Sexual Selection & Male Tactics

Question 1

What is natural selection?

Answer 1

Individuals with favourable traits are more likely to survive than those with unfavourable traits

Question 2

What is the “selfish gene” theory?

Answer 2

• Evolution occurs through the differential survival of competing genes
• Those alleles whose phenotypic trait is successful will increase in frequency in population.

Question 3

What is inclusive fitness?

Answer 3

The ability of an individual to pass on its genes, including genes shared with relatives.

Question 4

What is sexual selection?

Answer 4

• Selection for traits that are solely concerned with increasing mating success & production of offspring
• Acts on phenotypic characteristics that determine an organism’s ability to obtain a mate & successfully reproduce

Question 5

What is the operational sex ratio?

Answer 5

• Fewer females available for mating than males
• Due to more investment, e.g. egg production, parental care - usually female who cares for progeny

Question 6

What 3 things does the operational sex ratio depend on?

Answer 6

1. Overall sex ratio of adult population
2. Relative amount of care given to offspring by males & females
3. Degree of reproductive synchrony amongst members of a given sex

Question 7

When OSR is biased, with one sex ‘in demand’ and the other ‘in excess’, what does sexual selection lead to?

Answer 7

• Competition for mating opportunities
• Drives evolution of features to improve a male’s access to mates, e.g. extravagant plumage etc
• These features are usually sexually dimorphic & develop in males for courtship & mating

Question 8

In which mating system is sexual dimorphism most obvious in?

Answer 8

• Polygynous (or polyandrous)
• Where 1 male mates with number of females or vice versa

Question 9

Do females typically have an increased or decreased parental effort compared to males?

Answer 9

Increased

Question 10

Do males typically have an increased or decreased mating effort compared to females?

Answer 10

Increased

Question 11

What did Bateman (1948) predict?

Answer 11

That sexual selection will usually be a more potent force in the evolution of males than in the evolution of females

Question 12

Give 3 conditions for sexual selection

Answer 12

1. Variation in the sexual trait within the population
2. Trait must be heritable
3. Must be a relationship between trait & mating success

Question 13

What 2 ways can sexual selection act by?

Answer 13

1. Intra-sexual selection (WITHIN) - Favouring ability of 1 sex, usually males, to compete directly in contests (e.g. by fighting), the outcome of which determines mating success.
2. Inter-sexual selection (BETWEEN) - Occurs between males & females - favouring traits in 1 sex that make them more attractive to opposite sex, giving them a mating advantage.

Question 14

What 3 main categories do male tactics fall into?

Answer 14

1. Male rivalry or direct competition (fighting & ritualised contests)
2. Mate guarding
3. Sperm competition

Question 15

Why has ritualised contests evolved in male-male competition rather than direct fighting?

Answer 15

Less risky & chance of injury

Question 16

What may ritualised contests in male-male competition involve?

Answer 16

• Assessment of size
• Assessment of strength

Question 17

Give an example of male contests

Answer 17

• Red deer
• 1st phase is assessment:
• Males roar, starting slowly, then speed up. If harem/territory owner can roar faster than challenger, challenger usually retreats
• A stag has to be fit to roar, so gives an indication of whether it is a ‘worthy’ challenger
• If roaring is matched, stags walk parallel to each other which allows them to assess body size
• Most contests end here
• Fighting involves stags interlocking antlers & pushing against each other
• Larger stags usually win (unless injured or exhausted from other fights)
• 25% chance of injury in fights
• If they lose, may be replaced by younger stag
• Ensures most experienced & fittest stags reproduce & replaced when better males are present

Question 18

When is fighting most likely to occur?

Answer 18

When a female is ready to reproduce, e.g. in oestrus

Question 19

What is mate guarding?

Answer 19

Once a mate is found, males of some species will guard them to ensure no male competitor can access her

Question 20

Give an example of a species that exhibits pre-copulatory mate guarding

Answer 20

• Male magpies
• Guard mate from intruding males just before period of egg laying

Question 21

Give an example of acoustic mate guarding

Answer 21

• Has been found that physical aggression towards an intruder on part of a male has no deterrent effect
• However, males who sang a duet with mate had a significantly higher rate of paternity

Question 22

What is sperm competition?

Answer 22

Produce sperm that compete with those of other males in either numbers or viability

Question 23

In which 2 ways do males ensure their genes are successful in mating?

Answer 23

• Sperm competition
• Have adaptations to enhance own sperm fertilising the female & ensure any offspring are his

Question 24

Give an example of a species that have sexual organs that are adapted to remove previously deposited sperm

Answer 24

• Male black-winged damselfly
• Mates with several males over span of few hours & store sperm in special receptacle
• Males have specially adapted penis (recurved head) that removes previously deposited sperm

Question 25

Give an additional way that some animals ensure gene progression

Answer 25

• Kill offspring by rival male (infanticide)
• Seen in some mice, monkeys & notably in lions

Question 26

What feature of animals has been driven by sexual selection?

Answer 26

Sexual dimorphism