3. Polyandry (multiple males/female, male parental care)
a. few mammals or birds: jacana, some artic shorebirds
b. sea horses
c. 4 human societies
B. Causes of animal mating system variation
1. The ability of a male to provide parental care influences the payoff to a male
for remaining with a female versus seeking additional matings
2. Confidence of paternity influences whether a male is likely to provide care
to his young or another male's young.
3. The distribution of resources used by females determines the extent to which males
can gain access to or defend females.
a. When resources are rich, females have small territories and males can defend multiple
females, e.g. lizards
b. When resources are sparse, females have large territories and males either defend
single females, or wait for females to visit.
c. Male territory size is inversely related to female home range in grouse and antelope
d. Human societies are predominantly polygynous. Male access to wealth influences female reproductive success and level of polygyny
II. Sexual selection
A. Asymmetrical gamete size (anisogamy) means the sex with smaller gametes should
usually compete for access to the sex with larger gametes. This results in greater
variation among males than among females for reproductive success (Bateman's experiment).
Males should, therefore, fight over females and females should select for resources.
Bateman combined 3 males & 3 females, each with genetic markers, and scored individual reproductive success
1. Males have higher potential reproductive success
2. Males show greater variation in reproductive success
3. Elephant seals and humans show even more extreme variation in reproductive success
among males than Bateman's Drosophila
4. Human records: Female Muscovite from 1800's had 69 children in 27 pregnancies but the Emperor of Morocco, Monlay Ismail the Bloodthirsty, had 888 children.
B. Sexual selection will occur if the expression of a trait improves the ability of that individual to acquire mates, due to anisogamy this usually means that males develop secondary sexual characters, either through combat or female selectivity.
1. intrasexual selection -
a. premating: body size (many animals, including humans), horns, antlers (mammals and insects), and pollen number in outcrossing plants
b. postmating: sperm removal devices, testis size (sperm number)
2. intersexual selection - aesthetic traits - plumage, song, etc.
a. Direct benefits: females pick males because they increase fecundity
i. resource quality of territory
ii. provision female or offspring - e.g., hanging flies
b. Indirect benefits: females pick males for their genes
i. females may choose traits to get sexy sons
This should lead to a genetic correlation between a female's preference and the male's trait. Both are expected to increase until natural selection counteracts the effects of sexual selection by female choice. For example, bright coloration in guppies is attractive to females, but also attracts predators.
ii. Alternatively, females may choose traits that indicate genetic quality, such
as resistance to parasites or diseas. However, for such choice to be of value, there
must be heritable variation in disease resistance. Best example comes from barn swallows.
female barn swallows prefer males with elongated tail feathers
long-tailed males leave more offspring because they pair earlier and double-clutch
males which had their tails lengthened, have shorter tails in the next year, indicating
that long tails probably incur an energetic cost
long-tailed males have fewer ectoparasites (mites)
Cross-fostering experiments indicate that mite resistance appears to be heritable
c. Why don't males lie?
Costly signals ensure honesty, e.g. energy spent by sage grouse in display
