Session 4: Behavioral Ecology

Behavioral Ecology

• ecology-organism-behavior
• Neo-Darwinian theory

 

STAGE 1

• Basic evolutionary theory

 

Processes of evolution

• Mutation
• Gene flow
• Genetic drift
• Natural selection
• the importance of individual variation, population variation, and the tendency of populations to look like the better fit variants
• Neo-Darwinian evolution is at the level of the individual (genotype) across generations and visible at the level of the population

 

Terms- from the Neo-Darwinian economic model

• Fitness= contribution to net lifetime reproductive success relative to other genotypes
• Strategy= set of behavior patterns that result from selection (ie. Result in net increase in reproductive success)
• Ecology= an organism’s environment and its activity and patterns
• Adaptation= trait/behavior whose prevalence is the result of selection in response to specific eco/environmental conditions
• Function= the ultimate affect a behavior has on fitness…frequently used as shorthand for behavior that provides a net fitness benefit (“functional”)

 

Practical notes

• Metaphor of “active action” is generally used to present strategies
• No cognitive intent necessary for most cases
• Be careful not to let metaphor/model become theoretical reality
• Primacy of socially variable environments for primates
• How do we measure fitness?  Proximate measures of lifetime reproductive success

 

 

Natural Selection and function

• Fitness value= net benefit to lifetime reproductive success
• Fitness cost= net cost to lifetime reproductive success
• Cost/benefit analysis and behavioral “choices”

 

Optimality theory (Model)

• Selection will favor those strategies with the highest net fitness returns
• Competition amongst variants leads, over time, to optimal fit/strategies
• But……can organisms be optimal in variable environments?

 

• “Selfish gene” paradigm (Dawkins, Wilson)
• Derives from basic capitalist economic model- maximization of net benefit in open competition (optimality)
• Genes seek to replicate themselves over other genes (selection at the genic level)
• Organisms compete to pass copies of their genome into the next generation
• Highest net benefit goes to those organisms (gene complexes) that maximize fitness

 

Keep in mind…

• Organisms are constellations of traits, oftentimes with conflicting pressures
• Behavior is not directly controlled by genome (no gene⇒behavior links for primates)
• Potential and performance as concepts

 

Other factors

• Chance/stochastic events
• Exaptation (co-opting of existing traits for a new purpose)
• Spandrels by-product of structural change
• Bauplan
• Phylogenetic constraints

 

Altruism?

• Benefit to others at a cost to self
• Seemingly altruistic acts are observed
• But, under a selfish gene or a Neo-Darwinian paradigm these acts select against self…

 

Kin Selection (Hamilton)

• C< B x R (Cost to Individual fitness <  fitness reward to relative * coefficient of relatedness)
• Inclusive fitness
• Assumptions of value based on % of unique genetic component

 

Mutualism

• Even distribution of benefits? 
• Prisoner’s dilemma game or Hawk/Dove model and why this model is argued against
• But……

 

Evolutionary Stable Strategy (ESS) (Maynard-Smith)

• “strategy such that, if all members of a population adopt it, then no mutant (competing) strategy can invade the population via natural selection”

 

STAGE 2

• Primate behavioral ecology--the basics

 

The initial challenges (ala Denham, 1971)

• Energy acquisition
• Predator pressure
• Reproductive strategies
• Male-Male competition
• Kin selection

 

Wrangham’s Model

Food►	Defendable	Groups Larger	Females cooperate Females Bond Female relationships well defined	►Males adjust to female distribution
Food►	Not Defendable	Groups Smaller	Females do not cooperate No female bonding Loosely defined relationships	►Males adjust to female distribution

 

 

Van Schaik and van Hoof

• Net food intake (food distribution and density) + predation risk ⇒ optimal group size and social organization

 

Terborgh and Janson

• Introduced patch size to van Schaik and van Hoof model (clumped vs. dispersed)
• Janson added: encounter rate, per capita consumption/per bout consumption, quality of foodstuffs, time spent feeding

 

Foraging and Competition  synthesized

• Two forms of competition:

• Van Schaik refined: female intra-sexual relationships based on food density, population density, and between group contest competition
• Isbell: food abundance shapes intergroup relationships and food distribution shapes intragroup relationships

 

Sterck adds to van Schaik

• Competition varies on three levels

 

A nod to habitat alterations and social flexibility…

 

Rare events as prominent drivers of systems

• Dispersal
• Predation
• Infanticide
• Parasites
• Disease

 

Dispersal

• How does it work?
• Its relation to intrasexual aggression?
• Isbell (2004): locational and social costs of dispersal affecting social organization

 

Predation

• How much of an issue/pressure is it?
• Past/present pressures…

 

Parasites and Disease

• Minimization of risk=group stability and site fidelity
• Female choice and male health
• Sexually transmitted diseases?
• How does this work in real life??

 

Infanticide

• A case study in approaches to primate behavior

• The driving force behind social evolution? 

• Male primates sometimes kill young.  In some species it appears to be relatively common, in others it does not appear to occur at all.  While there are a number of hypotheses proposed to explain males’ killing of infants the majority of recent attention has focused on the "sexual selection hypothesis" which states that infanticide is an evolutionarily adaptive strategy for male primates.  The killing of infants is an occurrence that biologically, and culturally, calls out for explanation.
  

 

Infanticide (strategy/functional)	Infant Deaths(action/non-functional)
A strategy- genetic basis	A result of behavior/circumstance - no genetic basis
Adaptive, in at least some contexts	Deleterious
Males as primary actors	Both sexes as actors
Females adapt to ameliorate the threat	Not a selective force

 

• For a detailed analysis of infanticide in wild non-human primates, see Carel P. van Schaik, "Infanticide by male primates: the sexual selection hypothesis revisited."  In Infanticide by Males and its Implications.  ed. Carel P. van Schaik and Charles H. Janson.  Cambridge University Press, 2000.  Pp. 27-60.

 

The sexual selection hypothesis

• Males compete for access to females
• Males compete via sperm competition
• Males maximize investment in number of potential offspring
• Removal of competing offspring provides potential benefits

• Females are a limited commodity (female receptivity limited)

• Maximization of fertilizations increases male fitness
• Lactational amenorrhea and reproductive inhibition
• Competition between adult males and non-related offspring
• Females cannot effectively defend their offspring by themselves

 

The sexual selection hypothesis  (Hrdy, van Schaik)

1. If a female loses her young offspring she will cycle (estrus) again more quickly than if she nursed offspring to full term
2. Males have some mechanism by which they can potentially differentiate their offspring from those of other males (mating association, facial recognition, etc…)
3. On average if a male is able to kill an infant he will then be able to mate with the female and have a high likelihood of achieving fertilization

 

The sexual selection hypothesis

• Males should therefore weigh the various factors and attempt infanticide when possible
• Females should therefore adapt via behavioral associations

• These adaptations have resulted in the types and patterns of social associations that characterize primates
• Male-female bonds are adaptation to prevent infanticide
• Sexual coercion and the bodyguard hypothesis (Mesnick)

 

The sexual selection hypothesis for humans (Daly and Wilson)

• Human males are able to use aggression and violence to commit infanticide in situations where paternity is certain and costs are high
• Human females also commit infanticide as a preventative strategy
• This pattern has resulted in an imbalance of reproductive power in humans
• Sexual coercion

 

The data

• Mammals (rodentia, carnivora, primates)
• Humans--cross-cultural variability
• For nonhuman primates:
• • “conservative”--tentative, there are not enough data
  • “non-conservative”--affirmative, there are sufficient data

 

Nonhuman Primates: the data--observed cases

• 55 observed cases (12 genera, 17 species)
• 46% from two genera (42% from two species: The Hanuman langur (Semnopithecus (Presbytis) entellus) and The red howler (Alouatta seniculus)
• 40% of cases not clear if male mated with female subsequent to the infant death (13% of the cases the male did not mate afterwards)
• 73% of cases male and infant not related, 27% unlikely but possible genetic relationships
• Context varied but majority in take-over/immigration by male into group

 

Non-human Primates: the data--inferred cases

• 18 genera and 41 species
• 11 species only in captivity
• Context usually take-over or immigration

 

Non-human Primates: the data--van Schaik’s conclusions

• Time gain (female cycling) of .25 derived from combined observed/inferred data set
• Unclear costs associated with injury (but probably small)
• Observed cases and inferences support sexual selection hypothesis over other hypotheses
• “infanticidal tendency in males is very common, if not universal”
• There should be female counter-strategies

 

Other hypotheses to explain the occurrence of infants' killing by males

• Social pathology
• Infanticide as by-product of aggression (infant attraction)
• Cannibalism
• Resource competition

 

Infant deaths result from male aggression...

• Evolutionary role--what is necessary?
• Could it be a strategy?--how to test?
• For langurs? howlers only or more widespread?
• Perspectives:
• • “conservative”--tentative, there are not enough data , it is not fully supported and other hypotheses are still important venues for research investigation
  • “non-conservative”--affirmative, there are sufficient data, this is fully supported (and should be “accepted”), therefore other hypotheses are irrelevant

 

The debate: three positions

• It occurs and is more than well-documented.
• Does it occur?
• Is the SS hypothesis the most viable (supported) hypothesis based on the current data set?

 

What about Female infanticide?    (L. Digby)

• “The complexity and richness inherent in the social networks female primates forge for themselves has, too often, obscured a vital fact of their lives: that competition among females is central to primate social organization” –S. Hrdy 1981
• 50 species across 5 mammalian orders (rodents, lagomorphs, pinnepeds, carnivores, and primates): over 300 cases of documented female infanticide
• Exploitation and Resource competition hypotheses
• “direct” and “indirect” infanticide
• Indirect most common in primates (observed in 10 species)

 

Comments:

• Hrdy
• Sommer
• Why this contention?
• “Is” vs “Ought” and where does this play in to the discourse?

 

Forcing dichotomies/ ignoring variation in accessing primate societies?

 

• Patch size
• Dominance
• Infanticide
• Predation
• Anthropogenic impact

 

Biases, science and the quest for understanding...