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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 femalemate guardingmate assistancefemale enforced

Polygyny - 1 male, >1 femaleresource defensefemale defensescramble competitionexplosive breeding assemblagelek

Polyandry - 1 female, >1 malemale defenseresource 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 sharedapprox. 92% of all bird species

Polygyny (male mates with several females) parental care usually by femaleapprox. 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

Polygyny should be more common in patchy environments (where there is more variation in territory quality)

fitn

ess

resourse defensepolygyny

Polygyny Threshold Model: Gordon Orians (1969)

Predictions - A male's territory quality will be correlated with his mating success

fitn

ess

“ideal free distribution”

Polygyny Threshold Model: Gordon Orians (1969)

Predictions - A male's territory quality will be correlated with his mating success

fitn

ess

“ideal free distribution”

Polygyny Threshold Model: Gordon Orians (1969)

Predictions - A male's territory quality will be correlated with his mating success

fitn

ess

“ideal free distribution”

Polygyny (non-resource based):female defensescramble competitionexplosive breeding assemblagelek

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 predationgreater 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 resource-based 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 extra-pair) 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.