1. Ecology Lecture 10 Life History Patterns 1

2. Topics covered (both Life History Lectures)  Sexual selection  What criteria do individuals use to choose mates?  Mating systems  How many mates does an individual have  What factors determine this?  Parental care/parental investment in offspring  Allocation of resources  Quick reproduction vs. growth & long life (r and K strategists, etc…)

3. Relevance to Population Ecology  Life-history characteristics determine  Organization of individuals in space and time  Examples…  How the population perpetuates itself  discretely vs. continuously; rapidly vs. slowly, etc…  Which characteristics will be selected for and persist within the population (due to sexual selection)  Conversely, selective pressures (abiotic factors, distribution of resources, etc..) will affect the life-history patterns observed

4. Sexual selection  Defined:  Selection for characteristics/behaviors that maximize chances of mating and producing the most, and highest quality, offspring.  A category within “natural selection”  General pattern: Male-male competition and female choice  Common pattern with many exceptions!

5. Example: Satin bowerbird mating behavior  Each male build an elaborate bower where he conducts his courtship display  Bowers located near each other  Each female visits several times, finally chooses a mate

6. Bowerbird males: # of mates  Some males much more successful than others

7. Bowerbird females: # of mates  Only ~1/3 of females have >1 mate

8. Why do males usually compete, while females choose?  Hypothesis 1, A.J. Bateman: “Eggs are expensive, sperm is cheap!”   Amount of energy invested in a single gamete is much greater for females (eggs) than for males (sperm)  Female bird may invest up to 30% of body weight in eggs.

9. “Eggs are expensive, sperm is cheap.”  Difference in investment per gamete  Example: Fairy wrens  Males have 8 billion sperm in testes at once  Females lay six eggs maximum per clutch  Female limited by egg production,  Male limited by number of mates only (presumably unlimited sperm)  Operational sex ratio skewed toward males

10. Is sperm really cheap?  It often takes a large number of sperm to fertilize a single egg, due to  Hostile environment within female  Acid  Attacks by the immune system

11. Is sperm really cheap?  Sperm competition among males  Occurs when females have multiple mates  Possibly the predominant situation  Fitness advantages for female (will explore in next lecture)  Some males may actually run out of sperm…  Garter snakes, zebra finch, blue crabs, rams…

12. What if there is no sperm competition?   In sea horses, eggs are deposited into pouches, and there is not sperm competition.  Why not?   Male sea horses have relatively low sperm counts!

13. Is sperm really cheap?  Drosophila bifurca: one sperm with long tail  Sperm tail is 20x length of his body  His testes make up 11% of his body mass.

14. Why do males usually compete, while females choose?  Hypothesis 2 (R. Trivers): Competition vs. choice is based on individual with the most total parental investment  Often the female (example: mammals)  But in some species, male makes a greater total investment

15. Gulf pipefish  While male cares for a single brood, a female can produce two clutches of eggs  male has greater total parental investment  Operational sex ratio skewed toward females.  Males choose large, ( ornamented females over small, drab ones. Female Male

16. Types of sexual selection  Intrasexual selection: maintenance of traits that assist in competition within the gender  successful mating  Intersexual selection: maintenance of traits that are attractive to the opposite gender

17. Intrasexual selection 1: Adaptations to gain access to females  Dominance behavior & characteristics  Example 1: sexual dimorphism in elephant seals

18. Male-male competition and sexual dimorphism (seals)

19. Male dung beetle, Phanaeus vindex (Rattlebox photography) Intrasexual selection 1: Adaptations to gain access to females  Weaponry for fighting with other males.   Example: dung beetle!

20. Intrasexual selection 1: Adaptations to gain access to females  Sneaker strategies  Example 1: Plainfin midshipmen  Dominant male features and behavior  Nest building, singing, guarding  Sneaker male features and behavior  No nest, no singing, just sex…  Small fish, big balls!

21. Dung beetles: two morphs  Behavioral and morphologial differences similar to midshipmen  Large, dominant males with horns defend burrows  Small, hornless males with “big balls” sneak  Midshipmen  Genetically-based differences  Dung beetles  Nutritionally-based differences

22. Intrasexual selection 2: Adaptations favoring the use of one’s sperm  Displacing or inactivating rival sperm  Damselfly “scooper” penis

23. Intrasexual selection 2: Adaptations favoring the use of one’s sperm  Displacing or inactivating rival sperm  Example: Chemical sperm inactivation in fruit flies Photo: San Francisco Exploratorium

24. Intrasexual selection 2: Adaptations favoring the use of one’s sperm  Mechanisms to avoid sperm displacement  Mate guarding (Example: many crab species)

25. Intrasexual selection 2: Adaptations favoring the use of one’s sperm  Mechanisms to avoid sperm displacement  Prolonged mating and cannibalism (example: redback spider)   Female less likely to mate with another if she eats him   He has low likelihood of finding a new mate (high predation)  Andrade, 1996

26. Intrasexual selection 2: Adaptations favoring the use of one’s sperm  Mechanisms to avoid sperm displacement  Anti-aphrodisiac (Example: Heliconius erato)

27. Intersexual selection Focus on female choice  Material benefits: Nutrition Ex: hangflies)  Length of mating time depends on quality and size of “courtship gift”

28. Intersexual selection Focus on female choice  Material benefits: Anti-predator substances  Defensive compounds in arctiid moths Photo: butterfly-conservation.org

29. Intersexual selection Focus on female choice  Ability of males to provide sufficient sperm  Female fruit flies (some species) choose virgin males Photo: San Francisco Exploratorium

30. Intersexual selection Focus on female choice  Parental ability  Cannot assess directly  May be correlated with other features of the male  Example 1: Redwing blackbird  “Epaulettes” correlated with nest defense  Courtship effort correlated with feeding effort Photo: Vancouverislandbirds.com

31. Intersexual selection Focus on female choice  Parental ability (Ex: sedge warbler)  Size of song repertoire correlates with chick weight at fledging

32. Sedge warbler (cont.)  Female sedge warblers choose on the basis of repertoire size.  Thus they choose the most fit males

33. Intersexual selection Focus on female choice  Health/Genetic quality  Example: song repertoire in great reed warbler  Females chose males with larger repertoires  This was correlated with greater offspring survival (unrelated to parental care)

34. Intersexual selection Focus on female choice  Health/Genetic quality  Example: Bright coloration of sticklebacks negatively correlated with low parasite loads

35. Intersexual selection Focus on female choice  Health/Genetic quality (bright color negatively correlated with parasite load)  Advantages to females choosing these males  Avoid getting parasites while mating   Avoid transferring parasites to young  Are choosing healthier males; their health status may be related to “genetic quality”

36. Satin Bowerbirds: multiple signals of health and fitness (and good genes?)

37. Origin/maintenance of mate choice for “exaggerated” characteristics  In some species, why do males develop what appear to be extreme traits that actually can hamper their survival?  Example: Peacock’s tail

39. Origin/maintenance of mate choice for “exaggerated” characteristics  Hypothesis 1: Runaway selection (R.A. Fisher)  Directional that takes on a life of its own  Starts as an “honest signal”  more extreme.  Mechanism: Females choose males with large tails, multiple eyespots.  the next generation has a higher proportion of these males.  Will work even if his traits are not honest signals of quality. Why?  Evidence of arbitrary choices by females (bird band example)

40. Origin/maintenance of mate choice for “exaggerated” characteristics  Hypothesis 2: Handicap or “good genes” hypothesis (R.A. Fisher)  Exaggerated trait might decrease chance of survival, only males with superior genes can survive despite the handicap  Example: peacock tail as a handicap.  In this case, a female choosing a male with these traits would be improving her fitness. (His signal is an “honest signal” of fitness.)

41. Distinguishing between Runaway Selection and “Good Genes” (Petrie)  Methods  Males of different ornamentation/tail length and randomly bred them with females  Why random?  Young raised under identical conditions and then released  Results  Offspring of the “attractive” males weighed more at day 84.  Offspring of the attractive males were more likely to be alive after two years  Which hypothesis is supported by this data?

42. Petrie’s peacock data

43. Origin/maintenance of mate choice for “exaggerated” characteristics  How extreme can a characteristic become? Under what conditions will directional selection stop? (Think about costs vs. benefits…)