Sexual selection Flashcards
What is the main principle of sexual selection?
Reproductive success is the key. It is how many offspring an individual has that survive and reproduce themselves.
What are secondary sexual characteristics?
These are male/female distinctions appearing at puberty.
- size
- coloration
- weaponry
- shape
- ornamentation
What kind of morphological and behavioural traits are hard to explain using natural selection, mostly in males. Those traits and behaviours that hinder and do not promote survival?
These are things that might attract predators or lead to a slow escape. Examples are the plumage on birds of paradise, a peacocks tail and an elks enormous antlers.
Why was another mechanism needed to explain these features not explained by natural selection?
- Not easily explained by natural selection. They are not apparently beneficial to survival and sometime detrimental to survival.
- Secondary sexual characteristics don’t appear until adulthood.
What re the two mechanisms proposed by Darwin leading to the evolution of secondary characteristics?
Male-male competition
or
Female mate-choice.
However, these are not discrete categories, some sexual systems can have male-male competition with female mate choice as well.
What are the benefits of secondary characteristics in male-male competition for intra-sexual selection?
Secondary characteristics give males a reproductive advantage when in competition with other males
What are three types of male-male competition?
- Contests - aggressive fighting
- Scrambles - coming into contact and by chance you’re in competition.
- Endurance rivalry - outlasting the other males
There is variation in competition between animals. Give an example species that shows extreme male competition and the effects this has on female mate choice.
Elephant seals endure weeks of fighting for dominance. The females arrive after the battles are sequestered by dominant males which constrains female choice because females often can’t exercise choice if males are much larger than them. Especially if the baby is with them.
There is variation in competition between animals. Give an example species that shows extreme female choice.
Bowerbirds
Males construct and decorate elaborate structures.
Females then visit several bowers where males court females with vocals and visual displays.
Female then chooses mate(s), then leaves to lay eggs and rear chicks alone.
Outline the differences in the key aspects of natural and sexual selection
NS emphasises survival where SS emphasises reproduction.
NS emphasises getting to adulthood. Once in adulthood then SS kicks in.
NS natural environment is key although social environment can have an impact. But SS the social environment key.
NS operates on all individuals where SS operates more on males (in primates)
What external physical traits can indicate intrinsic qualities that are a product of “good genes”?
Body/ canine size or colour intensity.
These physical traits can indicate;
- testosterone level (Higher levels of testosterone has been linked to lower immunity. So despite this, if he has this bright red coloration and he is still healthy shows he is a strong male.)
- parasite load
- fighting ability
- age
- tenure stage
What are the two types of “good genes” model?
- “Handicap” models.
This is where ornamentation reduces average male survival. However females can assess a male’s ability to survive despite his burdensome ornament because the expensive signal = possession of excess energy. - “Viability indicator” models. This is where females can assess male quality by using characters that indicate a less apparent trait which can determine male fitness.
What are some typical male traits that may be viability indicators?
- behavioural displays
- acoustic displays
- ornaments
- weapons
- pelage/plumage
- skin colouration
- chemical signals
Give some examples of species showing these viability indicators
- Orangutans without fatty cheek (unflanged) pads are seen as subhordinate males. Unflanged males can rape females.
Flanges will lose fattiness once he cannot hold down his territory.
- Birds: male plumage brightness and song quality is reduced by chronic parasite infections.
- Red skin primates often represent testosterone level or male health. E.g Gelada males: red patch on chest fades when they lose rank.
Redhead Uakari monkeys fade when they are ill.
Dominant Mandrills have brightest colors and most fatted rumps. Females prefer to mate with most brightly colored males.
Why would females care about male testosterone levels?
Because it’s related to male success in agnostic competition.
Testosterone suppresses immune response so the males are superior because they’ve still survived even with a lower immune response.
Strength of immune function may be heritable and the females offspring could benefit.
