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. Males about 10% taller on average.

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 dimorphism Charles Darwin suggested sex provided a solution. If traits increase mating opportunities then this could more than compensate for reduced survival.

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

Amount of parental investment Differences in amount of parental investment 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 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 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 investment This disparity suggests sexual selection likely to be a more powerful influence on evolution of males than on females.

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.

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

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 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 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

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

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 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.

Contests between males to hold harems are common in mammals e.g. deer, lions, antelope, elephant seals.

Bull elk and harem Males that dominate other males can secure harems of females and obtain exclusive mating access to them. Bull elk and harem

Southern Elephant Seals Strong relationship between fighting success and reproductive success. Southern Elephant Seals

Northern Elephant Seals

Competition between males has led to extreme sexual dimorphism when males can potentially control large harems. Male and female fur seals

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

Asterisks indicate maximum body sizes that iguanas could maintain successfully in each of two study years.

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 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 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.

Territories of numbered male marine iguanas.

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 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 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 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 competition What factors influence success in sperm competition? Number of sperm produced. (lottery analogy).

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 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 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).

Figure 10.27 from Animal Behavior text Damselfly penis (note spines for extracting sperm).

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.

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 In many species males cannot monopolize females and males must advertise for mates. Females inspect multiple males before choosing one.

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

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 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 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 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 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 Frogs Playback experiment. Different length calls given from pair of loudspeakers. Female frogs allowed to choose between speakers.

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 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.

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

Female choice Both female barn swallows and gray tree frogs actively choose mates. Why do they do so?

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 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 genes Measured five aspects of offspring performance: larval growth time to metamorphosis mass at metamorphosis larval survival post-metamorphic growth

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 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 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.

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.

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.

(female rejected the food item)

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

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 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 roles. Other “sex-role reversed” species include: jacanas moorhens phalaropes spotted sandpipers giant waterbugs some katydids

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

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 plants In herb Wurmbea dioica males make larger flowers than females. Pollinated by bees and Vaughton and Ramsey (1998) found larger flowers were visited more often than smaller flowers.