Lecture 7 - the evolution of sex, sperm competition, cryptic female choice and alternative mating strategies

Question 1

what was the ancestral state of gametes like?

Answer 1

the 2 gametes would have been of equal size and any 2 gametes would have been able to come together and mate

Question 2

describe the graphical model used to understand the two sex reproductive system

Answer 2

• assume that a mutant half sized gamete arose from ancestral state
• fusion of the half size gamete and normal gamete will provide zygote of 75% size of normal zygote
• small rare mutant is in a population of normal gametes
• the 75% gamete is larger than half of the normal sized gametes
• so twice as many half size gametes give more surviving offspring

Question 3

when would the advantage of small gametes be reduced?

Answer 3

• as small gametes become more common
• if fusion is random the chance of 2 small gametes fusing will increase when there are more of them in the population
• if 2 fuse they would produce a half size gamete with relatively low survival
• the way to avoid this is by gametes evolving the ability of avoiding to fuse if they are the same size e.g. sperm cant fuse with sperm

Question 4

what is the issue with the graphical model?

Answer 4

• useful but somewhat unrealistic
• unlikely that gametes would just become half sized - just assumed in graphical model to make it easier to understand
• more realistically look at slightly smaller gametes to see if they would be at an advantage which they almost certainly would be

Question 5

what is sperm competition?

Answer 5

The competition between the sperm of two

or more males inside the female’s reproductive tract

Question 6

what was wrong with darwins theory of sexual selection?

Answer 6

it only went as far as competing for mates when actually males also compete for fertilisations

Question 7

2 examples of how males in species try to outcompete their rivals

Answer 7

• male damselflies have an elaborate penis to remove sperm of other males before inseminating the female
• in insects, polyandrous males produce a higher proportion of viable sperm than monogamous males

Question 8

do monogamous males partake in sperm competition?

Answer 8

• in the case of Male Adélie penguins they strategically allocate sperm to Extra pair females
  (case study)

Question 9

what do males need to do in addition to sperm competition?

Answer 9

males need to protect their females from extra pair copulations - they guard their mates and drive off rivals
e.g. male magpies follow females during entire fertile period

Question 10

describe mate guarding in seychelles warblers

Answer 10

• the males mate guard the females in the days running up to her laying her egg
• when the egg is laid the male no longer guards the female
• no extra pair copulations are successful up until fertile period when the guarding then begins
• study done placing a fake egg in the nest showed the males to stop guarding and successful extra pair copulations took place

Question 11

describe the study on blue milkweed beetles to see if guarding increases paternity more than just looking for other mates?

Answer 11

• Mate guarding males were removed from females.
• 25% of separated males found a new mate within 30 minutes.
• So guarding results in missed mating opportunities.
• 50% of females found a new mate after male removal
• So guarding prevents females from remating
• If the last male to copulate with a female fertilises >40% of the eggs, males gain by guarding the female rather than by searching for more females

Question 12

what is a method used by some species to protect paternity?

Answer 12

• Male Fulmars copulate over 50 times with their partners
• Frequent copulation ensures optimal timing
  of insemination relative to fertilisation
• strong evolutionary pressures on males to gain fertilisations

Question 13

what is cryptic female choice?

Answer 13

Female manipulation of sperm inside her reproductive tract so that a preferred male fertilises her eggs
- pressure on females to regain control over fertilisations

Question 14

describe cryptic female choice in chickens

Answer 14

Females eject the sperm of subordinate males from their cloacas and accept the sperm of dominant males

Question 15

give an example of why you need to be careful when studying cryptic female choice?

Answer 15

in the example of dunnocks -a male may peck at his partner’s cloaca if another male has been near her. In response she may eject a droplet of fluid containing sperm, however examination of this fluid shows this sperm is non-viable so it is more likely she is clearing her reproductive tract of useless sperm to make way for new ejaculate - need care when interpreting behavioural observations

Question 16

how can nuptial gifts affect sperm acceptance

Answer 16

Hanging fly (Mecoptera) males give a nuptial gift to the female. While she is snacking the male mates and transfers sperm.

• sperm transfer takes about 20 mins
• if males provide large gifts it will occupy the female long enough for full transfer of sperm
• the female terminate the copulation as soon as shes finished feeding - small food = copulation ends early

Question 17

2 reasons why females engage in cryptic female choice

Answer 17

Females may gain:
1) Higher quality offspring
(by cryptically choosing high quality fathers)
2) More offspring
(by cryptic choice of genetically compatible males)

Question 18

what are alternative mating strategies

Answer 18

a way for males that are unsuccessful in competition to achieve reproductive success

Question 19

alternative mating strategies can either be….. or …….

Answer 19

1. alternatives are not equally rewarding (best of a bad job)
2. alternatives that are equally rewarding

Question 20

describe the several male mating tactics in scorpion flies

Answer 20

Large males
Guard dead insects attractive to females. - 6 matings each

Medium-sized males
Produce salivary gifts to attract females. - 2 matings each

Small males
Force copulations on females - 1 mating each
- 3 tactics have different levels of fitness - if you remove large males from the population the medium males adopt their tactic of guarding
- when faced with competition from larger males, smaller males adopt tactics that reflect their poor competitive ability i.e best of a bad job

Question 21

describe alternitive morphs in male isopods

Answer 21

• males occur in three morphs with different mating advantages
  1) alpha - fighter - fights off beta and gama males
  2) gamma - hider - sneaks copulations
  3) beta - female mimic - pretend to alpha males they are female
• The three morphs have different genotypes.
• in different combinations of male and female frequencies different morphs were more successful e.g. 1 alpha, 1 beta and 1 female = alpha gets more copulations but 1 alpha, 1 beta and many females = beta is more successful
• in the end the 3 genotypes occur at frequencies at which their fitness is equal
  alternative strategies with equal fitness

Question 22

describe the mating tactics of blue gills

Answer 22

a) large males - territorial - attract females
b) little sneakers - small - release sperm at the same time as large male
c) sattelites - medium - same body colouration as females - hang around the nest and slip between territorial male and his mate when female is spawning
- territorial males father 98.7% of offspring and other 2 only father 1.3% combined
- best of a bad job example - they may eventually grow large and take on territory but it is best to try other tactics as you may end up with some offspring