Chapter 7: Mating Systems
Types of mating systems
 Mating system: the way in which organisms develop and maintain reproductive
partnerships. Ecological and social variables determine this.
 The four types of mating systems are:
(a) MONOGAMY. A male and female form a durable reproductive partnership. It may
last for only one breeding season, or it may last an entire lifetime. Males and females of
most monogamous species tend to have little sexual dimorphism (e.g., the males and
females are about the same size and morphology).
(b) POLYGYNY. Some males mate with more than one female (other males do not get
to mate at all).
(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--no long-term bonds are formed.
Monogamy
 Surprisingly rare among species, especially in mammals.
 For males it means: reducing his maximal reproductive rate to the slower rate of a female.
 It tends to evolve when male parental investment is necessary, or when the benefits of the
male moving are too low (ex: will have a hard time finding another female). A lot of
birds are monogamous because eggs require incubation from 2 parents.
 In mammals, monogamy is not as popular because females do a lot of the parental
investment (carry the child, nurse, etc.) and the males don’t have to. But males can
contribute by catching food.
The “Biological Marriage Contract”
 Males will only be monogamous if there is a good chance that the offspring is theirs.
 The contract means that males offer parental investment in exchange for female sexual
access and fidelity. “divorce” happens when someone doesn’t do their part of the
contract.
Lifetime vs. serial monogamy
 Lifetime monogomay: animals remain with one partner throughout all breading seasons.
 Serial monogomay: 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, males and females can be unfaithful.
 Males can gain more mating partners to reproduce with.
 Females can have an “extra-pair copulation” for reasons such as: good genes, immunity.
 The cuckoo is a well-known species in which males raise and provide for offspring that
are not theirs.
Assortive Mating
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Signd of health and good genes are the general criteria for mate selection.
Positive assortment/assertive mating is also used to select a mate- choosing partners that
are similar to someone (physically and behaviorally).
Dissorative mating for immunological type
 Dissociative mating occurs for the major histocompatibility complex (MHC).
 We prefer those that have a different genetic sequence for immunities, this can sometimes
be detected through scent.
Sexual Monomorphism
 In monogamous species, sexual selection isn’t so strong and the two sexes tend to me
monomorphic (similar in body)
 Animals do courtship displays to show their readiness and capability for commitment.
 One of the behaviors seen here is courtship feeding. Male offers the female a piece of
food, she refuses, they go back and forth, finally she accepts and they mate.
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
 It varies with population density and competition for resources.
 It is the balance of a male mating with many females and also being able to care for them.
 Females agree to this 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:
1. Females that enter less preferred breeding situations should raise fewer offspring than
females with optimum breeding situations.
2. 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,
3. 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 makes males larger and more aggressive.
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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.
Resource Defense Polygyny
 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.
 Socionomic sex ratio: the ratio of breeding males to breeding females.
Scramble Competition
 Males aren’t always the territory holders, can be women too, or both.
 scramble competition polygyny, the most successful males will be skilled at detecting
females ready to breed; successful males will also be able navigators who can negotiate
hazards of strange terrain to reach such females
 it results in less conspicuous sexual dimorphism than resource defense polygyny.
 In this competition it is behavioral rather than physical sex differences that sexual
selection tends to work on.
Female Choice Systems
Female choice polygyny
 Female chooses a mate from many potential suitors. Females make a direct choice about
whether they want male by comparing them.
 “sexy son” hypothesis/”Fisherian” model: females seek out mates who will have males
offspring who will be attractive to other females.
 It is a real disadvantage if one does not conform in a system like this. Female will not be
prosperous for long.
 “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
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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
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ward off not only predators he can see, but the numerous microscopic parasites which are
a constant threat to everyday survival”
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.
Polyandry
 One female mates with more than one male. Rare because it does not benefit males.
 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.
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.
Polygynandry
 Not every species has to fall into one of these categories. Polygynandry is a combination
of them.
 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
 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.