Download Chapter 11





Males and Females of a species look
different \
Does not make sense in light of natural
selection
Natural selection pressures apply equally
to male and female so ………
Why aren’t both sexes selected for in the
same way



Darwin recognized that individuals differ in
their success in obtaining a successful mating.
He called this sexual selection
Sexual selection is differential reproductive
success due to variation among individuals in
success at mating.
Evolutionarily speaking failure to mate and
leave offspring is equivalent to dying young

Females typically make a
larger parental investment.




It takes more effort to produce
eggs or pregnancies than to
produce sperm
in 90% of mammals the female
provides significant parental
care while the male provides
none
In most species neither parent
cares for the young but…
Females make a much larger
investment in constructing
the young


determined by number of eggs she can
make or pregnancies she can carry
Not determined so much by the number of
males she can convince to mate with her


more likely to be limited by the numbers of
females he can convince to mate with him
Not as much by the number of sperm he can
provide



Access to females will
be limiting factor to the
success of the male but
access to males will not
be limiting to females’
success (example of the
rough skinned newts in
text page 405-406)
Leads to an asymmetry
in the fitness limits of
the two sexes ….
which leads to different
mating behaviors by the
two sexes.



Males should be competitive with other
males
Females should be choosy in the males they
select
These differences are not inherent to
maleness and femaleness so much as they
are dictated by survival and fitness.

Intrasexual – where males can directly
monopolize access to females –



males will fight with each other.
Females mate with the winners
Intersexual –Where males cannot control
access to females –


the males advertise for mates
the female chooses


Combat
Sperm Competition

Infanticide

large body size
 weaponry
 armor




Females make a large investment in
producing and raising young (20% of body
mass into the eggs themselves) digs nest,
buries eggs and guards a few days
Males do nothing
In Iguanas, males can monopolize females
by staking out territories in which the
females graze and sun themselves
Females are not choosy
Territories are only a few square
meters.
Numbers identify the males which
own the territory.
59 and 65 were the most successful
males
Prime territories are near the water
Males fight over
territories, winner takes
all females in the
territory




Although large body size is disadvantageous in
and of itself on the islands because it takes so
much energy to maintain a larger body size
In fact selective forces are always working
against larger body size
Male iguanas get big because bigger males get
more mates and pass on their big-male genes
Sexual selection actually works in opposition to
natural selection in order to choose for
characteristics which give males a chance to
mate successfully



Occurs with internal fertilization where one
female mates with more than one male
within a short period of time
Sperm are in a race to the egg
Animals in this situation typically have
larger ejaculates with many more sperm
than males that are not in this type of
competition ( Example of the medfly in the
book)




prolonged copulation
copulatory plugs
application of pheromones to the female to
reduce her attractiveness to other males
Scoop out the sperm left by a previous
suitor



Animals that live in groups. Such as a pride of
lions
Pride composed of a group of closely related
females with 2 or 3 adult males related to each
other but unrelated to the females
average time a male holds a pride is a little
over 2 years


When a new male moves in to a new pride, he
will usually kill any young who are still
nursing. 25% of cub deaths in the first year are
due to infanticide. (10% of all deaths)
Spontaneous abortions also happen often when
new males move in. Female cuts her losses
because cub would be killed shortly after birth
anyway




Based on Female Choice
Female choice leads to elaborate displays by
males
Occurs when males cannot monopolize the
access to females
Females are highly selective in these situations


male and female build nest together and both
feed the chicks
since they both care for the young seems to
equalize parental investment but other factors
suggest they do exhibit sexual selection
not a monogamous species
sexual dimorphism suggests that there is
female choice involved
The quality of the potential parents may vary




He displays his tail while perching and flying
to the females after he sets up a territory and
builds a nest
Males with elongated tails attracted more
mates and mated more quickly. The longer the
tail the bigger the difference.
Also showed that if the female mated with a
shorter tailed male they were more likely to
seek other males to mate with as well
Also the longer tailed males were more likely
to convince more than one female to mate with
them



Males advertise with calls both the length and
speed of calls seem to be relevant to the female
choice
When males hear other males calling nearby
they increase both the speed and length of their
calls
Researchers have observed females actually
going to the more distant caller suggesting
selection was occurring



Used loud speakers to present calls to frogs
75% of females preferred long calls to short
calls, even when short calls were louder
Also 72% of females went past the closer
speaker to the more distant speaker which was
giving longer calls


Perhaps males that give better displays are
genetically superior . (This is also called
honest advertising)
This was tested in an experiment with the
Gray Tree frogs
Frogs from the two groups were maternal half siblings
Five aspects of fitness related performance were
monitored
larval growth rate
time to metamorphosis
mass at metamorphosis
larval survival
post-metamorphic growth
Table 11.2



Hangingflies example
males present food to females
if she accepts the food they mate while she is
eating the food


If not enough food she breaks off and goes
looking for another male bearing gifts
What benefits are there for the female when she
chooses males bearing large gifts?
1. Provides her with more nutrients, so she can lay more
eggs
2. Saves her from having to hunt herself, hunting is
dangerous. Males die in spider webs at more than twice
the rate of females.


Possible that selection on another trait may
make a female more responsive to certain
cues from males
In these cases the female preferences evolve
first and then the male mating display
follows and takes advantage of female biases
already in place.


have poor vision rely mostly on smell and touch
Hunt by doing a “net-stance” where mite stands on
a plant by its hind legs puts its front legs out to
form a net and waits


Male water mites mimic the pray in order to
find females.
The male gets the female’s attention by
mimicking the vibration of the prey species.
If the female turns and clutches at him
he deposits the spermatophore.


Males induce females to accept their sperm
sacks by fanning the water across the
spermatophore towards the female
Moving water carries the pheromones
towards the female
The female will then
detect the pheromone
and might pick up the
sperm packet

It is possible female choice is simply arbitrary
no real selection going on for any apparent
reason.




Once a particular male advertisement is favored
by a majority of the females, selection by
females will automatically reinforce selection
for that trait in male offspring (sons)
AND......
Daughters (female offspring) will also inherit
the preference for that same trait passed on to
them. Must show that preference is genetically
based to prove this hypothesis.
This is sometimes called the “sexy sons” or
“runaway selection” hypothesis.


Females choosing more fashionable males will
produce more fashionable sons and therefore
more grandchildren than females choosing
unfashionable mates
Example in book of stalk-eyed flies neatly
shows that

Selection by females for either long-eye stalks or
short eye stalks in males can produce an
evolutionary response in the females which selects
for the corresponding eyestalk length in future
generations.




There is no one hypothesis that is better than
another.
Individual species use different strategies
all of these explanations are mutually compatible
and in many species more than one strategy may
be involved
This is a very active area of evolutionary research
today.
Sex role reversal



sea horses and pipefish
In these organisms the female lays her eggs
directly into the male brood pouch and the
male supplies the eggs with oxygen and
nutrients until they hatch
In these cases:



Males invest a greater amount of energy.
access to male brood pouches limits the
reproductive success of females
females should compete with each other and
males should be choosy



In one species of pipefish the females are larger
and have dark blue stripes and skin folds on
their bellies which appear to be advertisements
for males
Skin folds are only present during mating
season
In captivity females only develop skin folds
when males are present
Number of males tested


Females show no
tendency to
discriminate between
males
Same sex-reversed
effects are seen in
other species as well





Plants are often sexually dimorphic
The seed parent (female) makes a much larger
reproductive investment than the pollen donor.
Mating involves successful pollination
Mating success = access to pollinators
Principles of sexual selection we have been
studying would predict that the success of pollen
donors is more limited by access to pollinators
than is that of the seed parent(female)





self-incompatible
white and yellow flowers; white shows simple
dominance over yellow
study population of 8 WW (white) and 8 ww
(yellow) plants
Monitored pollinator visits to each color of
flower. ¾ of the pollinator visits were to
yellow flowers.
Measured reproductive success through both
male and female function


For females simply count the number of
plants that produced fruits containing seed.
Essentially all plants produced seed.





The measure of the males success is a bit more
tricky
Could not do it by individual plant
Had to raise the seeds produced by the yellow
seed parent and determining how many white
and how many yellow flowers
WHY?
Because if the yellow parent (recessive) had
been pollinated by a yellow pollen it would
produce yellow plants but if by a white pollen
it would produce white plants
•Remember, Yellow flowered plants got ¾ of the
pollinator visits




If male reproductive success is limited by pollinator
visits then male pollen from yellow-flowered plants
should have gotten ¾ of reproductive success, since
they received ¾ of the visits.
¾ of the seeds did produce yellow flowered plants.
So male success was directly related to the access to
pollinators and where they delivered the pollen.
but seed parents (females) had equal success in
producing seed and it did not matter which male
was the pollen provider
Thus, reproductive success of males is more limited
by access to pollinators than is the females


we would predict that dioecious plants
would have dimorphic flowers and that
??? flowers would be showier?
male
Males do whatever they can to attract
pollinators and assure that their pollen will
be successful.
•Many studies bear this out


the size of flower parts is simply designed to
protect the reproductive parts, the larger the
reproductive parts, the larger the perianth
(sepals + petals)
In animal pollinated, however, we see a
variety of strategies …..





Males also seem to have stronger odors and
more flowers per inflorescence
Not all flowers are dioecious. In this case larger
flowers are usually more successful.
Larger flowers are visited preferentially by bees
and butterflies.
Larger flowers will be visited first and their
pollen, once delivered to the female, will have a
head start on forming a pollen tube in the pistil
of the flower it pollinates.
Meanwhile, females typically receive 4 times the
amount of pollen needed to produce seed
successfully



Male flowers actually train male bees not to
visit other males
The orchid also assures that no other pollen
coming to the same female flower at a later
time will be able to successfully pollinate
How do they do all of this?






Males produce their pollen in a pollinarium which
is held by a triggered mechanism
When a bee trips the trigger the pollinarium shoots
at the bee and sticks to the bee’s back
After being shot the bees avoid going to other male
flowers
When the bee visits the female flower the flower is
smaller and rubs the pollinarium off of the bee
and it lodges on the receptive stigma
In response the stigma swells and shuts out any
further pollination