Parental Investment

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Parental Investment

• Lecture 11

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Parental Investment

• So far we have focused on mating systems
• Producing fertilized offspring
• Choosing mates

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Parental Investment

• But parents must invest energy to raise young to
  the point at which they are independent
• Natural selection should favour investment
  decisions that maximize the production of
  offspring during an organisms lifetime

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Parental Investment

• Parents must decide how much of available
  resources to devote to
• Offspring survival
• Their own survival

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Parental Investment

• In this lecture well consider how parents
  decide to devote resources to
• Offspring survival
• vs
• Their own survival

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Parental Investment Definitions

• Parental care any parental behaviour that
  increases offspring fitness
• Parental expenditure expenditure of parental
  resources on parental care of one or more
  offspring
• Parental investment refers to the extent to
  which parental care of one or more offspring
  reduces a parents residual reproductive value

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Evidence that parental care influences fitness

• Fish that guard nest
• Removal of parents reduces survival
• Mammals
• Birth weight correlates with survival
• Egg size in birds
• Large egg size correlated with fledgling survival

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Long-lived mammals

• Primates (e.g. vervet monkeys)
• Females help defend offspring from dominant
  vervets
• Adult females with their mothers present have
  higher reproductive success

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Red deer

• Birth weight not only determines survival of
  calves
• But also the weight of the calves they will give
  birth to as mothers

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Hamsters

• Food deprivation (during first 50 days)
• Affects fecundity
• Mass
• And sex ratio of offspring
• But also mass of offspring of their daughters

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Parents have a limited amount of energy to invest

• Trade off between
• Investment per offspring
• Number of offspring

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Experimental evidence for this trade-off in
chickens

• Selection for larger eggsreduces egg laying rate
• Selection for increased egg laying ratereduced
  egg size

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Cost of egg care can be high

• In species where one parent cares for eggs alone
• Feeding is reduced
• Or ceases altogether
• Can reduce reproductive output in following
  seasons
• Birds 10-30 BMR to warm eggs

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Costs continued

• Birds
• Feeding nestlings up to 4X BMR
• Mammals
• Lactation 2.5 to 5X BMR
• Bats --food consumption by females 50-100 of
  body mass to meet energy demands of lactation

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Natural selection should

• Favour production of the offspring size/number
  combination that maximises lifetime fitness
• The best approach may vary with conditions

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If predation is high and size dependant

• Selection for small number of large offspring
• Primates
• deer

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If predation is high and independent of size

• Selection for large number of small offspring
• Some amphibians
• Insects
• Some fishes

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Incubation patterns in birds

• Some delay incubation until clutch in complete
• Other species begin immediately upon laying of
  first egg
• Delayed incubation

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Incubation patterns in birds

• Incubating eggs as soon as the first egg is laid
• results in staggered offspring size and an
  advantage for the first hatched offspring
• Hatchling
• Delayed incubation
• Results in equal-sized offspring

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Parental InvestmentWho cares for young?

• If number of young produced X probability of
  survival with parental care gt number of offspring
  produced by multiple matings X probability of
  survival without parental care
• Parental care should evolve

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Similar evolutionary decision for both sexes

• What one sex does with affect the equation for
  the other
• If for example care by at least one parent is
  required
• Can apply game theory or ESS models to predict
  what parental care system can evolve

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Parental care (Fish)

• Evolves in cases where offspring occur in harsh
  environments
• Marine fishstable environment low predation
  rates
• Parental care in uncommon
• Fresh watervariable environment high predation
  rates
• Parental care common

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Male vs Female parental care

• In freshwater fish male parental care is common
• Males defend nest sites that females need for
  spawning
• Little cost to males in terms of lost mating
  opportunities associated with providing parental
  care

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Female care in fish is rare

• Associated with species with short breeding
  seasons
• Cases where females defend territories from other
  females who cannibalize eggs

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Male parental care in pipefish

• Male carries eggs in brood pouch

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Male parental care in Phalaropes

• Roles are reversed
• Females are brightly coloured

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Parent-offspring conflict

• If parental care continues
• Offspring benefits
• Parental care terminated early
• Parent benefits by being able to reproduce again

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Evolutionary value of offspring changes with the
nesting season

• This should influence parental investment
  decisions especially if behaviours are risky
• e.g. predator defence

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Nesting stage
Previous investment is higher
Probability of re-nesting decreases
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Red-winged blackbirds

• Nest defence
• Risk taking increases
• From egg to nestling stage

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Offspring recognition

• Parents are under strong selection to recognise
  their own young
• In some species this is a huge challenge
• Mexican free-tailed bats
• 20 million babies in a colony

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Milk herd hypothesis

• Females randomly linked up with any unclaimed
  baby
• Difficult to explain how this could evolve

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How could mothers possibly find their offspring?

• Geographic land marks get them to the general
  area of the cave
• Vocal cuesmid-range
• Scent recognition to confirm ID
• Confirmed by marking pairs with LEDs (McCracken
  et al.)
• Genetic studies

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Gastric brooding frog