10. Mating systems Flashcards
What is sexual selection?
The effects of the “struggle
between the individuals of one sex, generally the males, for
the possession of the other sex” (Darwin)
How does sexual selection arise?
From differences in fitness
- associated with non-random variance in mating success (in one or both sexes)
What does sexual selection account for?
Most phenotypical differences between males and females
Evolution + maintenance of exaggerated traits (that don’t benefit individual survival)
What is the point of elaborate ornaments?
Attracting mates (mate choice)
What is the point of weapons for fighting?
Repelling competitors (same-sex competiton)
What does the example of male dung beetles show us?
Long horned males are better fighters, but have smaller eyes (tissue for horn construction becomes unavailable for building eyes)
- Exaggeration beyond optimum under natural selection (mating advantage)
How does sexual selection work?
- Eggs are costly, sperm relatively is not
- males fitness (nb offspring) is limited by access to females
- females invest more in gametes (+ pregnancy) —-> limited by rate of egg production and access to resources
+ females are 100% of maternity, males are not - result is competing males and choosy females (often)
What are the two components of sexual selection?
Same-sex competition (mate choice depend on who has better resources)
Mate choice (ornaments signal good genes)
What are some examples of same-sex competition?
Fur seal territoriality and harems
Wapiti stags bellowing & rutting
Male-male competition in bisons
What are some examples of mate choice?
Good resources (nuptial gifts in male hanging flies)
Genetic benefits (peacock trains)
Why does peacock train signify good genes?
Number of ocelli is correlated
with degree of symmetry (developmental stability)
Peacocks with the most ocelli acquire the most mates
What are some different mate choices in birds?
Physical attraction (peacock, golden pheasant, long-tailed widow bird)
Brains/talent (Bower bird: object collection, lyre bird: vocal imitation)
Creativity/dancing ability (Birds of paradise)
Why is it difficult to distinguish between mate choice for good genes vs resources?
Gaining good resources depends on good genes & vice versa
(AND good genes mean larger repertoire & subsequently a better territory)
How do pied flycatchers and sedge warblers differ in their mate choice?
Pied flycatchers choose males with best territory
Sedge warblers prefer males with most elaborate song repertoires independently of resources
What are the direct benefits of mate choice?
Territory quality
Good access
Male parental care quality and quantity
Sperm quality
What are the indirect benefits of mate choice?
Gains through offspring
Good genes hypothesis: females choose males bc their offspring will inherit genetic/health benefits by inheriting their fathers:
- vitality/viability (Zahavis handicap principle)
- parasite/disease resistance (Hamilton and Zuks principle)
No obvious benefits: for the sake of beauty/fashion —> Fishers runaway / sexy son hypothesis: females choose an ornament bc it is aesthetically pleasing. They gain grandchildren bc son’s ability to attract mates
What support is there for Hamilton and Zuks theory?
Møllers study of barn swallows
- Parasites reduce host fitness
- Parasite resistance is heritable
- Male tail length affects the time between arriving in the breeding area and attracting a mate >
- Females prefer males with longer tails >
- Males with longer tails fathered offspring with lower parasite loads
What are the different mating systems?
Polygyny/lekking: one male, several females
Polyandry: one female, several males
Monogamy: one female, one male (serial or life time)
(but social does not equal genetic mating systems)
What do mating systems depend on?
Operational sex ratios (fertile females/sexually active males) but sometimes only in theory
How are the sex roles in parental care across taxa?
95% of mammals have female only care
90% of birds have biparental care
Amphibians have either male or female only care
What’s an example of reversed sex roles?
Pipefish males (caring)
Here females are larger and more colourful
Mating systems factors
1) females > makes: polygyny (one male, several females)
- male-male competition and female mate choice
- larger male mortality rate
2) males > females: polyandry (one female, several males, more rare)
- female- female competition and male mate choice
- larger female mortality rate
Conclusion/take-home 1
Quality = viability (Zahavi)
Quality = health/resistance to parasites (Hamilton and Zuk)
Quality = attractiveness to females (Fisher)
Conclusion/take-home 2
Reproductive rate + investment in offspring production of males vs females/ willingness to mate + availability of the other sex
—> strength and direction of sexual selection:
- female or male mate choice and intensity
- female or male same-sex competition and intensity
- female or male parental care
- larger female or male mortality rates
—> variation between and within species
—> particular systems may arise for random reason
—> sperm competition and female choice often happen cryptically
——-> sexual selection can cause a reduction in (small) population viability
