SEXUAL SELECTION

Hareesha A S

University of agriculture sciences, Bangalore , GKVK

9739210339

hareesha329as@gmail.com

Sexual Dimorphism

Males and females often are strikingly different

in size and appearance.

Why does sexual dimorphism occur?

Size

Fig. 12-06

Teeth and size

Horns

Humans Males

are average

10% taller then

female

SEXUAL

SELECTION

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

Intrasexual

• Combat

• Sperm Competition

• Infanticide

COMBAT

• large body size

• Weaponry

• Armor

Walruspangolin-desert

Male marine iguanas in combat

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

Competition between males has led to

extreme sexual dimorphism when males

can potentially control large harems.

Male and

female

seals

weaponry

pangolin-desert

Armor

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

• 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

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

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.

Hanuman Langur (Semnopithecus entellus)

Intersexual

1.Gift

2.Display

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

2.Display

Frigate Bird

Female red-collared widowbird prefer long-tailed males

What is the motive for female choice?

What benefit is gained?

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

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.

Females get better genes

• Measured five aspects of offspring performance:

– larval growth

– time to metamorphosis

– mass at metamorphosis

– larval survival

– post-metamorphic growth

Results: Long calling males have

significantly higher fitness

Table 10.3

House mice (Mus musculus musculus)

Salmonella enterica

Results

Possibility -2

Choosy females may benefit through acquisition of resources

Hangingflies

• Randy Thornhill studied hangingflies.

• When a male hangingflycatches an insect it releases a pheromone to attract a female.

• Male offers prey to female. If she accepts, they mate while she feeds.

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.

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.

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.)

Possibility -3

• Choosy females may have pre-existing sensory biases

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.

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.