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Chapter 11.2: Sexual selection. Males and females often are strikingly different in size and appearance (sexual dimorphism). E.g long-tailed widowbirds.

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Presentation on theme: "Chapter 11.2: Sexual selection. Males and females often are strikingly different in size and appearance (sexual dimorphism). E.g long-tailed widowbirds."— Presentation transcript:

1 Chapter 11.2: 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. Peacock male is brightly c0lored with an enormous tail



4 Sexual dimorphism Less extreme sexual dimorphism occurs in humans. Males about 10% taller on average.


6 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.

7 Sexual dimorphism Charles Darwin suggested sex provided a solution. If traits increase mating opportunities then this could more than compensate for reduced survival. The goal of natural selection is to maximize reproductive success not life span. An individual that dies young but produces many babies is more successful (from the point of view of natural selection) than one who lives a long time but produces fewer offspring.

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

9 Amount of parental investment Differences in amount of parental investment by members of each sex is the key factor in determining which sex will be the choosier. Parental investment: includes all of the energy and time expended on rearing an offspring. Parents also take on a lot of risk e.g. in defending offspring from predators.

10 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. Because females invest more in the offspring they are picky about which males they mate with.

11 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.

12 Amount of parental investment This disparity suggests sexual selection likely to be a more powerful influence on evolution of males than on females. Because they must compete for matings males will as we shall see be selected to either fight to control females (male-male competition) or to be very attractive to them (female choice).

13 Examples of investment differences Rough-skinned newts: males compete to mate with females at ponds. Females visit pond, mate then leave. Females expend a lot of energy in producing eggs and so are picky about which males they choose to mate with. Males only contribute sperm and will mate with any female given the opportunity. Jones et al. (2002) sampled all males and females and used molecular analyses to assign paternity to all offspring.

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

15 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.

16 This pattern is true in many species. Males have highly variable reproductive success. Most males fail to mate, but a few are very successful. Females have less variance in reproductive success. Essentially all females are able to mate, but there is not much difference between females in how many babies they have. For example, in bitterling fish (next slide)


18 However, it is not always males that have the greatest variation in reproductive success. For example, in seahorses and pipefish males have less variance in reproductive success because they care for the babies and females have to compete for a limited supply of childcare.

19 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.

20 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

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

22 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 and higher variance in reproductive success [some individuals secure many mates, most males obtain few or none.]

23 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. Females choose only the most attractive males to mate with and reject most males.

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

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

26 Southern Elephant Seals Strong relationship between fighting success and reproductive success. The bigger stronger males have higher rank and they mate more often.

27 Northern Elephant Seals

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

29 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. The bigger the prize (in terms of mating opportunities) the more strongly selection has favored the evolution of bigger males.


31 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 (seaweed is a relatively low quality food), even though large iguanas can harvest more food.

32 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. Bigger males are common even though their life expectancy is reduced.

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


35 Male-male competition among marine iguanas. Why is male body size larger than we would predict based on maximizing survival?

36 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.

37 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.

38 Territories of numbered male marine iguanas.

39 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.

40 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.


42 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.

43 Sperm competition Male-male competition may continue even after mating is over. Remember fertilization, not mating is the ultimate goal. In many animals (including even humans, but very, very rarely) a female may give birth at one time to multiple offspring and these may have different fathers.

44 Sperm competition What factors influence success in sperm competition? Number of sperm produced. (lottery analogy). The more tickets (sperm) that an individual has in the lottery the better chance of winning (i.e. fertilizing the egg)

45 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.

46 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.

47 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). Increasing swimming speed of sperm. Faster sperm can get to the egg quicker and sometimes sperm collaborate to swim faster.

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

49 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.

50 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.

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

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

53 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

54 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.

55 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




59 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.

60 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.

61 Mate choice in Gray Tree Frogs Playback experiment. Different length calls given from pair of loudspeakers. Female frogs allowed to choose between speakers.

62 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.

63 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. They consistently preferred increasingly long calls.

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

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

66 Females get better genes One explanation is that choosy females mate with males with better genes. Welch et al. examined genetic superiority hypothesis.

67 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.

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



71 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. Thus, females who chose males who gave longer calls received better quality sperm and produced higher quality offspring.

72 Choosy females acquire resources In many species males provide resources that benefit female and her young. Resources can take many forms. It could be food or it could access to a territory, which contains areas to feed, and safety from predators or safe nesting nests Females that can choose better providers reap a direct benefit in resources provided.

73 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.

74 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.


76 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.

77 (female rejected the food item)

78 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.)

79 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.

80 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.

81 Diversity in sex roles. However, males are choosy and prefer larger females and those with larger skin folds



84 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.

85 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)



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

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