Sexual selection

Question 1

why is there a trade off between reproduction and survival?

Answer 1

characteristics which promote success in reproductive competition (sexually selected features) may be deleterious to survival
Ns and Sexual selection can act in opposition
Eg. Tree frogs croak all day to attract mates, but this attracts bats. trade off btw attracting opposite sex and NS costs of greater pred risk.

Question 2

define intra- and inter-sexual competition

Answer 2

intrasexual competition - comp btw members of the same sex

intersexual comp - between sexes.

Question 3

how did darwin and parker see intrasexual selection?

Answer 3

Darwin - comp btw males, have traits to help them access females.
Parker, 1970 - traits help ensure male paternity after copulation

Question 4

when does sperm competition occur?

Answer 4

if there is potential for >1 males sperm fertilising any egg.
Can be important in internal and external fertilisation.
EG - delay btw copulation and fertilisation, sperm staorge, multiple mating, EPCs.

Question 5

why are there so many sperm?

Answer 5

• Cohen 1973 - many defective sperm as meiosis is error prone. gives back ups too, but same rate of error in egg formation.
• Parker 1982 - greater chance of winning sperm competition if there are many sperm.

Question 6

why are sperm small?

Answer 6

trade off btw size and number. eg Stockley et al 1997 showed that fish with many sperm had smaller sperm.

Question 7

what use is a long tail?

Answer 7

if internal fertilisation, sperm must race to the egg. favours longer sperm tail and more power, which selects for larger size. found longer sperm tails in species with multiple mating.
Briskie et al found positive correlation in birds of sperm length and rate of EPC paternity. not adaptation for speed as females store sperm anyway. if higher EPC rate, selected for larger sperm storage tubules, so females can store/select sperm of preferred males. sperm length is in response to larger storage tubules so can fill them up.
2. drosophila - Pitnick, 1995 - sperm 20 x males own body length. one sperm per ejaculate.

Question 8

give an example of sperm dimorphism and female response

Answer 8

Butterflies- 90% of sperm in ejaculate is cheap and incapable of fertilization ‘Apyrene’. simply there to block it up. Eupyrene - can fertilise eggs.
Found by Cook and Weddell- Females have stretch receptors on tubules, and delay mating for longer if there is more apyrene sperm.

Question 9

what is accessory fluid

Answer 9

contain nutrients for sperm
recently found also contains toxins.
some act against rival sperm eg in drosophila, but confer costs to females, who have a lower survival rate as a result of mating.

Question 10

how does testes size vary?

Answer 10

when females mate with many males, larger testes.
eg chimpanzees - promiscuous mating and huge testes.
Gorillas in a harem with 1 dominant male, has tiny testes.

Question 11

what has penis shape evolved due to?

Answer 11

sperm competition

• damselfly - bristles to remove sperm of other males out of the vagina.
• Diversity in genitalia morphology used by taxonomists for classification.
• 2x greater diversity in species which mate multiply - Arnqvist, 1998.

Question 12

what is a strategy of the ghost crab?

Answer 12

injects glue to seal up the previous males sperm.

plugs and seals very common through many species.

Question 13

give 7 examples of how male beh is affected by sperm comp?

Answer 13

1. sperm removal, eg Dunnocks peck at females cloaca before mating if there is another male nearby, causing her to eject any previous sperm.
2. Internal fertilisation - Parker suggested it evolved as a tactic to increase certainty of paternity.
3. Prolonged copulation - Brimstone butterfly stays in copula for a week.
4. Repeated copulation reduces chances that another male might mate at the crucial time of ovulation. Lions mate 100x a day for a week when female is in oestrus.
5. Mate guarding - if time of ovulation is uncertain, guarding before copulation is important. imp factor in evolution of monogamy.
6. forced copulation - in birds, only seen ducks, as other species dont have an intromittent organ.
7. Damage to female - less chance of her reproducing after this, so she will put more effort into this offspring.

Question 14

give some examples of male damage to female

Answer 14

1. Weevils - Crudgington and Silva Jothy - spines on penis to prevent f from mating again.
2. Marine hermaphrodite flatworm - penis fencing, sperm injected directly into body cavity. can cause infection throught the wound

Question 15

why would females choose to mate with multiple males? 5 reasons

Answer 15

1. fertility insurance - got enough fertile sperm
2. extracare - dunnnocks trade paternity for male help, males feed young in proportion to how many times he has mated with the mother. females with 2 mates have higher fledgling success than monogamous ones.
3. it may be too costly to resist
4. increasing competition may mean egg gets fertilised by the sperm with the best genes. sons may be better at male comp themselves.
   5 increased gen diversity

Question 16

when does sexually antagonistic coevolution occur?

Answer 16

when the female and male optimum are not equal.
Rice’s experiments:
- 2 lab pops of drosophila. one breeds over many generations, allowing males and females to coevolve with each other. Other m and f mate with others from different lines, so coevolution doesn’t occur.
found females who mated with males they hadnt coevolved with suffered high mortality due to toxin in seminal fluid. in coevolved pop, not seen, as f evolved countermeasures.

Question 17

What are2 types of benefits from female mate choice?

Answer 17

1. non heritable/direct, boots current reproductive effort.

2. heritable/indirect - to do with choosing good male genes.

Question 18

What are two theories of heritable female benefits?

Answer 18

1. Fisher’s sexy sons.
   - attractiveness musty be heritable, so will have more and attractive sons. sons will also pass on female preference for the ornament, so preference and ornament spread together as a pos feedback loop.
2. Zahavi’s handicap process - the ornament is a signal of the quality of the male’s genes, and so a choosy female’s offspring have higher viability

Question 19

give an example for evidence of Fishers attractiveness theory

Answer 19

Bakker 1990, Preference for a red belly is heritable in sticklebacks.

Question 20

Describe an experiment which showed attractiveness and preference covary.

Answer 20

Wilkinson and Reillo 1994 - Stalk eyed flies - females prefer wider eyes and wider eyed males have higher mating success.
Selected one line for wide eyes and one for narrow eyes, over 11 gen. found in narrowline, males had narrower eyes and females preferred narrow eyes. vice versa.

Question 21

is great tit breast stripe width, seen by Norris 1993, a product of Fishers or Zahavi’s theory?

Answer 21

Fisher - breast stripe width is heritable and females prefer wider stripes.
Zahavi - in cross fostering experiments, male offspring of wide stripe males survive better. crossfostering showed the females of attractive males invest more in offspring so that’s why they survive better.

Question 22

what must be demonstrated for zahavi and fishers theory to be true?

Answer 22

Zahavi - for this principle to work, offspring of more ornamented males must have viability advantages. easily demonstrated by artificial insemination which removes the possibility of female biased investment and effects are purely genetic.
Fisher - both attractiveness and preference for the trait must be heritable.

Question 23

why might cross fostering not make a difference?

Answer 23

if females invest more in eggs.
eg Zebra finches, Gil et al, 1999, put coloured bands on males legs, which females find red attractive and green ugly. there is no preference of actual characteristics, and researchers found more testosterone was put by females into eggs of red band males, promoting faster growth in young.

Question 24

example of Zahavi

Answer 24

Evans et al, 2004 - Offspring of female guppies artificially inseminated by more attractive males evaded predation better.

Question 25

How could both fisher and zahavi theories be correct?

Answer 25

• A Fisherian ornament is likely to be bigger and better in a (genetically) high quality male, but isn’t this starting to signal viability?
• If females prefer a Handicap trait, then their preference is passed onto their sons, who spread it, and a viability signal starts to signal attractive benefits.
  the modern view, is that there is a continuum - Fisher-handicap continuum, Kokko et al. this shows how the cost of ornaments ismore for low quality makes than high quality males - marginal cost.

Question 26

example of how marginal cost affects extent of ornaments

Answer 26

Getty, Optimal investment rule.
Peacock growing a tail is an investment. optimal tail length brings more rep success and doesnt cost excessively in terms of other components of fitness. optimum is different for individuals as they have diff amounts to invest. A high quality male can afford to grow a big tail to attract females, and females are attracted because it is reliably associated with the male condition and all the genes that affect the condition. also, benefits of having sons which have big ornament.

Question 27

describe the Lek paradox.

Answer 27

questions why unattractive genes are maintained in pop.
Female grouse fly into a lek, mate with the best males and fly away. all they get is genes, no parental help etc. only a few males get to mate. females constistently choose good males so why are bad males maintained?

Question 28

what forces maintain genetic diversity?

Answer 28

1. recurrent deleterious mutations - the mutation rate is low but lots of genes affect condition so mutation rate per individual is non trivial.
2. host parasite co-evolution. condition dependent ornaments can signal heritable resistance to disease - handicap/optimal investment rule. parasites have a central role in evolution of sex, so must do in sexual selection. resistance is neg freq dependent selection.

Question 29

study of signalling parasite load in sticklebacks

Answer 29

Milinski 1990
Male sticklebacks get a red belly in breeding season from carotenoids in diet. female choice trial under white or green light. - white - always chose healthier over parasitised males. under green, chose randomly as red shades cant be discriminated. maybe fish dont act normally under strange lighting.

Question 30

study of signalling parasite load in red jungle fowl.

Answer 30

Thornhill and Zuk, 1990
in Male red jungle fowl, the comb is particularly affected by blood parasites. Out of the comb, wattles, long tails and colourful feathers, the comb is the best indicator of health, and was chosen for by females in unparasitised males.
however, perhaps was another feature.

Question 31

describe an experiment in tail length of swallows

Answer 31

Moller, 1988
investigates female mate choice using tail manipulations in swallows.
4 treatments: Shortened, lengthened, cut and re-glued, just captured and released.

found that long tailed males paired sooner, elongated tails took average 3 days to pair, shortened tails 12 days.
Also, long tails fledged more than young and get more EPCs from females with short tailed mates.

Question 32

why may having a long tail not really make him a high quality male?

Answer 32

elongated males - less likely to return from migration the following year, more fault bars in tail feathers. It boosts his fitness for that year but pays in lower survival next year.

Question 33

What benefit do females gain from picking males with long tails?

Answer 33

• not better care for young, as sexy males do less care
• long tails may be a signal of blood sucking parasite resistance, as shown by a half brood fostering experiment. The parasite load of young was best predicted by the genetic father.