The Evolution of Sex, Sperm Competition, Cryptic Female Choice, and Alternative Mating Strategies

Question 1

Evolutionary Origin of Two Sexes:
Large Gametes (females) & Small Gametes (males)

Answer 1

Ancestral state: equal sized gametes

Mutant with small gametes arises. Will it do better? Will it “invade”?

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

selection could favour a mutant individual (invading a population for the first time) that produces twice as many half-size gametes. However this advantage will only persist if the small gametes fuse only with large gametes and not with themselves, which is exactly what happens in sexually reproducing systems.

Question 2

Sperm Competition

Answer 2

The competition between the sperm of two

or more males inside the female’s reproductive tract

Question 3

Outcompeting Rivals

Answer 3

Male Damselflies remove the sperm of rivals

In insects, polyandrous males produce a higher proportion of viable sperm than monogamous males

Question 4

Strategic Allocation of Sperm

Answer 4

Male Adélie penguins strategically allocate sperm to EP females

Question 5

Protecting Paternity

Answer 5

Males may guard their mates and drive off rivals (magpie & zebra finch)

Question 6

Mate Guarding in Seychelles Warblers

Answer 6

Mate guarding prevents extra-pair copulations.

Question 7

Mate Guarding in Blue Milkweed Beetle

Answer 7

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 8

Protecting Paternity

Answer 8

Male Fulmars copulate over 50 times with their partners.

Frequent copulation ensures optimal timing
of insemination relative to fertilisation.

Question 9

Cryptic Female Choice

Answer 9

Female manipulation of sperm inside her reproductive tract so that a preferred male fertilises her eggs

Sperm in a sperm storage tubule in the reproductive tract of a domestic chicken

Question 10

Sperm Ejection in Chickens

Answer 10

Females eject the sperm of subordinate males and accept the sperm of dominant males
Pizarri and Birkhead (2000)

Question 11

Sperm Ejection in Dunnocks

Answer 11

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, but this sperm is non-viable. Fools male.

Question 12

Nuptial gifts and Sperm Acceptance in Hanging Flies

Answer 12

Hanging fly (Mecoptera) males give a nuptial gift to the female. While she is snacking the male mates and transfers sperm. Larger size of nuptial prey meals duration on copulation is longer and more successful.

Question 13

Why do females engage in Cryptic Female Choice?

Answer 13

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

Question 14

Alternative mating strategies

Answer 14

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

Question 15

Alternative Male Tactics in Scorpion Flies

Answer 15

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

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

Small males
Force copulations on females
N = 1 mating each

Alternative tactics with unequal fitness

Question 16

Alternative Male Morphs in an Isopod

Answer 16

Paracerceis sculpta males occur in three morphs with different mating advantages.

The three morphs have different genotypes. Alpha (fighter), Beta (female mimic), Gamma (hider)

And occur at frequencies at which their fitness is equal.

Shuster & Wade 1991

Alternatives strategies with equal fitness

Question 17

Alternative Male Tactics in Bluegills

Answer 17

Territorial (large) 98.7% of offspring is theirs, Little sneakers (small), Satellites w/ female markings (Medium)

Question 18

Reproductive Behaviour Conclusion

Answer 18

Passing on genes is the most important act in the life of any animal
Hence the wide variety of behavioural strategies to achieve reproduction