Sexual Selection Flashcards
Intrasexual Selection (and the 5 kinds)
traits that help members of one sex compete with each other for mates
- Sexual Dimorphism
- Evolution of Weapons
- Fighting and Contests
- Alternative Reproductive Tactics
- Sperm Competition
Intersexual Selection
traits that make individuals attractive to the opposite sex
Sexual Selection
A process by which secondary sexual traits become elaborated because they increase their owner’s ability to gain access to mates
Bateman’s Principle
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)
Differences in Parental Investment between sexes
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
OSR
Operational Sex Ratio – ratio of receptive males to females in a population
Sexual Dimorphism
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).
Evolution of Weaponry
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.
Elephant Seals
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.
Dung Beetles
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.
Why do animals fight?
- Limited resources
- Food, mates, shelter, territories
- Contest over an indivisible resource
Costs and Benefits of Fighting
Benefits – win resources
Costs – energy, time, predation risk, injury, death
Dominance and Social Hierarchies…
Reduces the amount of fighting.
Fight Progression
Threats and Assessment (no contact)
Assessment and Contact
All out fight
Factors that influence when to give up
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
How do animals fight in the real world?
- Minimize costs
- Threat Displays and Assessment
- Dominance Hierarchies
- Ritualization
- Injurious / Deadly Behaviour is rare
Alternative Reproductive Tactics (ARTs)
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.
Marine Isopod, Paracerceis Sculpta
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.
Bluegill Sunfish
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.
Ruffs (shorebird)
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.
When do ARTs evolve?
- 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)
How do ARTs evolve?
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)
Frequency Dependent Selection
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.
Condition Dependent Selection
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.
Strategy
Genetically based decision rule.
Tactic
A behavioural pattern played as part of a strategy. (Only one set of genes.)
Avoiding Sperm Competition
Mate Guarding
Mating plugs (plug up vagina, bides time)
Releasing substances so that females can’t mate
Increased Sperm Competition leads to increases in…
- Sperm number
- Relative testes mass
- Sperm density
- Ejaculate volume
Sperm Morphology
It varies.
We don’t know what strange morphologies really do.
Though, longer sperm is beneficial with more sperm competition.
Sperm Swimming Speed
Increased sperm competition leads to increased…
- Swimming speed
- Longer duration of swimming
Sperm Competition and Mating Systems
Low sperm competition when females DO NOT mate multiply (monogamy, polygamy)
High sperm competition when females OFTEN mate multiply (polyandrous, polygynous, promiscuous)
ARTs and Sperm Competition
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.)
Strategic Sperm Allocation
European Bitterling example. When rivals are around, males ejaculate more often. Subordinate males ejaculate even more when rivals are around.
