Sexual Selection

Question 1

Intrasexual Selection (and the 5 kinds)

Answer 1

traits that help members of one sex compete with each other for mates

1. Sexual Dimorphism
2. Evolution of Weapons
3. Fighting and Contests
4. Alternative Reproductive Tactics
5. Sperm Competition

Question 2

Intersexual Selection

Answer 2

traits that make individuals attractive to the opposite sex

Question 3

Sexual Selection

Answer 3

A process by which secondary sexual traits become elaborated because they increase their owner’s ability to gain access to mates

Question 4

Bateman’s Principle

Answer 4

Males – reproduction is limited by the number of eggs fertilized, which is dictated by the number of females a male can mate with.
Females – reproduction is limited by the number and quality of eggs produced, which is limited by resources.

1) Variance in number of offspring and mates in males exceeds that of females
2) Males show a significant correlation between number of mates and number of offspring, whereas females do not (mixed evidence)

Question 5

Differences in Parental Investment between sexes

Answer 5

Where one sex invests in parenting more than the other, members of the least investing sex compete among themselves to mate with the most investing sex.

In most…
Males – more mating effort
Females – more parental effort

Question 6

OSR

Answer 6

Operational Sex Ratio – ratio of receptive males to females in a population

Question 7

Sexual Dimorphism

Answer 7

When two sexes are different species. Sometimes due to natural selection, disruptive selection. But MOST is due to sexual selection (fighting for access to females).

Question 8

Evolution of Weaponry

Answer 8

Traits that have evolved for male-male competition.
- Structures used in combat with rivals.
- Diverge in size, shape, and the habitats in which they’re used.
- Evolve when one sex (usually males) are able to defend spatially restricted critical resources.
- Usually the most variable morphological structure. Can reflect quality.

Question 9

Elephant Seals

Answer 9

An example of sexual dimorphism. The most sexually dimorphic of mammals. Males of this species have a higher variance in fitness, most males die without ever reproducing.

Question 10

Dung Beetles

Answer 10

An example of weapons. Many use horns as weapons. They compete for access to tunnels (where females are). Males with larger horns win more fights (proportion of fights won increased with disparity between sizes).

Small horns still persist because small horn males use sneaking tactics to access females. This is an example of an alternative reproductive tactic.

Question 11

Why do animals fight?

Answer 11

• Limited resources
• Food, mates, shelter, territories
• Contest over an indivisible resource

Question 12

Costs and Benefits of Fighting

Answer 12

Benefits – win resources
Costs – energy, time, predation risk, injury, death

Question 13

Dominance and Social Hierarchies…

Answer 13

Reduces the amount of fighting.

Question 14

Fight Progression

Answer 14

Threats and Assessment (no contact)
Assessment and Contact
All out fight

Question 15

Factors that influence when to give up

Answer 15

1) Resource holding power (RHP) – absolute fighting ability (the bigger the body, the longer the fight)
2) Resource value (V) – Intruders do not last as long as residents, as the resource is more valuable to the latter, who holds it.
3) Aggressiveness – inherent property of the individual

Question 16

How do animals fight in the real world?

Answer 16

• Minimize costs
• Threat Displays and Assessment
• Dominance Hierarchies
• Ritualization
• Injurious / Deadly Behaviour is rare

Question 17

Alternative Reproductive Tactics (ARTs)

Answer 17

Discontinuous behavioural, morphological, and/or physiological traits selected to maximize fitness in two or more alternative ways. Individuals of the same sex achieve fertilization in very different ways.

Question 18

Marine Isopod, Paracerceis Sculpta

Answer 18

Alpha males – large, compete for access to a sponge
Gamma males – very small, stay around edges of sponge to reach females without alpha realizing
Beta males – female mimics, gain access to females by allowing alphas to treat them as females.

Question 19

Bluegill Sunfish

Answer 19

Territorial males (compete for access to females with other territorial males)
Satellite males (female mimics)
Sneaker males (sneak copulations) – grow into satellites

Young that belonged to sneaker males grew faster. Suggests their sperm may be more competitive.

Question 20

Ruffs (shorebird)

Answer 20

Darker territorial males vs. Lighter non-territorial satellite males
The former tolerate the latter, as females are more interested in going to display sites with two males.

Trait is genetic. Satellite males have satellite sons.

Question 21

When do ARTs evolve?

Answer 21

• When there it fitness to be gained by pursuing different reproductive tactics
• When there is investment to be exploited by same sex competitors
• When intermediate expression of a trait is not possible or selected against

(Disruptive selection)

Question 22

How do ARTs evolve?

Answer 22

1) [STRATEGY] Frequency Dependent Selection (alternatives have equal lifetime fitness, polymorphic genotype)
2) [TACTIC] Condition Dependent Selection (tactic adoption depends on condition, status, maternal condition… same genes)

Question 23

Frequency Dependent Selection

Answer 23

Fitness of a trait depends on how common it is in a population.
- Positive: fitness increases as the trait becomes more common.
- Negative: fitness decreases as the trait becomes more common.

F* = the point where each tactic is equal.

Question 24

Condition Dependent Selection

Answer 24

Tactics adopted by individuals differ due to diverging conditions.
Could change throughout an individual’s life depending on condition continuously, or could be determined (fixed) at one point in development depending on condition.

The second tactic is only better than 0. Making the best of a bad situation. No assumption of equal fitness.

Most evidence is for this model.

Question 25

Strategy

Answer 25

Genetically based decision rule.

Question 26

Tactic

Answer 26

A behavioural pattern played as part of a strategy. (Only one set of genes.)

Question 27

Avoiding Sperm Competition

Answer 27

Mate Guarding
Mating plugs (plug up vagina, bides time)
Releasing substances so that females can’t mate

Question 28

Increased Sperm Competition leads to increases in…

Answer 28

• Sperm number
• Relative testes mass
• Sperm density
• Ejaculate volume

Question 29

Sperm Morphology

Answer 29

It varies.
We don’t know what strange morphologies really do.
Though, longer sperm is beneficial with more sperm competition.

Question 30

Sperm Swimming Speed

Answer 30

Increased sperm competition leads to increased…
- Swimming speed
- Longer duration of swimming

Question 31

Sperm Competition and Mating Systems

Answer 31

Low sperm competition when females DO NOT mate multiply (monogamy, polygamy)
High sperm competition when females OFTEN mate multiply (polyandrous, polygynous, promiscuous)

Question 32

ARTs and Sperm Competition

Answer 32

Lower sperm competition for guarder / alpha males. They sometimes get to mate without a sneaker around. They don’t always have sperm competition.
Higher sperm competition for sneaker males, as they ALWAYS mate with another male present. They almost always have sperm competitioom.

As such, more investment in testes and sperm by cuckolding males. (Plainfin midshipman sneakers, for example.)

Question 33

Strategic Sperm Allocation

Answer 33

European Bitterling example. When rivals are around, males ejaculate more often. Subordinate males ejaculate even more when rivals are around.