Mating Systems and Sexual Selection

1. Mating Systems and Sexual Selection

2. What is the meaning of life?What is the purpose of life? As far as I can tell, life has no meaning. It is simply an artifact of the laws of physics and chemistry. The purpose of life is simple: Do it again.

3. A key to understanding mating systems, is understanding secondary sexual characters. These characters generally increase the male�s chance of obtaining copulations, and thus, his chances of reproductive success. They may reduce his probability of survival.

5. Dimorphic traits are a consequence of sexual selection. There are 2 components: There may be competition amongst males for access to females. Female Choice! (This idea, that females chose males, was controversial until recently).

6. Mating systems and sexual selection. Sexual selection is a form of directional selection. Sexual selection acts more strongly on males than on females. Why? Relative energetic cost of reproduction for males and females. Difference in the advantage of multiple matings by males and females. (a males reproductive output is a direct consequence of number of matings)

7. Male � Female differences Females invest a lot of energy in eggs and care for young. Females have relatively limited reproductive opportunities. Sperm are relatively inexpensive, and males usually invest little in offspring. Males are limited only by access to females.

8. Male � Female differences Variance of reproductive output for females is low. Variance of reproductive output for males is high. Overall mean for both sexes is probably equivalent. Males are more likely to by polygynous / promiscuous.

9. Mating systems Usually polygynous Short breeding season and large aggregations of females. Results in explosive mating assemblages and scramble competition among males for females.

10. Mating systems Breeding season long � females dispersed. A) find female and guard her from other males until mating. B) call in females via advertisement (eg leks) C) if necessary resource is limited, establish territory

11. Explosive mating aggregations Common amongst species that breed in ephemeral ponds: Scaphiopus. Favored by short life of the pond. Also favored by possibility of cannibalism by older individuals. Males usually arrive at pond first. It is a mad scramble for anything that moves.

12. Arrival of wood frogs at a temporary pond. Notice in both years, males arrive first and in greater numbers than females.

13. Spotted and mole salamanders produce spermatophores, that are then picked up by the females. Males produce multiple spermatophores, even on top of those of other males.

15. Snakes exhibit mating aggregations as well. Female garter snakes produce a pheromone that attracts males. Males form huge clusters, and actually ambush females as the emerge from hibernacula. If it moves, mate with it. Males deposit a waxy plug in the cloaca that makes the female undesirable to other males. Is this female choice or male-male competition? How does male and female success differ?

18. Mate searching If the species is dispersed throughout the habitat, mate searching over large areas is required. If the male finds a female, it is essential to protect the resource from other males.

20. Mate Searching When snakes emerge from hibernacula, the disperse to forage, and are for the most part solitary. How then to find a mate when breeding season starts? Males have larger home ranges than females, increasing their chances of encountering females.

21. Mate Searching Males initiate a search for females. How? Random movements? Straight line? Among prairie rattle snakes, almost all females will mate in a given season. Few males will manage to mate. They are long lived, and have multiple opportunities to mate within a life time.

22. Mate Searching The search strategy depends on the distribution of females. If females are congregated in some way (food resource etc.) than males can have smaller home range and spend more time on courtship displays. If females are dispersed, then males spend more time searching and less oncourtship etc.

23. Mate Guarding Amplexus is a form of mate guarding. Atelopus (Bufonidae) males amplex weeks before the breeding season starts in an effort to secure a female. Costs for male? Costs for female? Chase other males away. Push the female away from other males.

25. Mate guarding Size matters! Males will fight with intruding males, and the bigger animal usually wins. Male tortoises often attempt to flip other males over. Males will stay with the female for extended periods, even though this may reduce possible encounters with other females. Why?

27. Mate Guarding When females are rare, best to guard the female you have. When females are abundant, best to find another one. Some mate guarding occurs even after mating, because females can store sperm and or have multiple paternity within a single clutch. (bummer for the males)

28. Mate Guarding If you are going to guard a female, guard a large one. Why? Large females produce larger clutches of eggs than small females. Thus, your return on investment is greater. Male lizzards tend to select large females over small females, when they have a choice.

29. Mate Guarding Occurs in snakes as well. Prolonged copulation (up to several days) is a form of mate guarding.

30. The bigger snake almost always wins!Size matters.

31. Lek Systems Males aggregate, often defending small territories, in an effort to obtain females. The territories are not always near a definable resource, and are maintained only for the duration of breeding. Amphibian leks differ from avian leks because mating must take place in association with water.

32. Leks Since territories are not based on resources, females must choose males based on other male qualities, like display posture, coloration, vocalization etc. Triturus sp. Males form leks, and are alos amongst the most dimorphic of all salamanders. Males display not only w/ color, but also with posture.

34. Leks Many anurans form leks. The males form choruses near water or in water, but not necessarily by oviposition site. Female choice is based on call qualities. When chosen, the male enters amplexus with the female, and she carries the male to the oviposition site.

35. Leks Nile crocodiles form leks, American alligators do not (mate searching). Herbivorous Iguanids generally do not form leks. Why? Low energy food source, widely dispersed, and not defensible. Quality of territory probably conveys little information about the male.

36. Leks However, marine Iguanas do form leks. Why? There is a defendable resource. Namely, basking sites where the feeding females emerge from the water.

37. ABOVE: distribution of male marine iquanas. Open circles are territorial males, closed circles are peripheral males.BELOW:male reproductive success vs. lek size for marine iguanas.

38. Leks Large marine iguana males are selected over smaller males. What does large size indicate about the male? Even when population densities are low, males form leks.

39. Leks Salamanders are most likely to form leks, because they usually have internal fertilization, and thus oviposition may be some distance from copulation site. Exceptions: Andrias japonicus, Plethodon cinereus (both use resource defense). In the case of Plethodon, females choose male territories on basis of diet (they determine whether males are eating ants or termites based on their scat).

40. Resource Defense Green frog and Bull frog both exhibit resource defense. They have extended breeding seasons, making leks difficult. They maintain territories for up to 2 mo. Territories include oviposition sites. Quality of oviposition site is more important than male quality/size, but male quality/size is correlated with territory quality. Site quality is based partly on temperature.

41. Above: Green frog combat for access to females.Below: Green frog amplexus.

42. Yes, size matters.

43. Resource Defense Insectivorous sit-and-wait lizards are territorial during breeding season. Display, courtship, and site defense. Basking sites, display perches, food, and retreats are critical. A bigger territory means more access to females, and potential polygyny (depending on female home range size). (difference between home range and territory?)

44. Above: female density vs. female home range size�duh.Below: home range ratio (male/female) relative to female home range size for territorial (solid) and non-territorial (open) lizards. Why decrease? Cost of defense.

45. The Big Picture (sexual selection-wise) If males have many opportunities in a season to reproduce, some will and some wont. The variance in reproductive success will be large. If they reproduce only once, variance will be smaller. When the variance is large, sexual selection will be stronger.

46. The big picture Compare scramble competition for mates with leks, or say, compare prairie rattle snakes with red-sided garter snakes. Use the following figure to understand the role of sexual selection.

48. Males! A determinant of male mating success is ability to persist. Reproduction is expensive (that is, calling, looking, handling etc) and when you do that, you don�t feed, and consequently lose mass. Blow your wad on one night: A. maculatum and A. texanum � produce many spermatophores even though only 1 or 2 are used � but what choice do you have? Spread your wads out � produce one now and then like plethodontids and have almost everyone count.

49. The cost of being a male marine iguana. Mating occurs in early January.

50. Size matters In explosive breeding animals, male-male combat is important, and thus male size is critical. Large males have higher reproductive success. This also results in dimorphism and allometric differences in the components that matter.

53. Components critical to fighting other males: notice the hypertrophied pectoral appendage on the male.

54. Sexual dimorphism related to fighting: compare males and females for these 2 species.

55. Female Choice Resources controlled by male. Choice based on behavioral or morphological traits is more difficult. Once the trait arises, it can be maintained and increased through runaway selection. Trait is an indicator of male quality. Sensory exploitation model.

56. Female Choice In playback experiments, it has been shown that female anurans prefer longer, louder, more complex calls (they require more energy to produce). It turns out, there have been relatively few studies of female choice in herps.

57. Alternative Mating Strategies Sneak matings Includes forced copulations. Satellite males Female mimicry Sexual interference

58. Behavior of satellite males of the naterjack toad.

59. Sexual interference

60. Sperm Competition Multiple matings by females. Multiple paternity. Few observations of simultaneous multiple matings, but some species have sperm storage.

61. Arboreal egg mass is fertilized by multiple males. There is strong selection for increased sperm production by males, and in fact, the testes of these and some rhacophorids are very large.

62. Sexual Size Dimorphism Often a consequence of sexual selection. Can also be reverse sexual size dimorphism.