Chapter 12 Mating Systems
Chapter 12.1 Sexual Conflict And Environmental Conditions Affect The
Evolution of Mating Systems
• Mating Systems: a description of the social associations and number of sexual
partners an individual has during one breeding session.
• In some mating systems two individuals live in close association with one another
during the breeding season and are said to form a pair bond.
THE EVOLUTION OF SOCIAL MATING SYSTEMS
• Sexual Conflict: the differential selection on males and females to maximize their
• The other factor is ecological: the ways in which resource limitation and
distribution affect the fitness benefits and costs for each sex.
• Emlen and Oring’s model assumes that for a female will often be more strongly
limited by the resources she can obtain to invest in offspring than by the number
of her sexual partners.
o This idea follows from the assumption that females typically invest more
energy in offspring than males do.
• They suggest that for a female, selection will favour a mating system that provides
the greatest access to resources. This system might be monogamyone male with
one female if her sexual partner provides high levels of resources to the young,
such as highquality territory and a large quantity of parental care. It might be
polyandryone female with multiple malesif multiple males all provide care for
• In contrast, for males, selection will favour polygynyone male with multiple
femaleswen males that mate with multiple partners have higher fitness than
those that mate with a single female.
• Emlem and Oring suggest that this system might often be favoured in resource in
poorenvironments or when resources are difficult to obtain. Biparental care and
monogamy might also be favoured when predation on unattended young is high.
• When biparental care is not required, each sex benefits when the other provides
most of the parental care. When parental care is female biased, selection will
favour the evolution of polygyny. Polygyny is predicted to evolve when
environmental conditions lead to the aggregations of females, because such
aggregations are more easily defended from rivals.
o Two factors can promote female aggregations. First, females may
aggregate for reasons other than reproduction. For instance, aggregations
of females may experience lower predation than solitary females, or a
group of females may more successfully avoid harassment from males.
Female defense Polygyny: a single male monopolizes and mate
with two or more females.
o The second factor that can lead to aggregations of females is the
distribution of resources in the environment. If the resources required by females are clumped, males can monopolize females by defending
territories with large amounts of such resources.
Resource Defense Polygyny: a male defends resources and mates
with multiple females attracted to the resources.
o Sometimes resources and females may be more uniformly distributed in an
environment, rather than clumped. Alternatively, resources may be too
unpredictable in space or time, making them costly to defend.
o Leks: a location when males aggregate and display to females.
o Male Dominance Polygyny: In a lek system, only a few males mate with
Males on leks can benefit as a result of reduced predation risk, and
such aggregations may be more attractive to females, which will
increase encounters with them. Females, too, can benefit from the
existence of leks, because they reduce the time required to search
for a mate: females can quickly assess the relative quality of
different males on a lek to fin the best option.
• Emlen and Oring suggest that polyandry can evolve when it’s advantageous to
both sexes that females be freed from providing parental care, making parental
care male biased.
o One possible situation that might produce a polyandrous system is when
there is very high predation on offspring. If many offspring are lost to
predators, both sexes can benefit if a female can quickly reproduce again.
However, recall that egg production is costly.
o If a male provides more care, a female can provide less care and thus feed
more to quickly replenish her energy stores for reproduction.
• Polygynandry and promiscuity both involve multiple mating partners for each sex
but differ in regards to social associations. There are social associations in
Polygynandry, but not in promiscuity.
• Promiscuity evolves when the benefits of social living are low. Both females and
males are then solitary and pairbond formation provides no fitness benefit to
o Condition also favour its evolution when the defense of mates or resources
is uneconomical, as might occur when population density is high or when