Marina Kusaba
Lindsey Briggs
Natalie Weaver
Week 7 Topic - Mating Systems
Reading Summary
Types of mating systems
● Mating system: The way in which organisms develop and maintain
reproductive partnerships, determined by ecological and social variables.
● The four types of mating systems are:
(a) Monogamy- A male and female form a durable reproductive partnership,
that can last one breeding season or a lifetime. Males and females of most
monogamous species tend to have little sexual dimorphism.
(b) Polygyny - Some males mate with more than one female.
(c) Polyandry - Some females mate with more than one male.
(d) Promiscuity - Males and females may mate with more than one member
of the opposite sex, and no long-term bonds are formed.
Monogamy
● Rare among species, especially in mammals.
● Monogamous males must compromise by reducing their maximal
reproductive rate to the slower rate of a female. (Therefore, monogamous
males and females have the same fitness variances.)
● It tends to evolve when male parental investment is necessary, or when the
benefits of the male moving are too low (eg. will have a hard time finding
another female).
● A lot of birds are monogamous because eggs require incubation from 2
parents and the young require large amounts of food acquired by both parents
to survive.
● In mammals, monogamy is not as common because paternal contribution is
not as crucial for the survival of offspring and the reproductive consequences
aren't as high.
● In primates, monogamy leads to characteristic behaviors including mate
guarding, pair-bonding, and the formation of a family unit.
The “Biological Marriage Contract”
● Males will only be monogamous if there is a good chance that the offspring
is/are genetically theirs.
● This "contract" means that males offer parental investment in exchange for
female sexual access and fidelity. “Divorce” happens when one does not
uphold their part of the contract.
Lifetime vs. serial monogamy
● There are two types on monogamy in the animal world:
○ Lifetime monogamy - animals remain with one partner throughout all
breeding seasons.
○ Serial monogamy - animals switch partners with each consecutive
season.
○ Both involve shared resources, living arrangements, and economic
investments.
Infidelity (extra-pair copulations – EPCs)
● Even in monogamous relationships, infidelity occurs.
● Females can have an “extra-pair copulation” for the following reasons:
parental investment, good genes, immunity (due to different immunity
profiles).
● The cuckoo is a well-known species in which males are "cuckolded," meaning
they raise and provide for offspring that are not theirs.
● Male guarding may be done to prevent oppurtunities for females to mate with
other local males.
Assortive Mating
● Signs of health and good genes are the general criteria for mate selection.
● Positive assortment/assertive mating (choosing partners that are similar to
themself, both physically and behaviorally) is also used to select a mate. This
discourages outbreeding for optimal level of mating relatedness and produces
cooperative rearing by having similar preferances and lifestyle while raising
the young.
Dissorative mating for immunological type
● Dissociative mating occurs for the major histocompatibility complex (MHC).
● Mates that have a different genetic sequence (or dissimilar MHC sequence)
for immunities are generally preferred; this can sometimes be detected
through scent.
Sexual Monomorphism
● Courtship displays of males and females in monogamous species are frequently
nearly indistinguishable (they tend to be monomorphic).
● Animals perform courtship displays to show their readiness and capability for
commitment. These displays can continue even after mating has occurred, and is
known as pair bonding.
●
Courtship feeding occurrs when a male offers a female a piece of food, she refuses
it, and they pass it back and forth, until finally she eats it.
Polygyny
● This is the most common mating system in non-human animals. Here, certain
males attain more than one female mate, while some acquire no female mates at
all.
● The four types are: harem defense, resource defense, scramble competition, and
female choice.
The Polygyny Threshold
-The point at which it becomes more advantageous for a female to mate monogamously
in a poorer environment than it would be to mate with a male who has a better
environment/territory, but also has multiple mates with which resources must be
shared.
● It varies with female population density and competition for resources.
● Happens when a rich territory becomes too crowded to be more attractive than a
poorer territory
● Male parental investment is salient; if he invests parentally in his offspring, they
are more likely to survive if he is in a monogamous relationship (his energies are
focused on fathering a single family). If he mates with several females, though, he
may be unable to provide for his offspring in any way.
● Females agree to polygyny because the male can fertilize her egg and it is
important for the female that she be with a male that other females desire.
● These predictions are made about female mate selection:
● Females that enter less preferred breeding situations should raise fewer offspring
than females with optimum breeding situations.
● The males who achieve the first mating relationships should also be the first to
benefit from second and third mating relationships if females do indeed choose a
male based on overall territory quality; and finally,
● Females choosing to be a second mating partner of a male should do equally well,
reproductively, as females who chose monogamous relationships (Daly and
Wilson, 1983).
Harem Defense
● In this type of polygyny, dominant males acquire and keep groups of female
mates by frequently engaging in acts of aggression toward other males and, less
frequently, even toward those females in their group who exhibit signals that they
may leave the harem. The biggest, strongest males usually win the battle.
● This intrasexual selection results in sexual dimorphism (different body types and
behaviors) which typically makes males larger and more aggressive than females.
- ratio of breeding males to breeding females in a given species is known as its
"operational"/"functional"/"socionomic" sex ratio.
● Males can get hurt and even die in these battles to mate. 2 ways around this: put
on a show instead of actually fighting, and to not fight at all before their prime.
● In the later option, males will develop later than females of their own
species and once developed, will look more like a fully developed female.
This is called delayed maturation.
● Trivers-Willard Effect is the theory that more sons are born when survival
rate is high and more daughters are born when survival rate is low. This is
to maximize future reproductive interests.
Resource Defense Polygyny
-Occurs in areas where resources are highly concentrated; males monopolize these
resources and females engage in polygyny to gain access to these resources
● boundaried territories are guarded by males against invasion by other male
challengers.
● Females are allowed in only if they become mates of the territory holder. They are
attracted to males that can hold a large territory.
● Along with the providing of resources, holding a large territory helps to ensure
the success of any offspring sired.
● agility, vigilance, endurance, and the ability to advertise one's superiority and
ownership (control) of territory are more important than size and strength,
though size and strength do help in conflict situations
● Because territories that are both large and rich in resources are rare, it is difficult
to hold and retain a large, rich territory. This acts as a limiting factor in the
number of mates and offspring a male may have.
Scramble Competition
● Males aren’t always the territory holders, can be women too, or both, or the
species may even be non-territorial.
● The most reproductively successful males will be skilled at detecting females
ready to breed; successful males will also be capable navigators who can negotiate
hazards of strange terrain to reach such females
● The mate selection based on breeding capabilities rather than territory defense
typically leads to less conspicuous sexual dimorphism than resource or harem
defense polygyny.
● Decreased sexual dimorphism results in behavioral selection (i.e., based off
spatial skills, risk-taking behaviors, etc.) rather physical sex differences.
Female choice polygyny
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Female chooses a mate from many potential suitors. Females make a direct
choice about whether they want males by comparing them.
Instead of competing aggressively, males compete to see who can display
themselves as the best mate (attraction through pageantry rather aggression)
“sexy son” hypothesis/”Fisherian” model: females seek out mates who will have
male offspring who will be attractive to other females.
Because of this, The most attractive male often ends up getting all the females in
a given area, and other males do not get the opportunity to mate at all. This
means all other males are reproductive failures.
It is a real disadvantage if a female does not conform to the other females'
preferences in a system like this. Such a female may successfully reproduce, but
her sons will be unattractive to future females (compromising their ability to
mate and reproduce) and her daughters are likely to be attracted to the same
(nontypical) traits their mother was attracted to, continuing the cycle.
“runaway selection”: in such a process, traits which were once functional for
survival and thus attractive to females become attractive in and of themselves in
the long run.
The “Handicap” and “Parasite” Models
● Some traits that males develop for sexual selection, such as a peacock’s long tail,
can be a handicap because it points them out to predators.
● The handicap model says: “a male exhibiting an exaggerated trait which is in
itself a physical handicap (long, elaborate tail feathers; a huge beak, etc.) is in
effect displaying an advertisement. He is signaling to females the fitness of his
genes that enable survival, since he would need to have strong energy reserves,
sharp perceptual skills to find energy resources, and an efficient metabolism to
develop and maintain the handicapping trait, as well as swiftness and agility to be
able to elude predators despite the handicap”
● These good traits will be passed on to offspring but the handicap will not (in
females).
● The parasite model says: “when a male displays well-maintained features which
are obviously expensive metabolically, it implies that he has good genes, as well
as something else: it also reveals that he must have a healthy immune system to
be able to have this extra energy to keep up this extra expense, despite having to
spend energy to ward off not only predators he can see, but the numerous
microscopic parasites which are a constant threat to everyday survival”
● Parasite model close to handicap model, but able to protect self against
microscopic parasites
● Females make sexual choice based on strong immune systems but an animals
immune system may change from season to season
Testosterone is a handicap because it inhibits immune system functioning. But
females look for men with high levels of testosterone because it shows that they
can have a high level of testosterone and still have a good immune system.
● Symmetry is highly attractive in choosing mates; in general symmetrical animals
perform better and are stronger than asymmetrical animals
● Asymmetry features indicate that during development, the epigenetic process was
disrupted, which results in different outcomes from the genetic code.
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Polyandry
● One female mates with more than one male. Rare because it does not benefit
males.
● Father need paternity assurance
● Divided into: synchronous, classic, and cooperative. Synchronous: more than one
male mates with a female and they both do not care for the offspring. Classic:
females compete for mates and leave a mating partner for the next, males are
responsible for parenting, females are noticeably bigger and aggressive.
Cooperative: the female and 2+ males divide the parental responsibilities. There
is very little sexual dimorphism in the species that practice this form of mating.
The Reproductive Costs of Polyandry to males
● Females benefit from polyandry but males do not.
● Males invest time in caring for offspring and cannot reproduce more during that
time, so they have to make sure the female is healthy and will have healthy
offspring. Also has to make sure the offspring is his.
● Male must be healthy in order to insure that his offspring are healthy and that he
is not wasting his time incubating the eggs rather than finding another mate
● Polygynandry
● Not every species has to fall into one of these categories. Polygynandry is a highly
structured system and is a combination of monogamy, polygyny, and polyandry.
● Several females and several males in a territory mate and share in parental
responsibilities.
● A variety of primate species showed this type of mating system.
Promiscuity- No long term bonds
● In animals, promiscuity refers to absence of any pair-bonding contracts between
mates.
● Promiscuous doesn’t mean that there is no discrimination of choice in species
that use this mating strategy.
● Secondary sexual characteristics can be very elaborate showing a high level of
importance towards sexual mate selection.
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Brown-headed cowbirds are promiscuous and don’t have any parental care for
their offspring