Week 4, part 3- Prudent sperm allocation. Flashcards
Sperm competition theory- Within individuals:
What must males trade off?
What does risk mean?
Males must trade off costs of sperm production against risk of sperm competition.
Risk = risk that there is any competition at all.
Continuation from sperm competition theory- within individuals:
What does this theory predict?
Is this WITHIN INDIVIDUALS?
Predicts that individual males, when faced with varying levels of sperm competition, should adjust the number of sperm they deliver according to variations in the risk and intensity of sperm competition.
Yes.
Continuation from sperm competition theory- within individuals:
What should you expect to see?
What does this mean?
What can they not do and what should they rely on?
Prudent sperm allocation.
This is when males allocate their sperm in a sensible fashion- they save it when there is no competition and deliver more when there is competition to maximise their chances of success.
They cannot assess risk directly. They must rely on cues.
Cues of sperm competition risk:
Give an example of one cue.
Explain it more with questions.
Presence of one or more rivals prior to or during copulation.
Have there been other males around this female before a male copulates with her? + Are there other males around waiting to copulate with her?
Continuation from cues of sperm competition risk:
Give an example of a second cue.
Explain it.
Male mating status.
If a male has already mated this means he has been alive for so long that any female he meets have probably also already mated with someone.
Continuation from cues of sperm competition risk:
Give an example of a last cue.
Explain it.
Time spent guarding a female prior to copulation.
If a male spends a lot of time with a female before he copulates with her, that is time she could have copulated with another male.
Early evidence of prudent sperm allocation:
What did Geoff Parker predict?
When?
What happened in the late 80s and early 90s?
He predicted that males might adjust the number of sperm they deliver from one copulation to the next in response to cues.
He predicted this in the 1970s.
Empirical evidence started to accumulate which showed that males in some species can engage in prudent sperm allocation.
Cues of sperm competition risk- presence of rival males:
Who did this study and when?
What insect was this study on?
What do flower beetles do when they are mating with a female and there is a rival male there?
What do they do when other males are just there?
What do they do after copulation when another male is there?
Gage and Baker, 1991.
Flower beetles.
They deliver more sperm before and during copulation.
They are more eager to initiate mating.
Show more guarding behaviour, sticking with the female after copulation.
Continuation from cues of sperm competition risk- presence of rival males:
What insect did Gage and Bernard 1996 look at?
What did they find? 2 things.
Field and house crickets.
1) Males delivered more sperm when mating in the presence of rivals.
2) They increased the number of sperm in proportion to the number of rivals present.
Continuation from cues of sperm competition risk- the time spent with a female prior to copulation:
Who did this study? And on what?
What did the rats in this study do?
Were they actually mating?
Bellis et al, 1990 on norway rats (mammals).
They either mated with a female they never met before or they guarded one for 4 or 5 days prior to mating with her.
No- they were separated by a wired mesh.
Continuation from cues of sperm competition risk- the time spent with a female prior to copulation- Bellis et al, 1990 study:
What did they find?
What did they argue this was evidence of?
Explain why they thought this was prudent sperm allocation?
They found that males deliver more sperm when mating with a female they had not previously been guarding.
Prudent sperm allocation.
Because the lack of guarding was a cue of potential sperm competition risk, this risk is much lower in the guarding situation.
Continuation from cues of sperm competition risk- the time spent with a female prior to copulation- Bellis et al, 1990 study:
What are two limitations of the study?
How do norway rats live in the wild?
What might females do?
Do they fight?
In colonies, what is there no prospect of?
Limitations:
1) Unrealistic- rats do not live like this in the wild.
2) Small sample of rats.
They live in colonies. There can be hundreds of them.
Females might copulate sequentially with many males during a single estrus (fertile period).
No.
There is no prospect of a male guarding a female for many days, it just would not happen in the wild.
Continuation from cues of sperm competition risk- the time spent with a female prior to copulation- Bellis et al, 1990 study- Limitations:
What is one limitation and explain it.
1) The prolonged period of guarding prior to copulation is unlikely to occur in the wild (the novelty of meeting a new female could have made them ejaculate more).
Continuation from cues of sperm competition risk- the time spent with a female prior to copulation- Bellis et al, 1990 study- Limitations:
What is the second limitation? Explain it.
Why was it not within subjects?
2) No evidence of adjustment by individual males (not within subjects). It was between subjects.
They did not study the number of sperm produced by individual males from one population the next. They did not look at within subject changes.
Continuation from cues of sperm competition risk- the time spent with a female prior to copulation- Bellis et al, 1990 study- Limitations:
What is the last limitation?
Explain it.
Did they have a directional prediction?
3) Small sample sizes. They were randomly allocated. One group had 4 males, the other had 6.
Yes- they could have used a one-tailed test but it still not strong evidence.
