Mating Systems

1
Mating Systems

• Lecture 9

2
Mating success

• Determines how many offspring an individual can
  potentially leave behind
• Darwin recognized that competition for mates can
  be intense

3
Sexual selection

• Darwin proposed the idea of sexual selection
• Selective pressure imposed on one gender by the
  other
• Usually takes the form of
• Male Male competition
• Female Choice

4
Typically

• Males compete for mates
• Females choose or reject potential mates
• Why?
• Answer is related to relative investment in
  offspring.

5
Sexes invest differently in Reproduction

• In birds, mammals and some invertebrates
• The males energy contribution to offspring is
  minimal
• Sperm cells contain very little energy
• Females producing a clutch of eggs or litter of
  offspring require a large expenditure of energy
• Eggs and foetuses take much more energy to produce

6
Females

• Reproductive potential is limited by the number
  off eggs or foetuses they can produce
• During a season
• During their lifetime

7
Males

• Contribution that males make to an offspring is
  small
• Males can produce unlimited numbers of sperm
• Incur no significant energetic cost
• Theoretically fertilize unlimited numbers of
  offspring

8
Relative differences in energy invested

• Robert Trivers recognized that these differences
  should have important consequences
• And that selection should act differently on the
  sexes

9
Relative differences in energy invested

• Means females have more to loose if they make bad
  mating decisions
• Males have little to loose by wasting sperm
• Females cant produce unlimited numbers of eggs
• Females should be choosy gender

10
One of the consequences of sexual selection is

• That male reproductive success is much more
  variable that female reproductive success
• This has implications for preserving genetic
  diversity in species being managed as part of
  reintroduction projects

11
Sexual selection

• Can favour traits that would otherwise hinder
  survival
• Darwin recognised that elaborate ornaments that
  the males of many species use to attract females
  must impose survival costs these traits seem to
  be maladaptive.
• Traits that are important in male male combat
  may also impose costs

12
Examples

• Birds of paradise
• Peacocks tail

13
Female reproductive success

• Influenced by resources (e.g., food, shelter,
  high quality nesting sites)
• Females therefore distribute themselves according
  to resource availability
• Males distribute themselves based on the
  availability of females

14
Males

• Can compete directly for females
• Or they can compete for the resources that
  females require

15
Evolutions of mating systems

• Depend on female dispersion
• Whether male parental care is required to
  successfully raise offspring

16
Males can monopolize several females if

• Male parental care does not improve female
  reproductive success
• When females have small home ranges
• When females form small stable groups

17
When females are

• Solitary
• Have large home ranges
• Occur in large unstable groups
• Males are unlikely to monopolize more than one
  female at a time

18
Seals that breed on pack ice

• Breeding sites are abundant
• Therefore females are widely dispersed
• And males defend single females (or small groups)

19
Seals that breed on land

• Safe breeding beaches are limiting
• Therefore females occur at higher densities at
  these sites
• Groups are stable
• Therefore males defend harems of females

20
Rodents

• Beldings ground squirrel
• Females live in aggregations
• Males fight for dominance in areas of high female
  density
• Defend small mating territories
• Females mate with several males
• First male sires 60 of young

21
Rodents

• 13-lined ground squirrel
• Females also mate with several males
• Females are widely dispersed
• Males mate and then continue searching for
  additional mates
• Do not guard mates or defend territories
• Little aggressive behaviour
• Females mate with several males
• First male sires 75 of young

22
Rodents

• 13-lined ground squirrel
• Male search tactics determine mating success
• Beldings ground squirrel
• Ability to defend territories

23
Rodents

• Idaho ground squirrel
• Females also mate with several males
• Males mate and then continue guard mates for
  several hours
• This species lacks the first male advantage
• Males that guard mates sire greater proportions
  of offspring

24
These comparative studies

• Support the hypothesis that
• Resources determine female dispersion
• Female dispersion influences the distribution of
  males and the type of mating system that evolves

25
Experimental studies

• Voles (Clethrionomys rufocanus)
• Radio telemetry experiments
• Females non-territorial
• Experimental additions of food
• Led to reduces home range size of females

26
Voles (Clethrionomys rufocanus)

• Males defend territories
• Overlap several female home ranges
• Experimental additions of food
• No change in territory size of males

27
Voles (Clethrionomys rufocanus)

• Experiments with cages females
• Females individually housed in cages
• Males defended large territories when females
  were highly dispersed
• Territory size decreased when females were clumped

28
Lizards (Anolis)

• Females territory size changes with resource
  distribution
• Male territory size increases with body size
• A correlate of fighting ability

29
Spatial and temporal availability of mates

• Anurans (frogs and toads)
• Wood frogsexplosive breeders
• Bull frogsTerritorial
• Large males have a greater advantage when the
  time from pairing to spawning is longer