Class Notes (1,100,000)
CA (620,000)
York (40,000)
BIOL (2,000)
s (10)

BIOL 2050 Lecture Notes - Sperm Competition, Ceratitis Capitata, Mating Plug

Course Code
BIOL 2050

of 5
Chapter 11- SEXUAL Selection
Sexual dimorphism- difference between the males and females of a species
11.1- Sexual Dimorphism and SEX
Natural selection cannot explain the differences between sexes of a species
Sexual Selection- differential reproductive success due to variation among individuals in success at
getting mates
oA theory of evolution by sexual selection: If there is a heritable variation in a trait that
affects the ability to obtain mates, then variants conducive to success will become more
common over time
Asymmetries in Sexual Reproduction
Mothers typically make a larger parental investment in each offspring than fathers
Parental Investment- energy and time expended both in constructing an offspring and in caring for it
oIt is measured in fitness
oIncreases the reproductive success of the offspring receiving it
oBut also decreases the remaining reproductive success that the investing parent may
achieve in the future by way of additional offspring
In more than 90% of mammal species, females provide substantial parental care and males
provide little or none
In most animal species, neither parent cares for the young
Mated pairs of parents just make eggs, fertilize them, and leave them
When eggs are more expensive than ejaculates-when mothers make a larger parental investment
than fathers-the factors limiting lifetime reproductive success will often be different for males
versus females
A female’s potential reproductive success will be relatively small, and her realized reproductive
success is likely to be limited more by the number of eggs she can make (or pregnancies she can
carry) than by the number of males she can convince to mate with her
A male’s potential reproductive success will be relatively large, and his realized reproductive
success is likely to be limited more by the number of females he can convince to mate with him
than by the number of ejaculates he can make
Access to mates will be a limiting resources for males, but not for females
oUnder these circumstances, sexual selection (variation in mating success)- will be a more
potent force in the evolution of males than in the evolution of females
To understand sexual dimorphism; we must quantify the relationship between number of mates
and reproductive success for both males and females, is central to the theory of sexual selection
Asymmetric Limits on Reproductive Success in Newts and Pipefish
Read Page 405-407 (top paragraph)- This example might be used on exam. Better safe than
In newts: heritable traits that are associated, in males, with failure to mate will tend to disappear,
while heritable traits associated with mating success will tend to become common
oSexual selection is a more potent force in the evolution of males
In broad-nosed pipefish: heritable traits that are associated, in females, with failure to mate will
tend to disappear, while heritable traits associated with mating success will become more common
oSexual selection is a more potent force in the evolution of females
Behavioral Consequences of Asymmetric Limits on Fitness
In newts: for males reproductive success is limited by access to mates, and at any given time
there are more males than females in the pond looking for love
oIn such cases, males will compete with each other for opportunities to fertilize eggs
For female newts: reproductive success is limited by capacity to make eggs, mating involves the
commitment of a large investment, and there is an excess of willing partners
oTherefore females will be selective about which partners they accept and which they reject
When sexual selection is strong for one sex and weak for the other we can predict that:
oMembers of the sex subject to strong sexual selection will be competitive
oMembers of the sex subject to weak sexual selection will be choosy
Intrasexual selection- differential mating success among individuals of one sex due to interactions
with members of the same sex (ex. Fighting with each other over the opposite sex); for example,
differences in mating success among males due to male-male competition over access to females
Intersexual selection- differential mating success among individuals of one sex due to interactions
with members of the other sex; for example, variation in mating success among males due to female
choosiness ; competition by advertising for their mates by singing, dancing, or showing off bright colours
11.2- Sexual Selection on Males: Competition
Sexual selection by male-male competition often occurs when individual males can monopolize
access to females
Males may monopolize females through direct control of the females themselves or through
control of some resource important to females, such as feeding territory or nest sites
Male-male competition can also occur for no apparent reason beyond simply impressing females
There are three forms of male-male competition: outright combat, sperm competition, and
Outright combat is the most obvious form of male-male competition for mates
Intrasexual selection involving male-male combat over access to mates can favour morphological
traits including large body size, weaponry, and armour
Male-male combat also selects for tactical cleverness
In iguanas: after sexual interaction, females lay their eggs and go and hide it far away from
predators, guards the eggs for a few days, and then abandons it
oTherefore parental investment by females consists mostly of producing eggs, and parental
investment by males consists entirely of producing ejaculates
Read Example Combat of Iguanas Pg. 408-412
Sperm Competition
Male-male competition does not necessarily stop when copulation is over
The real determinant of a male’s mating success is not whether he copulates, but whether his
sperm fertilize eggs
If an animal has internal fertilization, and if a female mates with two or more different males
within a short period, then the sperm from the males will be in a race to the eggs
Male-male competition can take the form of sperm competition. If a female mates with
two or more males, the male whose sperm wins the race to the eggs has higher
reproductive success.
In sperm competition from two fathers, the more sperm an ejaculate contains, the more the father
has chance of the offspring being his
In an experiment of mating medflies, two males were put in a cage with a male and in another
cage a male and a female were placed. In the first cage, both males had their turn to inject their
amount of ejaculates into the female. The female was dissected right after the process, and the
numbers of sperms released by both males were counted. It appeared that males raised and
mated in the presence of a potential rival ejaculated more than 2.5 times as many sperm as
compared to only one male mating with a female.
Large ejaculates do contribute to victory in sperm competition and that male medflies dispense
their sperm to balance the twin priorities of ensuring successful fertilization and conserving sperm
In addition to large ejaculates, sperm competition has apparently led to various other adaptations
Males may guard their mates, prolong copulation, deposit a copulatory plug, or apply
pheromones that reduce the female’s attractiveness
During copulation in many species of damselflies, the male uses special structure on his
penis to scoop out sperm left by the female’s previous mates
This strategy is proven to be highly effective
In some species of mammals, competition between males continues even beyond conception
oAn example of this is lions: Male-male competition can take the form of infanticide.
By killing other males’ cubs, male lions gain more opportunities to mate.
oRead Example on Lions Pg. 414-415
11.3-Sexual Selection on Males: Female Choice
There is a great variety of species in which male reproductive success is limited by opportunities
to mate but in which males are unable to monopolize either females themselves or any resource
vital to females
oIn such species, males advertise for mates
oFemales typically inspect advertisements of several males before they choose a mate
oSexual selection by mate choice leads to the evolution of elaborate courtship displays
oBeyond allowing females to identify a mate of the right species, male courtship displays
were thought to function primarily in overcoming a general female reluctance to mate; once
ready to mate, a female would accept any male at hand
When males cannot monopolize access to females, they often compete by advertising
for mates
Female Choice in Red-Collared Widowbirds
Read example on Female red-collared widow birds on Pg. 416-417.
oFemales find males with long tails sexy
Female Choice in Gray Tree Frogs
Read example on gray tree frogs and their mating calls on Pg. 417-419.
oFemale gray tree frogs prefer males with longer calls than males with shorter calls.
Choosy Females May Get Better Genes for Their Offspring
The displays given by males are indicators of genetic quality
Five aspects of offspring performance related to fitness:
oLarval growth rate – faster is better
oTime to metamorphosis – shorter is better
oMass at metamorphosis – bigger is better
oLarval survival
oPost-metamorphic growth – faster is better
Choosy Females May Benefit Directly through the Acquisition of Resources
In many species the males provide food, parental care, or some other resource that is beneficial to
the female and her young
Hanging flies live in the woods of eastern North America, where they hunt for insects
After a male catches an insect, he hangs from a twig and releases a pheromone to attract females
When a female approaches, the male presents his prey; if she accepts it, the pair copulates while
she eats
The larger the prey, the longer it takes her to eat it, and the longer the pair copulates
oThe longer the pair copulates, the more sperm the female accepts from the male
oIf she finishes her meal in less than 20 minutes, the female breaks off the copulation and
flies away looking for another male and another meal
oThe males giving her large gifts benefits her in two ways:
Provides her with more nutrients, allowing her to lay more eggs
Saves her from the need to hunt for herself
oIf the female is still eating after accepting all the sperm she can, the male grabs his gift
back and flies off to look for a second female to share it with
Choosy Females May Have Preexisting Sensory Biases
The pre-existing bias, or sensory exploitation, hypothesis holds that female preferences evolve
first and that male mating displays follow
Read example on Water Mites on Pg. 423.