Sept. 22, 2010 1

Lecture 7

Mating Systems &

Social Behavior

Sept. 22, 2010 2

Today’s topics

• 1. Parental care– Investment– Conflict

• 2. Mating systems (Monogamy, Polygyny, Polyandry)

• 3. Social behavior– Cooperation– Group living– Selection theories

Sept. 22, 2010 3

Parental Investment

• Any behavior that increases the offspring’s chances of survival at the cost of the parent’s ability to rear future offspring.

• A tradeoff to maximize fitness• http://www.alaskaphotographyblog.com/2010/06/wolf-pack-kills-moose-calf/

Sept. 22, 2010 4

Uniqueness of Mammals

• While most animals provide no care for their offspring, one or both parents provide at least some care in ALL Mammalia species.

Sept. 22, 2010 5

Sexual differences• Females often invest more in each individual

offspring (gestation, lactation)– Because of limitations in number of offspring, females

are more “choosy” in mates (quality over quantity)

• Males often invest little. Many males only contribute sperm. – Investment not as trivial as you many think

• Limits to ejaculation rate• Remember sperm competition

– In most cases, quantity over quality

Sept. 22, 2010 7

Internal fertilization• Males of species that internally fertilize

can not confirm that their sperm actually fertilized the egg.

• The female knows the offspring is hers.

• Trivers (1972) hypothesis of why males may be less willing to invest.

63% of litters multiple paternity with 4 males siring offspring from 1 litter

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Precocial vs. Altricial

• Males contribute less in precocial systems.

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Common exceptions limited parental investment–

Carnivora, Primates, Rodentia

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Hardwiring?• Changes in hormone levels in some males lead

to parental care.• Pregnant females urine enhances paternal

behavior

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“K” vs. “R” selection

• K = stable environmental conditions, larger body size, develop more slowly, longer lifespan, lower mortality rates.– “K” is a reference to carrying capacity.

• R = fluctuating environments, high reproductive rates, rapid development, small body size, little parental care– “r” is reference to reproductive rate.

Sept. 22, 2010 14

Parent-Offspring Conflict

• Why do offspring resist the weaning process?

• Natural selection may operate differently on the 2 generations (Trivers 1974).– Mother wants to invest to a point, then move

on to the next offspring– Offspring wants the mother to continue to

invest twice the benefit

Offspring-Offspring conflict

• Born with fully erupt canines and incisors

• If litter mates are same sex, on is often killed by the other.

Sept. 22, 2010 15

Mating systems

• Polygyny – Males mate with multiple females

• Polyandry – Females mate with multiple males

• Monogamy – One male and one female

• Promiscuity – Absence of prolonged association and multiple mating by at least one sex.

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Monogamy• Relatively rare in mammals (<5%

mammals)

• The bulk of instances is found in Primates, Carnivora, and Rodentia.

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Polygyny

• Most common in mammals• Resource defense polgyny – males defend

good habitat important to females• Female defense polygyny – females herd

for protection from predators and males exclude other males from their harem

• Male dominance polygyny – males congregate and advertise their fitness using courtship signals

Sept. 22, 2010 18

Resource defense polygyny

Female Defense Polygyny

Male Dominance Polygyny

• Male display sites• Leks – more common in birds• http://www.youtube.com/watch?v=aTr0KimaN70&p=49CD6F80A4148072&playnext=1&index=6

Polyandry

• Rare in mammals

• Although genetics is changing theories

• Multiple paternity?

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Neuroendocrine control

• Higher levels of hormones implicate monogamy– Oxytocin– Vasopressin

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Social behavior

• Society – a group of individuals of the same species that is organized in a cooperative manner.

• Complex social behavior has evolved in almost all orders, but especially among carnivores, cetaceans, and primates

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Cooperative rearing (although not common)

• Individuals other than the young’s mother provide care– Lionesses share nursing– Subordinate wolves regurgitate food– Meerkats young may be raised by up to 30

helpers

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Why do some mammals live in groups?

• Benefits– Protection from physical factors (huddling from cold)

– Protection against predators – Finding and obtaining food – Group defense of resources– Assembling members to locate mates– Division of labor among specialists (rare in

mammals – mole rats)– Richer learning environment (dolphins and

primates)Sept. 22, 2010 26

Huddling from cold

specialists

Why do some mammals live in groups?

• Costs– Increased intraspecific competition (more strain on

local resources)

– Increase chance of spread of disease and parasites (lice, CWD)

– Interference with reproduction (new male takes over, infants are killed – common in lion prides)

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Social behavior theories

• The selfish herd

• Kin selection

• Reciprocal altruism

• Parent manipulation

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The Selfish Herd theory

• Aggregating reduces an individual’s chance of being caught by a predator– An individually “selfishly” moves to the center

of the group to avoid being picked off.

• Musk ox?

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Kin selection theory

• An altruistic gene’s success depends not on how the individual benefits, but on the gene’s benefit to itself.– Hamilton’s rule (b/c > 1/r)

• b = benefit to recipient• c = cost to altruist• r = coefficient of relationship

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Inclusive fitness

• Reproductive success of an individuals own offspring + reproductive success of relatives.

For kin selection to work, individuals need to be able to

determine relatedness

• 1. Familiarity

• 2. Phenotypic matching

• 3. Recognition of genes– Example = MHC

Reciprocal Altruism

• Individuals may cooperate and behave altruistically if there is a chance that they will be recipients of such acts later– “you scratch my back, I’ll scratch yours.”– Carma?

Parent Manipulation

• Parents manipulate the offspring the the parents advantage– Giving parental care so offspring have equal

chance of surviving and reproducing– Restricting parental care when resources are

scarce– Killing some offspring– Temporary or permanently sterilizing offspring

and enslaving them as helpers.