Term 2 Lecture 4: Courtship And Reproduction Flashcards
Courtship
Sexual selection leads to secondary sexual characteristics
Many are striking displays that use physical morphology in behavioural displays
Natural selection Vs sexual selection
Darwin (1859,1871) individuals (at least in sexually reproductive species) can improve their chance of reproducing in 2 ways:
1) compete successfully to survive to acquire resources allowing reproduction (aka struggle for existence and natural selection) adapting to survive
2) compete successfully for mating opportunities (sexual selection) chooses for:
A) aid of competition within one sex (usually male) for access to the other (intrasexual selection)
B) enhancing attractiveness of individuals of one sex (again usually males) to members of the other - sexual or epigamic selection - usually at a cost to other components of reproductive success e.g. adult survival.
Darwin: (sexual selection) depends on a struggle between individuals (usually males) for possession of the other sex. The result is not death for the unsuccessful competitor but few or no offspring
Intra Vs intersexual selection
Intrasexual selection:
-male on male fight often dependent on agility (hummingbirds) or bulk (seals)
- intense post-copulatory selection - competition of sperm from different males that have mated with the same female
Intersexual selection
-morphological display features
- e.g. bowerbird feathers and nest decorations
(Darwin: female birds select beautiful males producing marked differences in morphology over generations)
Sexual selection
- some view natural and sexual selection as separate
- but survival is only valuable if an individual can reproduce
- both processes act through differential reproduction
- balance to be found between how much to invest in a struggle to exist Vs struggle to mate depending on relative contribution each makes to the individuals lifetime reproduction output
Often traits selected for in a mate reflect their fitness e.g. to be a Dom male deer you must be large, to be large you must eat well so you must be good at foraging and holding territory
Sexually selected characteristics that may conflict with survival
- antlers - energetically expensive
- bright plumage- conspicuous to predators
- therefore a trade-off between investment in survival Vs reproduction
E.g. peacock plumage will look tatty if the bird has lice or is sick, a bird with lots of healthy plumage is a target for predators so survival shows fitness
E.g. redness is appealing in fighter fish, this requires the fish to ingest enough carotenoids to display as skin pigment as well as for use in the immune system
Some characteristics serve intra & inter sexual selection
E.g. size of antlers in deer and physical bulk in male seals
Why are females choosy?
Females generally invest more in offspring than males (especially if internal fertilisation required) :
- costly large gametes (eggs)
- internal development of young
-raising young following birth or hatching
- finite egg production - limited no. Of mating opportunities
- in species with high level of investment after birth or hatching the female tends to provide the bulk of care e.g. feeding in mammals
Why do males fight?
Males typically invest less in each individual offspring
- sperm is ‘cheap’ and ‘unlimited’ so unlimited mating opportunities
- uncertainty of partnering - as most females mate with more than one male
Thus driving lack of investment in offspring
-males are less choosy to mate as many times as possible
Compete for mates or resources of use to females by conflict displays of male-male intrasexual competition
Or compete to attract females (courtship display) - intersexual competition
Misleading metaphor of males ‘parasitising’ females reproductive investment
Cost of competition -
males must deliver thousands of sperm packaged together and the sperm must be very active, motile and high quality
Time and energy must be invested in learning displays as well as finding good nutrition for size and health to be selected intersexuality such as colouration, bulk or feather health
Males also risk physical injury defending or competing for mates
What caused the development of the difference between male and female investment? why did anisogamy evolve?
The evolution of anisogamy - reproduction with two distinct gametes - large well provisioned egg and small motile rapidly moving sperm.
Isogamy existed previously with male and female gametes of the same size as is still the case in most microorganisms (fungi, algae etc. are sexually anatomically similar)
Anisogamy seen in larger organisms at adult phase organisms that need more provisions to develop even right at the start.
This led to disruptive selection a larger gamete has the advantage of better provisioning for the offspring whilst a smaller gamete has the advantage of higher motility to locate large gametes - this led to selection against the mean and the evolution of small/large gametes pairs.
Anisogamy has a profound impact on subsequent evolution of sexes in traits and behaviour of mating and courtship driven by sexual selection
Females can be considered a ‘limiting factor’
Bateman’s principle: the limiting sex is the sex with the lowest maximum reproductive rate (usually females)
The sex with the higher reproductive potential:
-invests less in each reproductive attempt
- experiences greater competition for mates leading to stronger sexual selection (usually male)
So more elaborate secondary sexual traits are usually seen amongst males.
Sexual dimorphism and secondary sexual characteristics
Not all dimorphism is due to sexual selection
Other selective pressures:
Feeding ecology e.g. anopholes mosquito females are larger as they take the ‘blood meal’ to feed their eggs
Fecundity: in some species females are larger as it means they are able to produce more offspring (often seen in amphibians)
Primary sex differences: in birds females are often larger to be able to carry eggs
Functions of courtship
- at a simple level to recognise members of your species - to produce viable offspring
- to identify individuals from the local area- same deme better chance to raise healthy young as they know the local environment
-to recognise specific individuals - as in long-term pair bonds- to recognise your mate for social interacting also in parental bond - to attract a mate
-mate choice - assessment of potential mates traits, condition and quality
-in long-term pair bonding as in many bird species, reaffirming pair bonding and contributing to care of offspring
-to coordinate reproductive behaviour and physiology between sexes (synchronisation) encouraging production of gametes - as in spider species - correct behaviour essential on males to avoid being eaten by potential mate
Mate attraction - what do displays signify?
E.g. bird song
-a way for females to compare males
- more complex/longer songs indicate healthier males, better territories and enhanced parenting potential
^ an indicator of good genes - honest signal of male quality
What is a mating pattern (or mating system) ?
Mating pattern- describes the pattern we observe between individuals of a population
A mating pattern is a population/species wide description composed of individual mating strategies
A mating system usually exists to serve ultimate cause i.e. enhancement of both male and female fitness
^ no individual strategies are adopted to enhance individual fitness - a mating pattern is population wide ‘composite’ of individual strategies
