Transcript 23_MatingSystems

Announcements
Sexual selection underlies the evolution of male
competition and female choice.
In many species, males and females are similar in
appearance.
How do ecological factors contribute to differences
among sexes?
Mating systems
Monogamy - 1 male, 1 female
mate guarding
mate assistance
female enforced
Polygyny - 1 male, >1 female
resource defense
female defense
scramble competition
explosive breeding assemblage
lek
Polyandry - 1 female, >1 male
male defense
resource defense
Promiscuity
Sexual selection theory and coupled with low parental
investment of males suggest that polygynous mating
systems should be most common.
Why are males monogamous?
Resource-based Mating Systems
Emlen and Oring (1977) - the ecology of an organism may not
permit males to have more than one partner.
Females widely distributed and males cannot monopolize them.
Females may mate with another male so monogamy may serve
to guard the female.
If males help rear young, fitness increases through increased
young survival.
The evolution mating systems is driven by the distribution of
resources in the environment for both the male and the female.
From Sinervo, UCSC
Social monogamy relatively rare in mammals but
common in birds.
Eggs develop internally in mammals, externally in birds.
Differences in constraints and costs between sexes in
parental care.
Mating systems in birds:
Monogamy (pair bond between 2 individuals)
parental care shared
approx. 92% of all bird species
Polygyny (male mates with several females)
parental care usually by female
approx. 2% of all birds
Polyandry (female associates with several males)
parental care typically by males
fewer than 1% of all birds
Promiscuity (indiscriminant sexual relationships)
about 6% of all birds
Monogamy:
Pair bonds may last for a single breeding
attempt, a breeding season, or many
breeding seasons
Occurs when:
Male participation is essential for
successfully raising young
Males cannot monopolize resources
necessary for supporting extra mates
snow bunting
Polygyny:
Some males in a population regularly have two
or more mates
Why should a female pair with an already
mated male while there are still unmated males
available?
pied flycatchers
Polygyny Threshold Model: Gordon Orians (1969)
Predictions - A male's territory quality will be
correlated with his mating success
resourse defense
polygyny
fitness
Polygyny should be more common in patchy
environments (where there is more variation in
territory quality)
Polygyny Threshold Model: Gordon Orians (1969)
fitness
Predictions - A male's territory quality will be
correlated with his mating success
“ideal free distribution”
Polygyny Threshold Model: Gordon Orians (1969)
fitness
Predictions - A male's territory quality will be
correlated with his mating success
“ideal free distribution”
Polygyny Threshold Model: Gordon Orians (1969)
fitness
Predictions - A male's territory quality will be
correlated with his mating success
“ideal free distribution”
Polygyny (non-resource based):
female defense
scramble competition
explosive breeding assemblage
lek
Males compete for access to mates.
Females get only sperm from males (they raise young
elsewhere unaided by males)
In leks, males display 'status' in communal displays &
females choose among males.
Much variation in male mating success (one or a few
males copulate with many females; other males with
none).
The sons of successful males are more likely to be
successful themselves.
What conditions favor the evolution of leks?
'hot spots' - males gather at sites where
females are more likely to congregate
reduced predation
greater resources (undefendable)
amplify sexually selected characteristics
'hot shots' - females prefer to choose mates
from aggregations of males (i.e., group displays
facilitate comparisons).
The Lek Paradox
Since females usually mate with one male, why do other
males bother to come to the lek at all?
With one male getting most matings, why is there still
variance in traits?
Polyandry:
rare (sandpipers and jacanas, rhea)
typically involves sex-role reversal
(females larger & more brightly colored)
males incubate eggs & care for young
number of clutches may be limiting factor
(rather than size of clutches)
Polyandry:
1. females that abandon nests can lay
more clutches.
2. males care for clutches (alone),
have more nestlings survive.
3. females that defended resourcebased territories attracted more
males and have more offspring
Extra-pair copulations (EPCs)
Occur in monogamous species, polygynous
species, & polyandrous species
In many monogamous songbirds, the
percentage of extra-pair young has been found
to be about 10 - 25%. (as high as 80%)
Gibbs, Weatherhead and colleagues, 1990, Science
Benefits of EPCs for males:
Increased fitness
Possible future mate acquisition
Insurance against mate's infertility
Benefits of EPCs for females:
Fertility insurance
Genetically diverse young
Improved genetic quality of young
Access to resources
Not all individuals pursue EPCs
Possible costs for males:
Sperm depletion & ejaculate production costs
Increased risk of cuckoldry
Reduction in parental care
Increased likelihood of “divorce”
Not all individuals pursue EPCs
Possible costs for females:
Male retaliation
Risk of injury
Harassment from extra-pair (or potential extrapair) males
By mating multiply, the female not only gets offspring that
might be of superior genetic quality, but she also gets
offspring that are quite variable.
Increased genetic diversity of offspring might be favored by
selection. (particularly in a variable environment)
This explanation for multiple paternity in females, is the
same argument regarding the evolution of sex.