The Struggle for Food

Scavenging, Hunting, Avoiding predators

Avoiding becoming food is a huge selection pressure for animals

Predator Defense:

  • Group Defenses like the mobbing behavior by gulls: harass/chase to scare off predators (hawks) to protect eggs and juveniles as well as against other hawks/large birds in their area. The adaptation of mobbing is beneficial and they do not distinguish between predatory hawks or non-predatory birds. The adaptation is not perfect but increases fitness of the individuals in the population because all potential predators are scared off and the offspring are able to survive. Predator is more likely to get attacked as it moves closer into the colony of gulls.

The Comparative Method:

  • Look at two closely related species to study a behavior.
  • For example: To test if mobbing is a protection against predators, observe two populations of gulls.
    • one nests on the ground
    • the other nests in high cliffs where predators are less likely to attack so there is less predation pressure.
    • Measure through observations of the mobbing behavior of both populations.
    • Possible reasons:
      • Ground nesting populations moved to cliffs to avoid predators?
      • Divergent Evolution? Predict that cliff nesters should not mob if mobbing evolved as a defense against predation.
      • Or is there convergent evolution? Test if two unrelated species who live in high predation areas exhibit mobbing behaviors due to similar selection pressures.
        • For example: Gulls and colonial California ground squirrels exhibit mobbing behaviors.

Social Defenses:

  • Zebras live  together in large groups.
    • Their stripes make the group have “disruptive coloration” so predators have a harder time distinguishing one from another. Called the “confusion effect”.

      “Zebras” by Kathleen Steeden. (CC by 2.0)

  • Dilution Effect: probability of being eaten in a large group.
    • 100% chance of being eaten if alone
    • 10% chance of being eaten if in a group of ten
    • Increase group size will decrease the probability of being eaten.
    • Larger groups size costs: more visible, resource limitations
    • For example: mayflies swarming and coming all out together is explained by the dilution effect
  • Many Eyes Hypothesis: more individuals around means more can be searching for food/being lookouts for predators.
  • Many Legs Hypothesis: social spiders (Anelosims eximius) living in Peru together in one web and can feel the predator vibrations on the nest, more legs can feel the vibrations and lead attack against the predator.

    “Social Spiders (Anelosimus eximius) communal web” by Bernard DUPONT (CC BY-SA 2.0)

  • Flooding effect: starling murmurations fly in very large groups which creates a dilution effect, and they also exhibit mobbing behaviors.

Selfish Herd

  • Animals in the center of the heard are less accessible to predators, so individuals behaving selfishly they jockey for positions in the center. Old and weak individuals are typically shuffled to the outside of the herd and are typically picked off.

Game Theory:

  • The success of an individual depends on what its competitors within the group are doing.

Distraction Displays:

  • direct the attention of the predator away from a vulnerable prey to one that is more likely to escape.
  • Broken wing act of some birds to lure predators away from the nest
  • Sonoran Desert Toad can intake air to expand body size


  • Mimic background to hide from a predator. Camouflage, coloration and morphology allow the animal to blend into the environment to avoid predation.
    • For Example: the stick bug
  • Aggressive Mimic: predator blends into environment so it can attack prey.
    • Spiders that look like ants
    • Crab spiders that blend into flowers

Aposematism: bright coloration that advertise toxicity

  • Poison dart frogs
  • Sea slugs

    “Strawberry Poison Dart Frog (Oophaga pumilio)” by Carpar S (CC by 2.0)

Mimicry: when an organism resembles other species.

  • Batesian- harmless species mimics a harmful one
    • Hoverfly mimics a wasp
    • Bumblebee moth
    • Monarch Butterfly-aposematically colored and toxic due to the consumption of milkweed. They are mimicked by other species “Viceroy” butterfly. Blue-jays vomit up monarch butterflies due to the toxicity and learn/remember to not eat them again.
      • Ratio of mimics never gets too high because the bluejay will keep eating those non-toxic mimics.
      • System reinforcement by the memory of predator and negative effects of consuming a monarch.
  • Mullerian- two or more unpalatable (not edible) species resemble each other. Amplify signal and take advantage of the predators learning capabilities.
    • Queen butterfly looks like a monarch but it too is also toxic. It tapped into the learning mechanism of Blue-Jays.
  • Stotting Springbok– why make itself conspicuous?
    • Hypothesis:
      • A signal to conspecifics that a predator is close (alarm/social cohesion)
      • Confusion effect?
      • Signal unprofitability- it’s not worth eating me!!
    • Predictions:
      • Solitary gazelles will not stot
      • If stotting is a confusion effect then the rump will be. shown to predator
    • Observations have shown that solitary springboks will actually be seen stotting to signal to predator that it is too fast to be caught. The observations also show that predators fail or give up on stotting springboks.
      • Stotting is an honest signal because evolution tends toward honesty.



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