L9 Flashcards
Evolutionary Origin of two sexes:
- Large gametes (females) and small gametes (males)
- Originally two gametes were equal size and any individuals could mate
A mutant with small gametes arose, will this be selected for?
- If you assume small gametes are half size and that the mutant can make twice as many as a result
- Yes - if survival of smaller embryo is >50% of normal embryo
- No - if survival of smaller embryo is <50% of normal embryo
When it is rare small gametes only ever fuse with large as there are many large gametes
Resulting embryo of small and large gametes Is not as large as normal zygote, this will impact survival chances, but more are produced so this could be off set.
What are the chances of small gamete invasion?
- As size increases so does survival up to a point
- Steepest line is at an optimal size
Optimal size of gamete (in a fake world)
- Half of optimal embryo size so two can add up top optimally sized embryo
Would making gametes half this size be favoured?
Yes- it would be 0.75% times original embryo size
- Twice as many half size gametes more surviving offspring, at least when the small gamete is rare - Under different S shaped curves half sized gametes is not favoured - It is possible for smaller gametes to be at an advantage - Advantage of smaller gametes decrease as they get more common - This is because chance of two gametes fusing increases as there is more of them, this would produce an embryo with a low survival chance
How do gametes get around this?
- Small gametes evolve ability to avoid fusing with other small gametes
- We assume small gametes were half size of large gametes, it is too random half size
It is better to ask if gametes that were slightly smaller would be at an advantage
Sperm competition
- The competition between the sperm of two or more males inside the females reproductive tract
- Aims to maximise chances of fathering offspring by outcompeting rival males that have mated with the same female
Outcompeting rivals methods
- Male damselflies remove the sperm of rivals before inseminating female
- In insects, polyandrous males produce a higher proportion of viable sperm than monogamous males
- Monogamous males produce worse sperm as there is no competition
Extra-pair copulations (EPC)
- Paired males actively promote sperm competition by having extra pair copulations
- They simultaneously have to ensure their mate doesn’t have an EPC with other males
- Males may guard their mates and drive off rivals during the fertile period
When does mate guarding occur in Seychelles warblers?
- Mate guard during days prior to female laying their single egg
- Male doesn’t need to guard mate against rivals
How successful is mate guarding at controlling male paternity?
- Female fertile period is four days before egg-laying
- Insemination at this time could lead to fertilisation
- In the days before this fertile period many EPCs intrude onto females territory
- Many EPC attempts but little success
- As soon as fertile period starts pair male starts to mate guard and this reduces intrusions and EPC’s
- Some EPCs do occur during this period because the female is helping them to happen
Altering mate guarding experiment
- By placing a fake egg in the nest before the female lays her real egg, mate guarding behaviour can effectively be switched off
- When males stopped guarding during the fertile period their partner was exposed to many intrusions and EPC attempts and successful copulations
- Shows how effective mate guarding really is
Does time spent guarding from other males give more offspring than not guarding and attempting EPCs?
- Investigated in blue milkweed beetles
- Following copulation the male remains mounted on the female back so other males cannot copulate
- Experiment removed beetles
- 25% of separated males found a new mate within 30 minutes. So guarding has a cost in terms of missed mating opportunities
- 50% of females found a new mate after guarding male was removed. So guarding female prevents her from remating
Where do the costs and benefits of mate guarding overlap?
- Guarding is beneficial if the last male to copulate fertilises >40% of the eggs
- If the last male fertilises <40% of eggs it is better not to guard as the female isn’t worth guarding
- Guarding male will get a higher return in terms of offspring fathered
Protecting paternity
- Copulating frequently with partner to outcompete other males sperm
- Male fulmars copulate over 50 times with their partners even though just once fertilises eggs
- This frequent copulation ensures optimal timing of insemination relative to fertilisation
- These act to promote sperm competition and increase male fitness
- Strong evolutionary pressures on males to gain fitness
- Equally strong pressures on female to retain fertilisation control over males
Cryptic female choice for selected sperm
- Female manipulation of sperm inside her reproductive tract so that a preferred male fertilises her eggs
- Outcome is that a preferred male fertilises her eggs
- Sperm exist in storage tubules
Can the female manipulate sperm so a preferred male can fertilise her egg?
- Female chickens can’t always dictate which males will mate with them as they can force copulate
- Females have some control over whether they father her offspring
- Females can eject sperm after mating with a low ranking male
- Females more likely to reject sperm of low ranking males
Sperm ejection in Dunnocks
- A male may peck at his partners cloaca if another male has been near her. In response she may eject a droplet of sperm containing fluid
- This looks as if she’s ejecting low ranking sperm for new male
- In fact this sperm ejected is non-viable, more likely to be just clearing out reproductive tract but the first males sperm is already in her tubules
- Falsely tricks male into believing she is ejecting previous males sperm
Nuptial gifts and sperm acceptance in hanging flies
- Large nuptial gift of a dead insect in exchange for whole sperm transfer
- Only males providing large gifts transfer full complement of sperm as this takes time, it is done whilst she eats nuptial gift but after this is eaten she ends copulation
- If gift isn’t large enough copulation will end before full sperm transfer
Why do females engage in cryptic female choice (CFC) two reasons?
Females may gain:
1. Higher quality offspring 2. More offspring
Alternative mating strategies
How do males that are unsuccessful in competition and are not chosen by females achieve reproductive success?
Alternative mating tactics:
1. Alternatives are not equally rewarding (best of a bad job) 2. Alternatives that are equally rewarding
Scorpion flies Large males
- Guard dead insects attractive to females
- Gain more mating to males who can’t guard
Get 6 matings
Scorpion flies medium males
- Salivary gifts to attract females
- Not as attractive
- Less mating but still some
- 2 matings
Scorpion flies small males
- Force copulations on females
- Not successful
- 1 mating
What happens if large males are removed from the population?
If large males are removed from population then medium males adopt position of large males and use dead insect tactic, smaller males adopt tactics of medium males
Small males cannot compete so resort to a different tactic
Isopod’s
- Three male morphs
- Different in size
- Alpha = most dominant, fights off rivals to mate
- Beta = female mimic
- Gamma = hider, sneak mating
- Alpha can throw gamma out
- Beta male pretends to be a male and alpha males waste time attempting copulating with it
Are morphs different tactics or distinct strategies?
- Differences between morphs should be genetically determined if they are distinct strategies
- Should all have equal reproductive success in order to maintain morphs
- The three morphs have different genotypes
Reproductive success depends on males and females living together
- An alpha and a beta living with one female, the alpha fathered most offspring
- If an alpha and beta live with many females, beta fathers most offspring
- In some situations gamma is best
- Three morphs have equal fitness in different sex ratios
Alternative male tactics in bluegills
- Large males are territorial
- Sneaker males lie low and slip into spawning pair, then fertilize eggs before larger male does
- Satellite males have coloration of females, slip in between territorial male and fertilize when female is spawning, male is unbothered as believes it’s another female
- 3 males are morphologically different
Who fathers the offspring in bluegillsunfish?
- Territorial males father 98.7% of offspring
- Other two strategies combined father 1.3% of offspring
- Once they become bigger they may become territorial
Summary:
- Passing on genes is the most important act in the life of any animal
- Hence the wide variety of behavioural strategies to achieve reproduction
