1) Sexual dimorphism, or differences in size and appearance between males and females, often occurs due to sexual selection. 2) There are two main types of sexual selection: intrasexual selection, where males compete with each other for access to females; and intersexual selection, where females choose their mates. 3) Examples of traits that evolve due to sexual selection include weaponry used in combat between males, ornamentation used in displays to attract females, and gift-giving behaviors that benefit feeding females.

2. SEXUAL SELECTION
Hareesha A S
University of agriculture sciences, Bangalore , GKVK
9739210339
hareesha329as@gmail.com

4. Sexual Dimorphism
Males and females often are strikingly different
in size and appearance.
Why does sexual dimorphism occur?

5. Size

6. Fig. 12-06

7. Teeth and size

8. Horns

11. Humans Males
are average
10% taller then
female

12. SEXUAL
SELECTION

13. Types of sexual selection
• Intrasexual – where males compete for
females.
– males will fight with each other.
– Females mate with the winners
• Intersexual –Where males cannot control
access to females –
– the males advertise for mates
– the female chooses

14. Intrasexual
• Combat
• Sperm Competition
• Infanticide

15. COMBAT
• large body size
• Weaponry
• Armor
Walrus
pangolin-desert

16. Male marine iguanas in combat

17. Territories are only a few square meters.
Numbers identify the males which own the territory.
59 and 65 were the most successful males
Prime territories are near the water

18. Competition between males has led to
extreme sexual dimorphism when males
can potentially control large harems.
Male and
female
seals

19. weaponry

20. Armor
pangolin-desert

21.  Male-Male competition often does
not stop with successful mating.
There is often post-copulatory
competition.
This type of intrasexual competition
is called,
SPERM COMPETITION
DAMSELFLY

22. • Occurs with internal fertilization where one
female mates with more than one male within
a short period of time
• Sperm are in a race to the egg

23. Other strategies developed due to
sperm competition
• prolonged copulation
• copulatory plugs
• application of pheromones to the female to
reduce her attractiveness to other males
• Scoop out the sperm left by a previous
suitor

24. INFANTICIDE
• Animals that live in groups.
• Ex: Pride of lions
• Pride composed of a group of closely related
females with 2 or 3 adult males related to each
other but unrelated to the females
• average time a male holds a pride is a little over 2
years.

25. Hanuman Langur (Semnopithecus entellus)

26. Intersexual
1.Gift
2.Display

27. Gifts:
Male Hanging Flies present their
female partners with insect food
items. The size of the gift is
correlated with the duration of
copulation and the number of
sperm transferred.
HANGING FLIES

28. 2.Display
Frigate Bird

29. Female red-collared widowbird prefer long-tailed males

30. What is the motive for female choice?
What benefit is gained?

31. Possibility -1
Choosy females may get better
genes for their offspring
• The males that give better
displays are genetically
superior.
• This was tested in an
experiment with the Gray Tree
frogs

32. Females get better genes
• Collected eggs from gray tree frogs and
fertilized half with sperm from long-calling
and half with sperm from short-calling males.
• Reared half of young on generous diet, others
on restricted diet.

33. Females get better genes
• Measured five aspects of offspring
performance:
– larval growth
– time to metamorphosis
– mass at metamorphosis
– larval survival
– post-metamorphic growth

36. Results: Long calling males have
significantly higher fitness
Table 10.3

37. House mice (Mus musculus musculus)
Salmonella enterica

39. Results

40. Possibility -2
Choosy females may benefit through
acquisition of resources

41. Hangingflies
• Randy Thornhill studied
hangingflies.
• When a male hangingfly
catches an insect it releases
a pheromone to attract a
female.
• Male offers prey to female.
If she accepts, they mate
while she feeds.

42. Hangingflies
• The larger the prey the longer the female eats
and the more sperm the male transfers. After
20 minutes male has transferred the maximal
quantity of sperm.

44. Hangingflies
• Male must offer a meal that lasts at least 5
minutes or no sperm are transferred.
• If meal lasts less than 20 minutes female
breaks off copulation.
• At 20 minutes male breaks off copulation and
seeks another female to offer the remains of
the meal to.

45. Hangingflies
• Female preference for males providing large
meals benefits her in two ways.
• 1. Provides nutrients and energy that allows
her to lay more eggs.
• 2. Saves her from having to hunt for herself.
Hunting is dangerous. (Males twice as likely
to be caught in spider webs.)

46. Possibility -3
• Choosy females may have pre-existing
sensory biases

47. Choosy females may have preexisting
sensory biases.
• Proctor (1991,1992) work on water mites.
• Tiny freshwater animals that catch copepods.
Very poor vision. Depend on smell and touch.
• Females hunt copepods by standing on four
hind legs with four forelegs spread in net-like
fashion.
• Mating does not involve copulation. Instead
male deposits a spermatophore.

49. Water mites
• When male smells a female he approaches
female while lifting and vibrating his front
legs. Frequency of vibrations similar to
copepod frequency.
• Female turns towards male and clutches. Then
male deposit spermatophore.