BSC 385 Lecture 8: Chapter 6 Continued

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Chapter 6 Continued
Interactions Between Sexual and Natural Selection
Balance hypothesis
o Male traits are exaggerated by female choice until their overall fitness cost is too
o Example of the evolutionary trade-off
o Elaborate phenotypic traits of males are an indicator of overall fitness
o Help females choose a superior male, thereby increasing the quality of the
Ecological Factors that Influence the Mating System
The mating system is the set of relationships between males and females during
o The number of mates an individual copulates with during the breeding season
o The relative contribution of males and females to parental are
o How long the relationship between males and females lasts
Types of Mating Systems
Monogamy: males mate with a single female
o Serial monogamy: monogamic relationship lasts for a single breeding season
Polygyny: each male can mate with more than one female during a breeding season
Polyandry: each female can mate with more than one male during a breeding season
Ecological Determinants of Monogamy
If successful rearing of the young requires both parents, monogamy is advantageous for
both sexes
If reproduction is highly synchronous (e.g., due to the seasonality and resource
phenology), providing parental care is more advantageous to a male than seeking
additional copulations
Monogamy most common type of mating system in passerine birds
Types of Monogamy
Social monogamy
o Female bonds with one male
o Male and female share in parental care
Genetic monogamy
o All progeny of a socially monogamous pair is produced by the male and the
female of that pair
o Rare (extrapair copulations are common, e.g., sandpiper)
o A few exceptions: e.g., the African hornbill, Peruvian poison frog
Ecological Determinants of Polygyny
Spatial distribution of females
o The potential for males to control and ensure access to more than one female is
dependent on the spatial distribution of females (e.g., their aggregation)
Spatial distribution of other critical resources
o If critical resources are distributed such that males can defend them, the males
may obtain a territory that attracts multiple females
Behavioral Determinants of Polygyny
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