Optimality Theory

Predicts that animals should behave in a way that…

  •       Maximize benefits
  •       Minimize costs
  •       Maximize net energy gain

                Central Assumption: the more net energy an individual gains, the greater reproductive success (fitness)

  • A range of costs and benefits for a particular behavior that has various phenotypes.
  • Adaptations occur when the behavioral phenotype has more benefits than costs.
  • The phenotype with the highest benefit relative to cost will be the adaptation compared to what is happening in the rest of the population.
  • Benefits > costs =maintained trait

Costs and benefits of group size:

Costs: Competition for resources/mates

Benefits: group protection

Peak benefits when “covey” size is at optimal (not too small/not too big)

“covey” = group of birds

“ibis group” by Michelle Galloway (CC BY 2.0)

Optimal foraging:

Maximize amount of energy gained, minimize cost  of obtaining food.

(Ex.) Reto Zach’s observations: crows only chose large whelks (snails); fly to about five meters up before dropping onto the rocks to break shell, continue to fly and drop until it breaks (even if many trials are required)

  • Predictions: if crows are foraging optimally then…
    •   Large whelks should be more likely than small ones to shatter after a drop of 5 meters
    •  Drops of less than 5 meters should have lower breakage rate
    •   Drops of more than 5 meters should not greatly improve cracking whelk
    • Probability of breaking is independent of the number of drop
  • Results:
    •  At any height the large whelks take less drops to break
    • Increased height of drops does not significantly change number of drops needed
    • The hypothesis that crows forage optimally is supported.

“Whelk” by Bernard Spragg. NZ (CC0 1.0)

Profitability of prey = energy gained per unit handling time

  • Young garden skinks lower their foraging success in order to reduce the risk of predation. Forage on crickets, hide from predators (snakes).
    • The benefit of avoiding predators outweighs the cost of losing food.
  • Risk of predation/parasites/disease affects optimality model.

“Skink” by Robyn Jay (CC BY-SA 2.0)



Alternative Foraging Strategies:

  • Differences within populations
  • Evolutionary process
  • Genetically based strategies maintained by frequency-dependent selection. Two distinct strategies are maintained. Both strategies both have fitness benefits.
    • Fitness depends on rarity- as you become more frequent you become less fit. Fluctuations between the best fit behavioral genotypes.
  • Conditional strategies based upon environmental conditions
  • Geographical separation and divergence
  • Conditional strategy: inherited mechanism that gives an individual the ability to be flexible
    • Neospintahrus trigonum are predators and eat host spiders
      •  kleptoparasites -steal prey
      • They also steal webs from host spiders
      •  commensals -take food that is unavailable to hosts-when they are babies
      • scavenge for deserted webs when it is a baby.
      • Environmental pressure influence
      • More likely to steal webs when there are many hosts
      • More likely to be kleptoparasites/commensal when the hosts are large.

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