1. Mating Systems & Social Behavior
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
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.
Sept. 22, 2010

8. 63% of litters multiple paternity with 4 males siring offspring from 1 litter

9. Precocial vs. Altricial
Males contribute less in precocial systems.
Sept. 22, 2010

10. Common exceptions limited parental investment– Carnivora, Primates, Rodentia
Sept. 22, 2010

11. Hardwiring?
Changes in hormone levels in some males lead to parental care.
Pregnant females urine enhances paternal behavior
Sept. 22, 2010

12. “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
Sept. 22, 2010

15. 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

16. 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

17. Monogamy Relatively rare in mammals (<5% mammals)
The bulk of instances is found in Primates, Carnivora, and Rodentia.
Sept. 22, 2010

18. 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

19. Resource defense polygyny

20. Female Defense Polygyny

21. Male Dominance Polygyny
Male display sites
Leks – more common in birds

22. Polyandry Rare in mammals Although genetics is changing theories
Multiple paternity?
Sept. 22, 2010

23. Neuroendocrine control
Higher levels of hormones implicate monogamy
Oxytocin
Vasopressin
Sept. 22, 2010

24. 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

25. 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

26. 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

27. Huddling from cold
specialists

28. 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

29. Social behavior theories
The selfish herd
Kin selection
Reciprocal altruism
Parent manipulation
Sept. 22, 2010

30. 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

31. 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

32. Inclusive fitness
Reproductive success of an individuals own offspring + reproductive success of relatives.

33. For kin selection to work, individuals need to be able to determine relatedness
1. Familiarity
2. Phenotypic matching
3. Recognition of genes
Example = MHC

34. 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?

35. 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.