Lecture 7
Mating Systems &
Social Behavior
Sept. 22, 2010
1
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
2
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
3
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
4
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
5
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.
Sept. 22, 2010
7
63% of litters multiple paternity with 4 males
siring offspring from 1 litter
Precocial vs. Altricial
• Males contribute less in precocial systems.
Sept. 22, 2010
9
Common exceptions limited
parental investment–
Carnivora, Primates, Rodentia
Sept. 22, 2010
10
Hardwiring?
• Changes in hormone levels in some males lead
to parental care.
• Pregnant females urine enhances paternal
behavior
Sept. 22, 2010
11
“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
12
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
Sept. 22, 2010
14
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.
Sept. 22, 2010
16
Monogamy
• Relatively rare in mammals (<5%
mammals)
• The bulk of instances is found in Primates,
Carnivora, and Rodentia.
Sept. 22, 2010
17
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?
Sept. 22, 2010
22
Neuroendocrine control
• Higher levels of hormones implicate
monogamy
– Oxytocin
– Vasopressin
Sept. 22, 2010
23
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
Sept. 22, 2010
24
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
Sept. 22, 2010
25
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
Sept. 22, primates)
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)
Sept. 22, 2010
28
Social behavior theories
•
•
•
•
The selfish herd
Kin selection
Reciprocal altruism
Parent manipulation
Sept. 22, 2010
29
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?
Sept. 22, 2010
30
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
Sept. 22, 2010
31
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.