Chapter 10 sexual selection l.jpg
This presentation is the property of its rightful owner.
Sponsored Links
1 / 94

Chapter 10: Sexual selection. PowerPoint PPT Presentation


  • 235 Views
  • Updated On :
  • Presentation posted in: General

Chapter 10: Sexual selection. Males and females often are strikingly different in size and appearance (sexual dimorphism). E.g long-tailed widowbirds male is black with long tail, female is dull brown. Sexual dimorphism. Less extreme sexual dimorphism occurs in humans.

Download Presentation

Chapter 10: Sexual selection.

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Chapter 10 sexual selection l.jpg

Chapter 10: Sexual selection.

  • Males and females often are strikingly different in size and appearance (sexual dimorphism).

  • E.g long-tailed widowbirds male is black with long tail, female is dull brown.


Sexual dimorphism l.jpg

Sexual dimorphism

  • Less extreme sexual dimorphism occurs in humans.

  • Males about 10% taller on average.


Sexual dimorphism5 l.jpg

Sexual dimorphism

  • Why does sexual dimorphism occur?

  • Many of the traits seen in the showier sex seem likely to reduce prospects of survival.

  • Evolution by natural selection cannot explain showy traits.


Sexual dimorphism6 l.jpg

Sexual dimorphism

  • Charles Darwin suggested sex provided a solution.

  • If traits increase mating opportunities then this could more than compensate for reduced survival.


Sexual selection l.jpg

Sexual selection

  • Sexual selection: differential reproductive success due to variation among individuals in obtaining mates.


Amount of parental investment l.jpg

Amount of parental investment

  • Differences in amount of parentalinvestment by members of each sex are key in determining which sex will be the choosier.

  • Parental investment: energy and time expended on offspring.


Amount of parental investment9 l.jpg

Amount of parental investment

  • In general, mothers invest more heavily in offspring than fathers.

  • In 90% of mammals, females provide substantial parental care and males little or none.


Amount of parental investment10 l.jpg

Amount of parental investment

  • In general, because of difference between sexes in investment, a female’s lifetime reproductive success will be limited by the number of young she can rear.

  • In contrast, a males will be limited by the number of matings he can obtain.


Amount of parental investment11 l.jpg

Amount of parental investment

  • This disparity suggests sexual selection likely to be a more powerful influence on evolution of males than on females.


Examples of investment differences l.jpg

Examples of investment differences

  • Rough-skinned newts: males compete to mate with females at ponds. Females visit pond, mate then leave.

  • Jones et al. (2002) sampled all males and females and used molecular analyses to assign paternity to all offspring.


Slide13 l.jpg

Note scales on y-axes are not the same for males and females


Rough skinned newts l.jpg

Rough-skinned newts

  • Most males failed to mate and there was much greater variation in male mating success. In contrast, all females mated at least once.

  • Most males fathered no young and a few males fathered almost all of them. All females reproduced.


Broad nosed pipefish l.jpg

Broad-nosed pipefish

  • In pipefish and their close relatives the seahorses males provide all the parental care.

  • Male has a brood pouch in which females lays eggs. Male tends eggs until they hatch.


Broad nosed pipefish16 l.jpg

Broad-nosed pipefish

  • In this species females compete for access to males and access to their pouches.

  • As a result, males have less variance in mating success than females


Slide17 l.jpg

Note scales on y-axes are not the same for males and females


Broad nosed pipefish18 l.jpg

Broad-nosed pipefish

  • Based on the preceding data we can conclude that the sex that invests more should be the choosy sex.

  • Conversely, in the sex that invests less there should be intense competition to mate.


Forms of sexual selection l.jpg

Forms of sexual selection

  • Two ways in which process of sexual selection may develop.

  • Males may fight among themselves to control a resource important to females or to control a group of females. Male-male competition.

  • Males may advertise for females by displaying or singing: Female choice.


Slide20 l.jpg

Contests between males to hold harems

are common in mammals e.g. deer,

lions, antelope, elephant seals.


Slide21 l.jpg

Males that dominate other males can

secure harems of females and obtain

exclusive mating access to them.

Bull elk and harem


Slide22 l.jpg

Strong relationship between fighting success

and reproductive success.

Southern Elephant Seals


Slide23 l.jpg

Northern Elephant Seals


Slide24 l.jpg

Competition between males has led to

extreme sexual dimorphism when males

can potentially control large harems.

Male and

female fur

seals


Slide25 l.jpg

In seals there is a strong relationship

between harem size and relative sizes

of males and females.

In harbor seals, harems are small and

sexes similar in size.

In elephant seals, harems are large and

males much larger than females.


Male male competition among marine iguanas l.jpg

Male-male competition among marine iguanas.

  • Natural selection acts strongly on body size of male marine iguanas on Galapagos Islands.

  • Intermediate size males survive better than larger or smaller males.

  • Reason is that a large body is expensive to maintain and obtaining enough food can be difficult, even though large iguanas can harvest more food.


Male male competition among marine iguanas28 l.jpg

Male-male competition among marine iguanas.

  • Maximum male body size consistently exceeds the body weight that can be sustained, but female body weights do not.


Slide29 l.jpg

Asterisks indicate maximum body sizes that iguanas could

maintain successfully in each of two study years.


Male male competition among marine iguanas31 l.jpg

Male-male competition among marine iguanas.

  • Why is male body size larger than we would predict based on maximizing survival?


Male male competition among marine iguanas32 l.jpg

Male-male competition among marine iguanas.

  • Female iguanas lay one clutch of eggs per year and mate only once.

  • Females invest as much as 20% of their body mass in a clutch, so they invest much more than males. Males compete to fertilize females.


Male male competition among marine iguanas33 l.jpg

Male-male competition among marine iguanas.

  • Male iguanas stake out territories on rocks where females bask between feeding bouts and fight other males to defend their territories.

  • Territory holding males much more attractive to females.

  • Male mating success strongly related to his ability to hold and defend a territory that females like to use.


Slide34 l.jpg

Territories of numbered male marine iguanas.


Male male competition among marine iguanas35 l.jpg

Male-male competition among marine iguanas.

  • Territories held by males 65 and 59 were strongly preferred by females for basking.

  • Male 59 was the largest male in the colony and to claim the territory had to eject 4 other males.


Male male competition among marine iguanas36 l.jpg

Male-male competition among marine iguanas.

  • Male 59 had more than four times as many copulations as any other male in the colony.

  • For the colony as a whole mean body size of males who got to copulate was significantly larger than mean body size of all males who tried to copulate.


Male male competition among marine iguanas38 l.jpg

Male-male competition among marine iguanas.

  • Because body size is heritable and confers such a huge advantage in mating, male marine iguanas male marine iguanas have been sexually selected to have large body size despite the survival costs.


Sperm competition l.jpg

Sperm competition

  • Male-male competition may continue even after mating is over.

  • Fertilization, not mating is the goal.

  • In many animals (including humans, but rarely) a female may produce a brood fathered by more than one male.


Sperm competition40 l.jpg

Sperm competition

  • What factors influence success in sperm competition?

  • Number of sperm produced. (lottery analogy).


Sperm competition41 l.jpg

Sperm competition

  • Gage (1991) tested idea that males might adjust number of sperm adjusted depending on risk of sperm competition.

  • Experimental male Mediterranean fruit flies reared either alone or with another male. Then allowed experimental male to mate with a female.


Sperm competition42 l.jpg

Sperm competition

  • Males mating in presence of another male produced 2.5X as many sperm as males reared alone and mating in absence of potential competitor.


Sperm competition43 l.jpg

Sperm competition

  • Other male strategies for success include: mate guarding in which males deter other males from copulating.

  • Blocking female genital opening with a plug.

  • Removing other males’ sperm from female (male damselflies use hooked penis to scoop out sperm).


Slide44 l.jpg

Figure 10.27 from Animal Behavior text

Damselfly penis (note spines for extracting sperm).


Infanticide l.jpg

Infanticide

  • In some animals infanticide is practiced as a way to enhance mating success.

  • In lions males that take over a pride kill all the cubs to bring females back into estrus. Otherwise they would have to wait to mate and males usually hold prides for only a couple of years.


Infanticide46 l.jpg

Infanticide

  • Female jacanas (a long-toed bird that can walk on water lilies) also practice infanticide. Female jacanas defend territories and lay eggs for multiple males.

  • If a female loses her territory, new female kills any young or destroys eggs to free up males to tend her young.


Female choice l.jpg

Female Choice

  • In many species males cannot monopolize females and males must advertise for mates.

  • Females inspect multiple males before choosing one.


Female choice48 l.jpg

Female Choice

  • Females evaluate large number of possible male traits: display, song repertoire, physical appearance.


Female choice in barn swallows l.jpg

Female choice in Barn Swallows

  • Male Barn Swallows have longer tail streamers than females.

  • Males display their tails from small display territories and while flying.

  • Female visits several males before choosing one to nest with. Male assists with feeding of young


Female choice in barn swallows50 l.jpg

Female choice in Barn Swallows

  • If males and females cooperate in rearing young why would there be sexual selection?

  • Males might be able to secure extra-pair copulations (EPCs).

  • Anders Pape Moller tested hypothesis that tail length is a sexually selected trait and that females prefer males with longer tails.


Female choice in barn swallows51 l.jpg

Female choice in Barn Swallows

  • Moller monitored male mating success after manipulating tail length.

  • Four groups in experiment.

    • Shortened tail feathers

    • Tail cut, but glued back (control I)

    • Tail uncut (control II)

    • Elongated tail feathers


Female choice in barn swallows55 l.jpg

Female choice in Barn Swallows

  • Elongated males attracted mates more quickly and had greater mating success.

  • Females mated to short-tailed males were significantly more likely to seek extra-pair copulations from males than females mated to males with longer tails.


Mate choice in gray tree frogs l.jpg

Mate choice in Gray Tree Frogs

  • Males call to attract females. Males differ in the length of their call (number of trills given).

  • Gerhardt et al. tested hypothesis that females prefer longer calls.


Mate choice in gray tree frogs57 l.jpg

Mate choice in Gray Tree Frogs

  • Playback experiment. Different length calls given from pair of loudspeakers.

  • Female frogs allowed to choose between speakers.


Mate choice in gray tree frogs58 l.jpg

Mate choice in Gray Tree Frogs

  • 75% of females preferred long calls regardless of volume.

  • 72% of females also went past closer speaker giving short call to approach distant speaker giving longer call.


Mate choice in gray tree frogs59 l.jpg

Mate choice in Gray Tree Frogs

  • Also quantified female preferences for different calls precisely by comparing them to a standard length call.

  • Females strongly discriminated against short calls. Consistently preferred increasingly long calls.


Slide60 l.jpg

(Y-axis should read phonotaxis score not phototaxis score)


Female choice61 l.jpg

Female choice

  • Both female barn swallows and gray tree frogs actively choose mates.

  • Why do they do so?


Females get better genes l.jpg

Females get better genes

  • One explanation is that choosy females mate with males with better genes.

  • Welch et al. examined genetic superiority hypothesis.


Females get better genes63 l.jpg

Females get better genes

  • Collected eggs from gray tree frogs and fertilized half with sperm from long-calling and half with sperm from short-calling males.

  • Reared half of young on generous diet, others on restricted diet.


Females get better genes64 l.jpg

Females get better genes

  • Measured five aspects of offspring performance:

    • larval growth

    • time to metamorphosis

    • mass at metamorphosis

    • larval survival

    • post-metamorphic growth


Females get better genes67 l.jpg

Females get better genes

  • In 18 comparisons over two years of study offspring of long-calling males performed better in all 6 cases where there was a statistically significant difference.

  • Offspring of short calling males never did better.


Choosy females acquire resources l.jpg

Choosy females acquire resources

  • In many species males provide resources that benefit female and her young.

  • Females that can choose better providers reap a direct benefit in food provided.


Hangingflies l.jpg

Hangingflies

  • Randy Thornhill studied hangingflies.

  • When a male hangingfly catches an insect it releases a pheromone to attract a female.

  • Male offers prey to female. If she accepts, they mate while she feeds.


Hangingflies70 l.jpg

Hangingflies

  • The larger the prey the longer the female eats and the more sperm the male transfers. After 20 minutes male has transferred the maximal quantity of sperm.


Hangingflies72 l.jpg

Hangingflies

  • Male must offer a meal that lasts at least 5 minutes or no sperm are transferred.

  • If meal lasts less than 20 minutes female breaks off copulation.

  • At 20 minutes male breaks off copulation and seeks another female to offer the remains of the meal to.


Slide73 l.jpg

(female rejected the food item)


Hangingflies74 l.jpg

Hangingflies

  • Female preference for males providing large meals benefits her in two ways.

  • 1. Provides nutrients and energy that allows her to lay more eggs.

  • 2. Saves her from having to hunt for herself. Hunting is dangerous. (Males twice as likely to be caught in spider webs.)


Choosy females may have preexisting sensory biases l.jpg

Choosy females may have preexisting sensory biases.

  • Females use sense organs for tasks other than mate selection.

  • Males may exploit sensory biases of females that make them particularly responsive by incorporating these stimuli in their displays.


Choosy females may have preexisting sensory biases76 l.jpg

Choosy females may have preexisting sensory biases.

  • Proctor (1991,1992) work on water mites.

  • Tiny freshwater animals that catch copepods. Very poor vision. Depend on smell and touch.

  • Females hunt copepods by standing on four hind legs with four forelegs spread in net-like fashion.


Water mites l.jpg

Water mites

  • Female mite stands in position until she detects vibrations then turns towards them and clutches.

  • Mating does not involve copulation. Instead male deposits a spermatophore and tries to induce female to accept it.


Water mites79 l.jpg

Water mites

  • When male smells a female he approaches female while lifting and vibrating his front legs. Frequency of vibrations similar to copepod frequency.

  • Female turns towards male and clutches. Male then fans legs which carries pheromone towards female who may then pick up the spermatophore.


Diversity in sex roles l.jpg

Diversity in sex roles.

  • In a few organisms males invest more in offspring than females do.

  • In pipefish male brood eggs in pouch. Limiting resource is time because females can produce eggs faster than males can rear them.


Diversity in sex roles82 l.jpg

Diversity in sex roles.

  • Because males invest more, expect they should be choosy.

  • In pipefish, Nerophis ophidion, females are larger than males and have larger skinfolds.

  • In paired-choice tests females did not discriminate between different sized males.


Diversity in sex roles83 l.jpg

Diversity in sex roles.

  • However, males are choosy and prefer larger females and those with larger skin folds


Diversity in sex roles86 l.jpg

Diversity in sex roles.

  • In another pipefish, Syngnathus typhle, males and females are similar is size and appearance.

  • Females can change color to emphasize zig-zag pattern on sides.


Diversity in sex roles87 l.jpg

Diversity in sex roles.

  • Females competing over males display their dark colors.

  • Females initiate courtship and mate more readily than males.

  • Males are choosy. Males prefer females showing fewer black spots (which indicate parasitic infection)


Diversity in sex roles90 l.jpg

Diversity in sex roles.

  • Other “sex-role reversed” species include:

    • jacanas

    • moorhens

    • phalaropes

    • spotted sandpipers

    • giant waterbugs

    • some katydids


Sexual selection in plants l.jpg

Sexual selection in plants

  • Male flowers produce pollen. To fertilize a female flower pollen must be transported to it.

  • Males must attract pollinators to transport their pollen to female flowers.


Sexual selection in plants92 l.jpg

Sexual selection in plants

  • Expect male flowers to be more attractive to pollinators than female flowers.


Sexual selection in plants93 l.jpg

Sexual selection in plants

  • Study of wild radish plants found that males with bright yellow flowers more successful at attracting pollinators than males with white flowers.

  • Flower color did not affect female reproductive success.


Sexual selection in plants94 l.jpg

Sexual selection in plants

  • In herb Wurmbeadioica males make larger flowers than females.

  • Pollinated by bees and Vaughton and Ramsey (1998) found larger flowers were visited more often than smaller flowers.


  • Login