BIO208/REHNBERG
Optimality in Biology: A Cost-benefit Analysis
of Feeding in Painted Lady Butterflies
Optimization is the process of attaining the best possible compromise between
minimizing costs and maximizing benefits. When shopping, consumers trade off price
and quality. For a given price range, it makes sense to look for the highest quality
product currently available. For a given quality level of product, the strategy is to find
the lowest price. Evolution by natural selection is a process of optimization. In an
evolutionary context, natural selection trades costs and benefits in ways that move
phenotypes in the direction of optimality.
An example of an important trait in the life of a bird is the porosity of its egg shell.
Shells have microscopic pores that permit exchanges of gases between the atmosphere
and the respiring embryo. Pores that are excessively small will limit respiration, but very
large pores will desiccate the embryo by promoting evaporative water loss. Natural
selection has favored an optimal pore size that represents a compromise: pores of a
size sufficient for aerobic respiration but not large enough to upset the egg’s water
budget.
Animal behaviors often have associated costs and benefits and can therefore be viewed
in terms of optimality. Baleen whales obtain energy by straining microorganisms out of
the water column as they swim. Suppose that in waters near the surface there are very
high densities of small plankton whereas in a deeper strata the plankton are less
abundant but of greater size and energy content. Where should the whales swim?
Natural selection will favor choices and behaviors that promote efficient acquisition and
processing of food energy and nutrients. In times of food scarcity, efficiency could
mean the difference between living and dying. During times of food abundance,
efficiency could permit an animal to make a very large investment in personal
reproduction.
Butterflies feed on nectar produced by flowers. Nectar is a complex solution of sugars
and amino acids. The butterfly has a proboscis (see sketch) that allows it to reach
nectaries deep inside the flower. When feeding, the uncoiled proboscis pulls nectar
upward by a process called siphoning. The ease with which the nectar rises in the
proboscis depends on several factors including solution viscosity and proboscis size.
What are some of the important relationships that relate to foraging on nectar? Intuition
tells us that the rate of fluid flow decreases as viscosity increases. From experience,
we know that we can drink water or soda through a straw faster than a super-thick milk
shake. As sugar concentration of nectar increases, so does viscosity ... but so does
energy content. Is low flow of energy-rich nectar preferable to a high flow of energypoor nectar? This sound like a problem in optimality.
In this lab we will investigate the ingestion of sucrose solutions by the painted lady
butterfly (Vanessa cardui). Our test solutions of 8.75%, 17.5%, 35%, 50%, and 80% will
therefore represent varying concentrations, viscosities, and energy contents.
MEASURING INGESTION
Each group will have a small collection of butterflies. Each butterfly will be used in our
experiment only one time. Butterflies are relatively delicate insects and anything that
we can do to reduce mechanical damage and general stress will be to our advantage.
Butterflies should be gently picked up by grasping the folded wings between your index
and middle fingers. Don’t be alarmed if a butterfly gets away from you. Once they
come to rest they can be easily captured.
Our strategy will be to determine the volume of ingested solution by weighing butterflies
before and after feeding. Weights will be measured on an analytical balance to four
decimal places. Place a butterfly in a weighing container and allow it to become inactive
before taking the reading. After weighing, remove the butterfly from the container and
present the sugar solution. Allow the tarsi (last segment) of the walking legs to touch
the solution. Tasting the sugar in this way should cause the proboscis to uncoil. You
may find that if you hold the butterfly near the bench top, it will extend its legs. You can
then walk the butterfly into the sugar solution. It may help ergonomically to brace your
elbow or wrist on the lab bench. If the proboscis will not uncoil naturally, you may have
to use a small probe to help it reach the fluid. IMPORTANT: Do not dunk the body of
your butterfly into the sugar solution. A wet body will distort your estimates of ingestion
weights.
Feeding will be assumed to occur between the time that the proboscis enters the fluid
and the time that it leaves the fluid. Carefully record this interval with a stopwatch. It is
preferable but not necessary that feeding bouts are continuous. Stopping and restarting
the stopwatch is OK (avoid accidentally zeroing your stopwatch!). During a typical
feeding session, a butterfly may consume 30-40 uL of sugar solution. In order to attain
this, try to measure ingestion over the following intervals: 8.75 % sucrose - 2 min, 17.5
% - 2.5 min, 35 % - 3 min, 50 % - 4 min, and 80 % - 5 min. When feeding stops,
reweigh the butterfly and then place it in the holding tank for used subjects. Record the
data. Repeat this entire process with other butterflies and concentrations according to
your research design.