Sexual selection 1 Flashcards
(14 cards)
What is sexual selection?
Sexual selection acts on an organism’s ability to obtain and successfully copulate with a mate (and can trade-off with survival).
‘depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction’
Why do patterns of sexual selection differ between males and females?
Eggs are expensive to produce (sperm are cheap)
Females become a limiting resource
Triggers competition among males, allowing females to chose mates
The result is different secondary sexual characteristics (traits not directly involved in reproduction e.g. deer’s antlers or the peacock’s tail)
Male competition vs female choice
Male Competition - males compete for access to females (Intra-sexual selection) -> armaments
Female Choice – females chose their male sexual partners (Inter-sexual selection) -> ornaments
Examples where sexual symmetry is reversed?
There are examples where the asymmetry in sexual reproduction is reversed e.g. pipefish, where males care for young and competition is stronger in females
These sex-role-reversed species have considerable untapped potential and will continue to contribute to sexual selection research in the decades to come (untangling demographic and ecological factors driving sexual selection).
How is male competition categorised?
Contests – direct fights (weapons)
Scrambles – rapidly locating a mate (often first to arrive mates)
Endurance Rivalry – persistence increases success (longer fights, extended mating periods/seasons)
Southern Elephant Seals
Give birth out of water
Females haul to beach to give birth & mate
For three months, huge concentrations of females
Males can defend large harems of females
Males can be up to six times heavier than females!
Largest male win most fights (contest & endurance)
Less than one third of males copulate (highly skewed)
Small males can try and sneak matings. Female will call and protect, attaching to a dominant male
Describe male mating strategies/life histories
Elephant seals - small males try to sneak matings
Probably opportunistic, young males, still growing
Could still become massive harem holders…
But all males appear to have same/similar life-histories
Coho salmon - small males (jacks) can try and sneak matings
Jacks migrate to sea for < 1yr = small
Hooknoses can spend years at sea feeding = large, hooked jawed & armour
Hooknoses fight to fertilise eggs - the first fertilizes the most eggs
Males irreversibly become a jack or hooknose (they cannot be both)
Describe sperm compeition
Growing evidence the quantity, quality and even morphology of sperm are under sexual selection
Can view each sperm as a ticket in a lottery – the more sperm/tickets, the higher the chance of fertilizing the egg! So ejaculate size/quantity may be important…
Growing numbers of species have been shown to have sperm heteromorphism (two or more sperm morphologies), that may ‘’block’ other sperm or even mechanisms for genetically similar spermatozoa to cooperate…
Give some examples of sperm competition
Mate guarding (and prolonged copulation) – a defensive behavioural trait that occurs in response to sperm competition; males try to prevent other males from approaching the female and copulating
Copulatory plugs inserted immediately after a male copulates with a female, which reduce the possibility of fertilization by subsequent copulations from another male, by physically blocking the transfer of sperm.
Damselflies use a scoop on their penis to remove sperm left by previous males and Drosophila males use toxic chemicals that act as an anti-aphrodisiac causing a dejection of subsequent copulations, and also stimulate ovulation and oogenesis…manipulating the female
Describe evolution of the human penis in relation to sperm compeition
…the shape human penis may have been selected to displace seminal fluids implanted in the female reproductive tract by a rival male…humans have large testis (relative to some other primates), perhaps reflecting higher levels of multiple mating in females and more intense selection for sperm competition…
Describe infanticide
Male-male competition doesn’t just stop at conception…
High turn-over of males controlling prides
Females do not breed until cubs weaned
When new males take over will try to kill cubs
This increases male reproductive success
Reduces female reproductive success – sexual conflict
Estimated 10% of all lion mortally due to infanticide!
What is sexual conflict?
When the two sexes have conflicting optimal fitness strategies concerning reproduction it leads to sexual conflict (sometimes called inter-sexual conflict)…
Give an example of sexual conflict
Consider the Drosophila males with chemicals (accessory gland proteins) released with their sperm that stimulate ovulation and oogenesis – so the females produces more eggs he can fertilize.
For females this may not be optimum, allocating more energy to reproduction now may reduce her survival and life time output of eggs/fitness (a trade-off between survival & reproduction).
But this cost to the female is not paid by the male, as later reproduction would probably mean the eggs are fertilised by another male…the two sexes may have conflicting reproductive strategies…
If we experimentally manipulate the Drosophila into monogamy the strategies align – a life time reduction on egg production in the female now reduced her partners/males fitness too!
Holland & Rice bred two types of fly lines for 47 generations, enforcing monogamy in set and allowing normal mating in another as a control…
In monogamous lines the female survival and reproductive output (no. mature progeny produced) increased, just as we would predict if sexual conflict were operating…
The results suggest conflict between males and females is reducing reproductive output by ~ 20%
Conflicts between evolutionary theorists
