Evolution Lecture 13: Sexual Selection

1. Evolution Lecture 13: Sexual Selection

2. Sexual Dimorphism • Differences between males and females=dimorphism • In this case selection is operating differently in males and females • Plumage in male birds is often brighter than females, size is often larger in males, etc • Occurs in a great variety of organisms

4. More examples here: https://www.mnn.com/earth-matters/animals/blogs/9-most-dramatic-examples-sexual-dimorphism

5. Human Dimorphism: for 200+ human societies, men vs women heightAlso obvious looking at non-human primates too

6. Why sexual dimorphism? • Can reduce competition among sexes (each sex is better adapted to a more specific niche) • Produce different ecological niches for each sex • This can still be explained via natural selection

7. Males and females feed on different flowers; the beak of the male vs female allows each sex to better exploit the food source they use.

8. Can also be related to differences in intrinsic reasons, such as reproduction. The snout of the female weevil allows the female to bore a hole into a flower to deposit eggs.

9. But some dimorphic traits are harder to explain via natural selection If this tail improves survival, why only in males? And, how could this tail possibly improve survival? (Energetically expensive, slows you down so predators can catch you…)

10. Sexual Dimorphism-Sex provides another explanation • Sex provides the solution • Sex complicates life by adding a challenge of finding a mate and persuading them to reproduce • Differential reproductive success due to variation in success at getting mates = sexual selection • https://www.youtube.com/watch?v=4j7GSu99LmY

11. Consider Parental Investment • Energy and time expended both in constructing and caring for offspring=parental investment • E.g., for many mammals, females generally have the largest investment • Males produce cheap gametes (relative to eggs) and often provide little parental care • Therefore it is important for a female to choose the best male • Since male gametes are cheap, and females can always find a male to reproduce with, then access to mates will be the limiting factor for males • Therefore, sexual selection will generally be a more potent force for males

12. Even in species where there is no or little parental care, females invest more in offspring Energy used to make eggs and sperm (y axis) plotted against body mass (x axid)…eggs are expensive and sperm are cheap

13. So females are often choosey in which males they mate with, considering they typically cannot breed with as many mates and are putting more investment into every breeding/each offspring… Females have reproductive limits that males often do not. Male strategy is to mate with many females since gametes are cheap to make Female strategy is to pick only the highest quality males, since gametes are expensive In these cases, the access to mates is what limits the reproductive success for males but not females.

14. Limits of reproductive success • We would assume that for the most part, females will always breed and males may not • Experiment that genotyped fertilized eggs from newts and parents revealed:

15. For male newts, more mates means more offspring!

16. Role reversal If males take care of the offspring or put in the greatest investment, then we see the opposite trend This is true in Broad-nose pipefish (seahorse family), where males are the limited and choosy sex, rear the young, and choose the females.

17. Pipefish: Males are blue, females red

18. Assymetric limits on fitness and behavioral consequences • For males reproductive success is limited by access to females • Therefore, we expect males to compete among themselves for mates • If fitness for females are not limited by the capacity to mate and females provide a large investment of resources making and rearing offspring, then females will be choosy (meaning males will compete for access)

19. Intrasexual Selection • Males often fight with each other using various structures or methodologies • The females often mate with the winners • We use the term intrasexual selection to describe competition within same sex

20. Intersexual Selection • If females choose the showiest males or the best display (singing, dancing) then we describe this as intersexual selection • The determination of reproductive success is determined by interactions between the sexes

21. Male-Male competition • Combat • Satellite Males (sneakers) • Sperm Competition • Infanticide Males may either control access to good resources (territory) or females directly or fight to show off to females https://www.youtube.com/watch?v=Gy0yuOSUSEs

22. Combat among marine iguanas

23. Combat among marine iguanas Large iguanas lose more energy while foraging than smaller iguanas Selection tends to favor medium-sized iguanas Males get bigger than optimum for survival though…

24. Larger more aggressive and combative males are better territory owners and thus mate more 59 and 65 favorite sunning spots for females

25. Sperm competition • If an animal has internal fertilization, and if females mate with more than one male within a short period of time, then the sperm will be in a race to the eggs • Males may compete with the size of the ejaculate • Male Medditeranean flies raised around other males produce 2.5 times more sperm compared to those raised alone individuals Other strategies: • Guard mates • Prolong copulation • Deposit a copulatory plug • Apply pheromones to reduce female’s attractiveness • Produce specialized structures to scoop out competitors sperm (damselflies)

27. Infanticide Competition may occur even after conception between males • Lions form prides with the core revolving around the females and related males • Newly mature males usually move on to new prides • Very difficult • If they displace the resident males, they generally kill the cubs that are not weaned • Females return to reproductive state 8 months earlier with no cubs to nurse • The females make the best of a bad situation, by: • Defending cubs • Abort pregnancies

28. Female Choice Males may not be able to control resources or directly monopolize females • Females choose • Females choose males that apparently have the best genes • Choose winning male • Females may acquire resources (gifts) • Preexisting sensory biasis • Sexy-sons hypothesis

29. Widowbirds Males have long tails, females brown and no extra tails Researchers altered tail-lengths of birds prior to nesting season (some left long and some shortened) Recaptured birds throughout season

30. Widowbirds Males with long tails lose a lot more weight throughout season

31. Widowbirds Short and long tailed males set up similar territories Females prefer the long tails though

32. Female Choice Why do females choose certain (non-survival related) traits? Can be arbitrary to start with (and “runaway”)

33. Female Choice May indicate other signs of vitality though (if you grow long tails, you are healthier, have “Good” genes resistant to parasites?) Will pass on to offspring and those sons will get a lot of mates (sexy sons hypothesis)

34. Female Choice Treefrog calls indicate good genes

35. Female Choice Offspring (with the same mother) of long calling males do better

36. Female Choice Hangingflies-males present a gift, the longer it takes her to eat it, the longer she copulates

37. Sexual Selection on Females Females may be polyandrous (mate with multiple males) More successful when female has multiple mates (bars have number of females per category)

38. Sexual Selection on Females Male pipefish choose larger females Prefer large skin folds Prefer few parasite spots Males avoid spotted females even if the spots are faked with tattoos

39. More Sexual Selectionhttps://www.youtube.com/watch?v=hx-Q1k_9SwA