Chapter 17: Sexual selection.

1. Chapter 17: 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.

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 parentalinvestment by members of each sex are key in determining which sex will be the choosier. • Parental investment: energy and time expended on offspring.

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.

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.

13. 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.

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. • Females have less variance in reproductive success. • 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

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.

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

25. Males that dominate other males cansecure harems of females and obtainexclusive mating access to them. Bull elk and harem

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

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 sizesof males and females. In harbor seals, harems are small andsexes similar in size. In elephant seals, harems are large and males much larger than females.

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, 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.

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. • 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.

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

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).

48. Figure 10.27 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.