition (high rank) have more sons
Gomendio (1990) Nature
More fertile males produce more male offspring
Gomendio et al (2006) Science
IN THE NEWS
(2010)
…in 2003 showing that expectant mouse moms who consume lowcalorie diets tend to carry more females to term than males—
indicating that male fetuses are the more sensitive sex in utero and
miscarry at higher rates. That made Rosenfeld wonder if diet causes
genes to behave differently in wombs with male or female fetuses.
Resource defence polygyny – Roe deer
Social groups: Solitary, forming small groups in winter.
Territory:
males mark and defend area from April
Scent glands: preorbital, tarsal, metatarsal.
Damage to trees and shrubs
Roe deer
Cleaning of antlers, marking
Carranza & Mateos-Quesada,
Oecologia (2001) 126:231–238
Fraying and scratching
around the stem of bushes
Defoliation by leaf eating
Within clearings
(2700 m2) more than 50% of
leaves were eaten
Resulting in:
Cover of shrub layer within
clearings diminished.
Function?
Enhanced visual signals?
Antipredator strategy?
Farming?
Roe deer
Territory size: few square km
Overlaps with homeranges of 1.5 – 3 females
Reproduction system: territorial (resource defence) polygyny
Mating in July – August
Male chases female, female circles
conflict
Age and antler size together affect territorial status
Hoem et al. Eur J Wildl Res (2007) 53: 1–8
Conflict between bucks seldom escalates into direct fight.
Resident buck usually wins. If escapes may still hold the territory.
For resident: loss is great, takes great risk.
For intruder: gain is unlikely, does not take risk
Optimal size of territory:
Optimal: maximum profit,
maximal difference
between cost and income
Cost is increased by
size of territory
population density
Benefit is increased by
size of territory
quality of terr.
Optimal size: X or X’
Effect of density on territory
size
Kjellander et al. Oecologia (2004) 139:
478–485
Home range
Many roe: black bars
Few roe: blank bars
buck > doe
winter >= summer
Few roe > many roe
Fluctuating asymmetry (FA)
In a roe deer population
FA is high with high density
Alternative male tactics in lizards
side-blotched lizard (Uta stansburiana)
Costs of territorial defence
Common side-blotched lizard
(Uta stansburiana)
3 fenotypes
Orange: large territories
Blue: strong pair-bond
Yellow: female mimic
(stone – paper – scissors)
Survival
of territorial males
Barry Sinervo
is low
Yarrow's spiny lizard males received testosterone
capsules. How did territorial behaviour change?
több időt töltenek a védelemmel
Survival of treated males was law if no extra food was given
Costs of territorial defence
Direct costs: time, injuries, energy, prredators
Indirect costs: highy testosterone → low
immunocompetence
IN THE NEWS
Men have a hard time
being just friends.
Sexual attraction is
regarded cost more by
women than men.
Discussed in
context of
differential
reproductive
strategy
IN THE NEWS
Singing Mice May Join
Humans and Songbirds As
Vocal Learners
Male mice produce courtship
ultrasonic vocalizations
with acoustic features
similar to songs of songlearning
birds. cortex region active during singing,
includes
a motor
that projects directly to brainstem vocal motor neurons
and is necessary for keeping song more stereotyped and
on pitch
depend on auditory feedback to maintain some
ultrasonic song features, and that sub-strains with
Figure 1. Brain systems for vocalization in birds and mammals.
Arriaga G, Zhou EP, Jarvis ED (2012) Of Mice, Birds, and Men: The Mouse Ultrasonic Song System Has Some Features Similar to
Humans and Song-Learning Birds. PLoS ONE 7(10): e46610. doi:10.1371/journal.pone.0046610
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046610
IN THE NEWS
Spontaneous human
speech mimicry by a
cetacean. Ridgway et
al. (2012)
A white whale, Beluga,
imitated human voice
Resource defence polygyny in an African cichlid
African cichlid fish Lamprologus callipterus exhibits extreme sexual
dimorphism (males 13X times larger than females).
Females deposit eggs inside an empty snail shells and guards eggs and
larvae
Resource defence polygyny in an African cichlid
Males gather shells into large collections and defend them from rival males.
The more shells the male can collect the more nesting sites it provides for
females. Up to 86 shells have been recorded in one collection and up to 14
females at once. Extremely large male body size has been selected for because
it enables males to collect shells and to defend their territories.
Alternative Reproductive Tactics (ARTs)
Nest male (bourgeois male): holds territory
Dwarf males (parasitic): small male, it's size is 2% of nest male, 25% of female
•Hides in the shell with female and seeks to fertilise eggs
•92% of young are fathered by nest male
•Genetically fixed
Sneaker males: mid size males seek to fertilise eggs when nest male is busy
•Transitional, opportunistic
Alternative Mating Strategies (ARTs)
Lake Tanganyika shell-brooding cichlid Telmatochromis
vittatus,
Les Bourgeois
Sneaker males
Pirate males
Territorial males + 2 parasites
Ota et al. (2010): territorial males living under the risk of pirates
and sneakers face increased risk of sperm competition and
therefore should produce high-quality sperm
experiments demonstrated that a visual stimulus of a pirate was
enough to induce an increase in sperm longevity in territorial males
Sepia apama
4-11 males fight for one female
The largest male wins, consorts (c)
Alternative tactics: male (m) assumes
female colouration and behaviour (takes
10-15 minutes), approaches,
inseminates
Success rate 30 times in 62 observed
cases (DNA fingerprinting)
Hanlon (2005) Nature
videó
Bluegill sunfish
Treatment: small sneakers were kept
by the tank of territorial males.
Control: territorial males did not see
sneakers
Test: predatory fish was introduced to
territory in plastic sack
Data: number of attacks to predator
Male can identify own offspring by smell at fry age but not in
eggs
Lek polygyny
Fallow deer males either defend territory or move to common
rut place „lek”
LEK: Swedish leka "to play"
Each male defends and marks a small stand within lek
Females approach rut place
Dominant males in centre position copulate.
Lek polygyny
Variable: may start with lekking followed by harem holding
Lek polygyny (McElligot & Hayden, 2000, Behav. Ecol.
Sociobiol.). Life-time mating success of males:
only 10% mates
cca 3% gained
73% of all matings
Lek polygyny
Wild Turkey (Meleagris gallopavo)
Several males gather, display
cooperatively, attract females,
dominant male mates with all females.
Subordinate males can either go solo, or join coalition.
What's in it for subordinates in coalition?
Lek polygyny
Krakauer (2005) found that cooperative subordinates are
related to dominant by r = 0.42
Kin selection
Lek polygyny
Great bustard (Otis tarda)
Males 3x bigger than females
„Extended lek”:
up to 200 males may gather
to perform their courtship displays
Larger male territories than in classical leks
Females choise is based on male quality and not on territory
Olea et al. (2010) found that found that the male birds tended
to direct their tail feathers
towards the Sun
in the morning.
Hypotheses for leks
Hotspot
males aggregate to maximize female encounter rate
Hotshot
females prefer to select mates in aggregations
males aggregate around most attractive male (hotshot)
• Expect center male to have highest mating success
Kin selection
Males aggregate around relatives
Black hole
females avoid sexual harassment by young males
Why Leks?
Benefit to
- females:window shopping
escape harassment by young males
reduced predation risk
- dominant males: many matings
- subordinate males:
kin selection AND/OR
few matings if possible AND/OR
gaining experience
Hierarchy within group
Wolves: reproduction exclusively by alpha pair
(pack is a family)
Spotted hyenas: females are dominant over
males, high ranking females mate with high
ranking males.
Pseudopenis
(enlared clitoris) is used
in communication
Pseudopenis
Pseudopenis is
displayed in
communication,
however,
Pseudopenis
Pseudopenis is
displayed in
communication,
however,
females urinate,
copulate and give
birth through
clitoris
Hierarchy + sperm competiton
Pre-copulatory choice. Cooperation of females is needed
for copulation (erect penis through flacid clitoris)
→ female choice
Hierarchy + sperm competiton
Post-copulatory sperm choice:
Females mate with multiple males
female's reproductive tract is long, and full of blind alleys and dead
ends
Only the best sperms reach the ova
http://www.vet.cornell.edu/labs/place/publications/11cunha%20et%20al.2003.pdf
(Social) mating systems
Monogamy
Polygamy: polygyny or polyandry
• Scramble competition
• Resource defence (e.g. territorial)
• Female (male) defence (e.g. harem holding)
• Hierarchy (e.g. lek polygyny)
Sperm competition (sometimes used also for polygynandry)
Polygynandry: more males with more females within a
social group
Polygynadry
Hyaenas can drive female and subadult lions off their
kills provided they outnumber the lions by a factor of
four.
At Savuti, groups of female and subadult lions lost
almost 20% of their food to hyaenas
Larger, year-round stable groups fare better
.
Polygynadry
Hyaenas can drive female and subadult lions off their
kills provided they outnumber the lions by a factor of
four.
At Savuti, groups of female and subadult lions lost
almost 20% of their food to hyaenas
Larger, year-round stable groups fare better, with males
even better. Cooper (2008) African J. Ecol.
.
Polygynadry
Males cooperate to monopolise female groups
Reproduction:
Oestrous lasts 5 days
Lioness may copulate with more than one male
Copulation rate: 1/20 min. for 5 days = hundreds of
copulations!
Why waste energy
for copulation?
.
Polygynadry
Sperm competition
• Male lions should cooperate to monopolize female group
• Direct conflict (fight) would be detrimental for male
coalition
Sperm competition: males transferring more sperm have
more offspring.
.
Polygynadry
Infaticide
Young males leave pride and form coalition
Male coalitions attempt to take over territory
When successful, males kill all cubs
.
Polygynadry
Infaticide
(10% of total lion mortality is due to infanticide!)
Good for the species? Good for whom?
Females get into eostrous when cubs die
Males can rule pride for about 2 years, first year is important
Cooperation among males and among females
DNA analysis: females of pride are allways close relatives,
males and females are NOT related
males in large coalitions are related
male repr. success increasingly skewed with increasing
coalition size → relatives act as helpers
. Packer et al. 1991.
Sperm competition
Chimpanzees
• Food is densely
distributed and plentiful
• No need for territoriality
for females
• Multi-male polygyny
polyandrogyny,
promiscuity, sperm
competition
Sperm competition
However, there are 3 male mating strategies
•
opportunistic mating, when females mate
consecutively with many males with little overt
competition between males
•
possessive mating, when a dominant male actively
attempts to prevent other males from copulating
with a female
•
consorts, when a female and male travel apart from
the group for hours or days
Sperm competition
Male reproductive success is related to rank
(priority-of access model) because dominant males
copulate with females in periovulatory period
Influence of female strategy: females may follow a
mixed reproductive strategy, being selective when
conception is likely and more promiscuous when
conception is unlikely.
Stumpf and Boesch 2004.
Females prefer dominant males
and males who shared meat
with them
Bluethroat (Luscinia svecica)
Immun response of offspring from
Extra Pair Fertilisation is superior
Sperm competition
Because of Extra Pair Copulation sperm competition
occurs in all mating systems.
Males use various techniques to ensure paternity
And to counteract those techniques
Females may be able to choose among sperms (spotted
hyena)
Copulation in the black-winged
damselfly enables the male to remove
a rival’s sperm before transferring his
own by his brush like penis
Another female counterdefence
Dunnocks (heggemus): females
eject sperm of
low-status males
Davies Nature 1983
In the news (2011)
Scientists have discovered that female
chickens have a remarkable ability to
choose the father of their eggs. Working
with feral fowl in Sweden, the scientists
found that many matings were forced, as
the roosters are twice the size of the hens.
Even when unforced, the females still exercised their right to choose by opting to
eject the sperm of males they considered to be at the bottom of the pecking order.
With the reproductive odds stacked against them, these low-status roosters have
fought back by developing larger ejaculates in the hope of increasing their chances
of passing on their genes.
.
Degree of sexual dimorphism
Forms of polygyny:
•Female defense (red deer harem)
•Lek polygyny (fallow deer)
•Resource defense (roe deer)
Degree of polygyny? Can you guess?
Pheasant
Partridge
Degree of sexual dimorphism
Degree of polygyny
Variance in male reproduction rate (number of offspring)
High variance – high competition.
Bigger males (larger antlers) win.
Antler size depends on body size (within and among species)
Degree of sexual dimorphism
Body size:
sexual dimorphism
Degree of
sexual
dimorphism
correlates
with party
size in deer
species
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?
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.
Polygyny threshold model
Polygyny threshold model: pied flycatcher
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.
Male Pied Flycatchers clearly
try to deceive females into
polygyny.
Not clear yet if females really
fooled or have no better
alternative.
male territory quality
ranked according to shade
Polygyny Threshold
e.g. Lark Bunting
choose on the
basis of M territory
Fitness benefits
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day of female arrival
Conflicts
Parents are not related genetically
Offspring is related to itself by r = 1, to parent by r = ½
Offspring demand more than what parents wish to provide
Full siblings are related to each other by r = 1/2
Parent-offspring conflict (POC)
Parents pass their genes to next generation by offspring.
Is survival of offspring equally beneficial for for parents and
offspring? Not.
Offspring carry half of the parent genes.
R (offspring – offspring) = 1
R (parent – offspring) = 1/2
Robert Trivers: parent-offspring conflict over the amount of
food and duration provisioned to young. At some point, a
parent will prefer to reserve investment for future offspring
rather than investing in the current one, while the current
offspring will disagree.
Parent-offspring conflict
Benefit: to current offspring
Cost: reduction in future
offspring.
The period of weaning
conflict ends when both
offspring and parent agree
that future investment by the
parent would be better
directed at future offspring
rather than to the current
offspring.
For full siblings,
this is when the benefit to cost ratio drops below ½.
Parent-offspring conflict
For half siblings when B/C ratio drops to 1/4, because the
current offspring is less closely related to future offspring.
Conflicts
Parent-offspring conflict
Prediction: in the conflict zone

mothers tend to reduce provisioning

offspring tend to demand more care

Offspring tend to manipulate parents (begging)

Should be an honest signal (energy demand)
Conflict among siblings
Prediction: less competition among full siblings
Parental favoritism and siblicide
In many cases parents actively discriminate
against certain offspring and either allow them to
starve or allow their siblings to kill them.
In egrets, boobies, pelicans and other birds older
siblings attack and drive younger offspring out
of the nest where they starve to death.
Young great egrets
fight while their parent
ignores the behavior.
At a Brown Booby nest the
older chick (under its parent)
has driven its smaller sibling from
The nest where it will die of
exposure and starvation.
Parental favoritism and siblicide
Parents assist siblicide
In Black Eagles incubation begins as soon as the
first egg is laid.
The first egg hatches 3-7 days before the second
and so the older offspring has a huge size
advantage over its younger sibling and can easily
kill him.
Parental favoritism and siblicide
In many birds the female spike the earlier laid
eggs with high doses of androgens causing high
aggressivity in older chicks.
Possible functions:

Insurance against failure

Environmental uncertainty