Week 11: Defense Feeding – CH6 p183-216, CH 7 p219- 232 & 235 – 238
Pg: 219 - 232
Optimal Foraging Behaviour
Optimality theory – the attributes of organisms are optimal, i.e. better than others in terms of
the ratio of fitness benefits to cost; the theory is used to generate hypotheses about the possible
adaptive value of traits in terms of the net fitness gained by individuals that exhibit these attributes.
Beachcombing northwestern crow – picks up clam (size varies)and flies up about 5m and drops
the clam on a sharp rock to crack open and eats the insides, if it doesn’t open, they try again as many
times until it does.
Exp. Zach tested the drop by crows by erecting a 15m pole on a rocky beach. He adjusted the
height, got whelks (another food source of crows) of different size (S, M, L) and dropped them all at
various heights. Large whelks broke at lower heights, with fewer tries and the times dropped were
independent of each other (always ¼ chances). The optimal theory here is that it would be a waste for a
crow to find another whelk if the one it had wasn’t breaking because the chances are the same and it
would require more time and energy. Also, the crow will always go for the large ones since it takes less
energy to break and more reward when it does.
In another study it was proven that the better foragers and ones who ate more large
mussels/whelks had higher reproductive success. Also took longer to lay their first egg when foraging
was not easy.
Red Knots (a sandpiper) – the number of individuals decreased as humans overharvested their
prey, from 57,000 to 27,000 in 2 years. This uncontrolled experiment shows how foraging can have
major impact on reproductive success (or fitness all together).
How to Choose an Optimal Mussel
Eurasian Oystercatcher (shorebird) – Meire & Ervnyck developed a maximization hypothesis to
apply to oystercatchers feeding on mussels. Calculated profitability of the size (fitness benefit) and time
required to open (fitness cost). In theory the large mussels require more energy but they provide the
most calories, and result in a net gain, and the birds should focus on these mussels, however they don’t.
1) profitability of very large mussels is reduced since some cannot be opened at all,
reducing the average return. Suggests that oystercatchers should favour the 50mm mussels but in
actuality they prefer, on average, the 30 – 45mm, so this hypothesis fails.
2) Many large mussels are not even worth attacking since they are covered with
barnacles, which make them impossible to open. It suggests that the birds should focus on 30-45mm
mussels, which they do. Criticisms of Optimal Foraging Theory
Animals don’t always hunt for food as efficiently as food. Optimality theory is not to make
statements about perfection in evolution but to make it possible to test whether one has correctly
identified the variables that have shaped the evolutions of an animal’s behaviour. Ecological factors as
well as level of risk for predation also affect the outcome of optimality theory and foraging behaviours.
Animals should take a small-caloric sacrifice for longer survival when predatory risk is high.
Ex. Dugongs (marine mammal (Pokèmon!)) - feed on sea grasses while avoiding tiger sharks.
Two styles of feeding: 1) cropping – quickly strip leaves from standing sea grasses. 2)Excavation – stick
their snouts into the sea bottom to pull out the sea grasses’ underground components (rhizomes), which
provide more energy. In feeding style 2, they cannot see well with most of their head in the ocean floor
and thus do not see predators (sharks) but in feeding style 1, they can swim by eat and still watch for
predators. Another ex. Is with elk living in Yellowstone National Park – probability of having and
offspring in the summer has fallen, and also the offspring surviving through the winter has fallen due to
the foraging change. This kind of behaviour is what Creel calls a “risk effect” - in order to reduce the