1. Sexual Selection and Reproductive Behaviour

2. Sex vs. Reproduction
Sex - process by which a genome is changed by rearrangement and combining of genes
Reproduction - process by which individuals are added to a population
Sex -changes in kinds of individuals in populations
Reproduction -changes in the number of individuals in populations

3. Reproductive Behaviour
1. What is ‘sex’?
2. What are some of the costs and benefits of sex?
3. Why are the sexes generally in a 1:1 ratio?

4. Reproductive Behaviour
Anisogamy
Frequency of producers
Gamete size

5. Reproductive Behaviour
Anisogamy
- lots are produced by not enough mass for cell division
- fewer produced but are competent
- very few produced but are very competent

6. Reproductive Behaviour
Anisogamy +
- lots of contact but won’t divide properly +
- will divide but fewer contacts +
- will divide but very few contacts +
greatest success
best combination of numbers and competence

7. Reproductive Behaviour
Costs of sex
1. Cost of meiosis
must combine genes with another organism
lose 50% every generation relative to asexuals
-often referred to as cost of males
- females can always do better asexually

8. Reproductive Behaviour
Costs of sex
2. Cost of recombination
take a functioning genome, split it and combine it with another one
-chance of deleterious combinations

9. Reproductive Behaviour
Benefits of sex
1. Produce new combinations
Unique offspring can cope with different niches x

10. Reproductive Behaviour
Benefits of sex
2. Muller’s ratchet
- in asexual species, deleterious mutations accumulate
Frequency of mutant gene
Mutation A
Mutation B
Mutation C
Time

11. Reproductive Behaviour
Benefits of sex
2. Muller’s ratchet
- in sexual species, such mutations tend to be eliminated
Frequency of mutant gene
Mutation A
Mutation B
Mutation C
Time

12. Reproductive Behaviour
Sex ratio -why 1:1?
Excess males
-some males won’t mate
-females that produce more females are favoured
1:1 sex ratio
-some females won’t mate
-females that produce more males are favoured
Excess females
Time

13. Darwin - Theory of sexual selection
Natural selection - Observations and deductions
Observation
- all species can increase exponentially but don’t
- more are produced than can survive
Deduction
- there is a struggle for existence
Observation
- there is variation in all organisms that is heritable
Deduction
- some are better equipped to survive than others
- advantage is passed to offspring
Differential survival = Natural selection

14. Darwin - Theory of sexual selection
Two parts
1. Intrasexual selection (Male-male competition)
All those structures and behaviour patterns employed by males to fight other males for the chance to mate with females
- claws, antlers etc.

15. Darwin - Theory of sexual selection
Two parts
2. Intersexual selection (Female choice)
All those structures and behaviour patterns employed by males to attract females
- plumage, song.

16. Reproductive Behaviour
Males and females have different reproductive interests
What does each sex have to provide to produce offspring?
Females
Males
Manufacture eggs
(expensive)
Manufacture sperm
(cheap)
2. Incubate eggs or fetus
(expensive)
3. Post partum care
(expensive)
4. Loss of mating opportunity when pregnant
(expensive)
Males invest less in any offspring

17. Reproductive Behaviour
In more general terms
Females
Difference in reproductive investment
Males
higher investment
lower investment
lower reproductive rate
higher potential reproductive rate
lower levels of mating activity
higher levels of mating activity
Biased operational sex ratio
Selection among potential mates
Competition for mates
Best mate = best fitness benefit
Achieve greatest number of matings

18. Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio
The ratio of the number of males to the number of females in the population
Operational Sex Ratio
The ratio of the number of males to the number of females in the population who are available for reproduction

19. Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio (close to 1:1)
Males
Females

20. Reproductive Behaviour
Operational Sex Ratio
Males
Females
Remove males for:
Remove females for:
1. Pre-puberty
2. Sterile
( Post-menopause)
3. Age
4. Pregnant
5. Lactating
Operational sex ratio

21. Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio (close to 1:1)
Males
Females
Operational Sex Ratio (very different from 1:1)
Competition in more abundant sex
Selectivity in less abundant sex

22. Reproductive Behaviour
Evidence??
Bateman (1948)
-Drosophila melanogaster - nearly all females mated
but not all males
-variance in male reproductive success - higher
Males
Bateman gradient
# of offspring
Females
# of mates

23. Reproductive Behaviour
Evidence?? - Lions
Females
% of breeders 10
# of surviving offspring

24. Reproductive Behaviour
Evidence?? - Lions
Females
Males
% of breeders 10
# of surviving offspring

25. Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource
e.g. Nuptial gifts

26. Intersexual Selection
Size of prey is critical
20 min
Duration of
copulation
# sperm transferred 8 25
16 mm
Size of prey
Time in copulation

27. Intersexual Selection
2. Good genes models
Female should look for best genetic complement in male
Traits should reflect genetic quality

28. Intersexual Selection
2. Good genes models
Fluctuating Asymmetry
Horns in oribi - Ourebia ourebia
Arese Anim.Beh. 48:1485

29. Intersexual Selection
2. Good genes models 3 2 1
Fluctuating Asymmetry
Harem size
Symmetric
Asymmetric
Damaged

30. Intersexual Selection
2. Good genes models
Good genes and parasites
Number
of matings
Clean
Infected
No choice

31. Intersexual Selection
3. Runaway Selection
Assume 2 genes
Gene A - Trait ‘X’ in males - more colourful feathers
Gene B - In females - preference for trait ‘X’
A + B
Expressed in males as the trait
A + B
Expressed in females as a preference for the trait

32. Intersexual Selection
3. Runaway Selection
Assume 2 genes
Gene A - Trait ‘X’ in males - more colourful feathers
Gene B - In females - preference for trait ‘X’
Frequency
in males
Frequency in females

33. Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Variation in males
Female

34. Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Two experiments
1. Standard disruptive selection
every generation - allow shortest and longest stalked
males to mate with randomly selected females
Short stalked
Long stalked
13 generations
Stalk length
Stalk length

35. Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Two experiments
2. Female choice
Female offspring prefer
short stalked males
Female offspring prefer
long stalked males
Stalk length

36. Intersexual Selection
Other factors affecting mate choice - Learning
Sexual imprinting -already discussed this
2. Mate choice copying
- chances of a male being preferred as a mate at time 2 increases as a result of being preferred at time 1.
Male 1
Male 2
Model female
Test female

37. Intersexual Selection
Other factors affecting mate choice - Learning
Sexual imprinting -already discussed this
2. Mate choice copying
Frequency
Near ‘chosen’
male
Near ‘non-chosen’
male
Time spent by test fish

38. Intrasexual Selection - Male/Male Competition
Competition before mating
Fiddler Crab (Uca)
1. Direct Fighting
Female digs burrow
Male
Male enters burrow and mates
Female
Male defends burrow and female
No evidence of female choice
Male leaves when female ovulates

39. Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
-male tries to interfere with copulating pair

40. Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
Elephant seals (Mirounga)
-females incite males to fight via interference
Males fight for dominance
-8.3% of males mate

41. Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
Logic - if subordinate male tries to mate
- female protests
- dominant interferes
-female gets ‘best’ mate
% of matings protested by female
Alpha
Adult male
Subadult male
Rank of male

42. Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Bluegill sunfish
Males set up territories
where females lay eggs

43. Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Three kinds of males
1. Parental - larger, aggressive territory holders
2. Sneaker - smaller
3. Satellite - look like females
spawning
Male digs nest
Female lays eggs
Male fertilizes eggs

44. Intrasexual Selection - Male/Male Competition
Male digs nest
Female lays eggs
Male fertilizes eggs
Parental male
Parental - larger, aggressive territory holders
Female
Sneaker - smaller - rush in before parental male
and fertilize eggs
Satellite
Satellite - look like females - spawn with pair

45. Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition
Females of many species can
Store sperm
Mate with several males before fertilization
Raises probability that sperm will compete for fertilizations
Males can a) reduce the chances that a second male’s sperm is used
(first male adaptations)
b) reduce the chances that a first male’s sperm is used
(second male adaptations)

46. Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘First male adaptations’
a. Postcopulatory mate guarding

47. Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘First male adaptations’
a. Postcopulatory mate guarding
Stick insects

48. Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘First male adaptations’
Postcopulatory mate guarding
Antiaphrodisiacs
Mating plugs

49. Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘Second male adaptations’
a. Sperm removal
Dunnock
Damselfly Penes

50. Intrasexual Selection - Male/Male Competition
Competition after mating
2. Bruce effect
- Mice - strange male (or male odour)
-females abort fetuses and become receptive

51. Intrasexual Selection - Male/Male Competition
Competition after mating
3. Infanticide

52. Summary - Sexual Selection
Intersexual Selection
1. Direct Benefits
Intrasexual Selection
Premating
2. Good Genes
a. Male fighting
3. Runaway Selection
b. Cuckoldry
c. Interference
2. Postmating
a. Sperm Competition
b. Bruce Effect
c. Infanticide