Sexual Selection

Sexual Dimorphism

• 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

Sexual Selection

• 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

Differences in selection pressures between males and females

• 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

This leads to a large difference in factors that effect the reproductive success of males vs. females

REPRODUCTIVE SUCCESS IN FEMALES

• 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

REPRODUCTIVE SUCCESS IN MALES

• 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

THEREFORE • Access to females will

be limiting factor to the male but access to males will not be limiting to females ( example of the rough skinned newts in text page 377-379)

• Leads to an asymmetry in the fitness limits of the two sexes ….

• which leads to different mating behaviors by the two sexes.

There are some exceptions to note• Pipefish example on

pages 378-379

• Males rear eggs to hatching in their pouches, females lay eggs in the pouches.

• Here males invest more and femalesfemales success is limited to the number of male pouches available.

Behavioral consequences of this asymmetry of fitness limits

• 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.

Types of male/male competition

• 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

Three types of male—male competition / Intrasexual

• Combat

• Sperm Competition

• Infanticide

Intrasexual - COMBAT

• large body size

• weaponry

• armor

EXAMPLE Of MARINE IGUANAS Galapagos

• 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

Males fight over territories, winner takes all females in the territory

Territories are only a few square meters. Numbers identify the males which own the territory. 59 and 65 were the most successful malesPrime territories are near the water

The bigger the male iguana is the more likely he is to win a good 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 to choose for characteristics which give males a chance to mate successfully

Intrasexual- SPERM COMPETITION

• 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)

Other strategies developed due to sperm competition

• 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

Intra sexual - INFANTICIDE

• 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

If females are nursing cubs, they will not return to breeding condition until the cub is weaned

• 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

Male - Male Competition / Intersexual

• 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

BARN SWALLOW EXAMPLE

• 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 sexually dimorphic suggests that there is female choice involved The quality of the potential parents may vary

Male uses tail length to attract females. • 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

GRAY TREE FROGS

• 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

Experimental results

• 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

What is the motive for female choice? What benefit is gained?

Possibility #1 Choosy females may get better genes for their offspring

• Perhaps males that give better displays are genetically superior

• This was tested in an experiment with the Gray Tree frogs

Compared tadpoles from the same mother which were sired by either long-calling males or by short calling males

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 10.3 results, Long calling males have significantly higher fitness

Table 10.3

Possibility #2 Choosy females may benefit through acquisition of resources

• Hangingflies example

• males present food to females

• if she accepts the food they mate while she is eating the food

The larger the food is the longer they copulate and the more sperm that are deposited

• If not enough food she breaks off and goes looking for another male bearing gifts

• Benefits 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.

Possibility #3 Choosy females may have pre-existing sensory biases

• 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

Example of water mites • 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

When pray swim close by they cause vibrations in the water and the mite turns towards the vibration and clutches at the vibration

• 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

Other explanations for female choice • It is possible female choice is simply

arbitrary • Once a particular male advertisement is

favored by a majority of the females, selection on females will automatically reinforce selection for that trait

• called sexy sons or runaway selection hypothesis

Why sexy sons do you suppose?

• 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.

Which is the most likely reason why females are choosy.

• 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

Diversity in Sex Roles

Sex role reversal

In some species it is the male who provides all the parental care

• 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 access to male brood pouches limits the reproductive success of females

• females should compete with each other and males should be choosy

Pipefish example

• 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

Studies show that males appear to prefer larger females with larger skin folds

• Females show no tendency to discriminate between males

• Same sex-reversed effects are seen in other species as well

Sexual Selection in plants

• 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)

Wild radish example

• self-incompatible • white and yellow flowers; white is dominant • 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

Results – Female success

• For females simply count the number of plants that produced fruits containing seed.

• Essentially all plants produced seed.

Results - Male success• The measure of the males success is a bit more

tricky• Could not do it by individual plant at all • Had to raise the seeds produced by the yellow

seed parent and determining how many white and how many yellow flowers

• WHY?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

• The reproductive success of the yellow seed parents can be used as an estimate of the relative reproductive success of both parents.

– The fact that seeds are produced (and the number of seeds produced) gives reproductive success of females.

– The number of yellow versus white offspring gives reproductive success of the males.

So what happened?

Results

• If male reproductive success is limited by pollinator visits then yellow-flowered plants should have gotten ¾ of reproductive success, since they received ¾ of the visits.

• ¾ of the seeds diddid 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

•Remember, Yellow flowered plants got ¾ of the pollinator visits

If all of this is true then….

• we would predict that dioecious plants would have dimorphic flowers and that ??? flowers would be showier, and do whatever they can to attract pollinators and assure that their pollen will be successful.

malemale

•Many studies bear this out

In wind pollinated species ….

• 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 often have larger and more showy perianths even though their reproductive structures are much smaller than the female’s

• Males also seem to have stronger odors and more flowers per inflorescence

• Larger flowers are visited preferentially by bees and butterflies.

• Larger flowers will be visited first and their pollen will have a head start on forming a pollen tube in pistil of the flower it pollinates.

• Meanwhile, females typically receive 4 times the amount of pollen needed to produce seed successfully

The devious orchid

• Male flowers actually train male bees not to visit other males

• The orchid also assure that no other pollen coming to the same female flower at a later time will be able to pollinate

• How do they do all of this?

Catasetum orchids are dramatically dimorphic • 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

The end of Chapter 9