Exam 2 Key

Animal Behavior ‑ Second Midterm ‑ Fall 2006 Name______KEY______

READ THESE INSTRUCTIONS: The test has three sections. You will answer 3 questions (33 points each); you will answer the single question from Section I, one from Section II and one from Section III.

For every question, don't assume I can make the same leaps in logic you might make. Explain yourself. You won't get rewarded for volume of words, but you need to tell me everything I need to know to understand your argument. If you can answer the question in a single word (doubt it), great. However, if you need more space, it should not exceed 2 pages.

This is an open book exam. You should use a word-processor to write your answers. Print your test such that each answer is on its own page. Bring your printed answers to class on Wednesday.

Section I. You MUST ANSWER this question:

A. What does it mean for an animal to behave optimally? What are the steps involved in constructing a model of optimal behavior? What must be measured in order to construct any such model? Illustrate your answer with an example of such a model and how it can be used to make testable predictions.

Answer A. Body of theory at heart of modern behavioral ecology. Behavior is assumed to be shaped by natural selection for behaviors that maximize fitness. Explains behavior in terms of a benefit/cost approach. There are four steps to building an optimality model. First, you must specify the kinds of behaviors possible in a given situation. Only look at possible, or plausible, behaviors. Second, choose the "currency" that is being maximized. This currency should be related to fitness, either directly (as in number of offspring produced), or indirectly (as with foraging models that assume maximizing some aspect of energy acquisition results in greater fitness for the organism). Third, determine the costs and benefits of each possible behavior in terms of the currency identified. Last, compare the possible behaviors in terms of their benefit/cost ratios and identify the behavior will maximize fitness relative to the other possible behaviors. This allows us to make testable predictions about behavior. Examples of particular models help explain these ideas, particularly with respect to what you can measure in terms of currencies, etc.

Section II. Answer of ONE the following (B or C, not both):

B. The Tasmanian native hen (Gallinula mortierii) is a flightless bird in which males often outnumber females. In this system, either a pair (male and female) raises offspring, or a trio (a pair helped by a subordinate male) raises offspring. The subordinate male in a trio is usually the brother of the male of the pair. Given the following data:

a. First year breeding pairs with no helpers average 1.1 offspring produced during that year.

b. First year breeding trios (Male and Female + subordinate male) produce 4.1 offspring on average.

c. Experienced pairs produce about 5.5 offspring per year without helpers

d. Experienced pairs with helpers (i.e. a trio) produce about 6.5 offspring per year.

If a first year male is faced with three choices -- breed on its own, help a brother in his first year, or help an older, experienced brother -- which should it choose and why? In answering the question, explain Hamilton's rule in detail, as well as kin selection, show your work and calculations, and state which of the three options is best, second best, and worst. (Remember, helpers are helping raise nephews/nieces, not siblings).

Answer B. Apparently altruistic acts can sometimes be explained by kin selection, in which an individual (the donor) sacrifices its own survival or reproduction in favor of another individual (the recipient) to which it is related by common descent. Fitness can be either direct (producing one's own offspring) or indirect (helping produce relatives' offspring). The indirect fitness component is calculated based on the increase in reproductive success resulting from the help provided by the donor weighted by the relatedness of the donor to those offspring. Hamilton's rule provides the basis for calculating the costs and benefits of helping. In its simplest form, Hamilton's rule is: r*B>C, where r is the coefficient of relatedness, B is the benefit of help to the recipient, and C is the cost to the donor. When calculating whether "altruism" will be selected for through kin selection, it is useful to define B and C in term of offspring and weight each by the coefficient of relatedness to the donor. This changes the equation to be:

r_B * B > r_C * C or, written another way B/C > r_C / r_B

We can use this form of the equation to solve the Tasmanian Hen problem.

Relatedness of donor to own offspring (r_C) = 0.5

Relatedness of donor to brother's offspring (r_B) = 0.25

C = cost in terms of own direct fitness = 1.1 offspring on average

B_{helping first year brother}= 4.1 – 1.1 = 3.0 offspring (extra offspring produced via help)

B_{helping experienced brother}= 6.5 – 5.5 = 1.0 offspring (extra offspring produced via help)

At this point, you can either compare ratios from the above equation, or you can just figure out proportion of genes transferred to the next generation by a first year male in all three of these choices. These are:

1.1 * 0.5 = 0.55 (cost – also benefit of breeding along – direct fitness)

3.0 * 0.25 = 0.75 (benefit of helping first year brother – indirect fitness)

1.0 * 0.25 = 0.25 (benefit of helping experienced brother – indirect fitness)

Either method has the same result, helping a first year brother gives the greatest benefit, followed by breeding alone, then by helping an experienced brother.

C. The black hamlet fish (Hypoplectrus nigricans) lives in the Caribbean. Individuals are simultaneous hermaphrodites, which means every individual has both male and female gonads. Spawning is a lengthy process in which two individuals take turns releasing eggs and sperm. Although it would be more efficient for one individual to release all its eggs at one time, while the other releases all its sperm, these fish take turns. The problem is that eggs are far more costly to produce than is sperm such that whichever individual first deposits eggs is at a potential disadvantage. Thus, not all eggs/sperm are deposited at once. Instead, mating is a lengthy, iterative process in which each individual takes turns expending more "effort". This is an example of what kind of altruism? What model might explain this? How does it fulfill the predictions from that model for such altruism?

Answer C. Hamlet Fish – This is an example Reciprocal Altruism. One individual cooperates with another in the anticipation that the other individual will also cooperate. In this case, one individual contributes eggs and the other sperm in mating. Each individual can only act as male or female, not both simultaneouslyl. Eggs are more costly to produce than are sperm, so the individual contributing eggs (the female) is at a disadvantage. Without cooperation, the individual contributing sperm might leave without also contributing eggs for the first individual to fertilize with sperm. The situation is similar to that of the Prisoner's Dilemma, in which two interactors are presented with the behavioral options of cooperating and cheating. In the solution to the prisoner's dilemma, the highest payback is to cheat, the lowest to be "suckered". Cooperating is a mid-range payoff. In this case, cooperating is alternating the contribution of eggs and sperm. Cheating is to withhold eggs and only contribute sperm. The solution to such a problem is the Tit for Tat (TFT) strategy which has been shown to be an Evolutionarily Stable Strategy (ESS). An ESS is a strategy that can't be displaced by other strategies, once it is established. In TFT, the cooperative behavior has a higher expected return than does cheating, so long as certain conditions apply. The TFT requires that interactors have a high probability of future interaction and that cheaters can be punished. In TFT, the strategy is to always cooperate first (lay down eggs, in this case), then always follow the other indivdual's action with the same action. This provides immediate punishment for cheaters and longterm alternating of roles/cooperation. Your book describes the strategy in terms of "be nice", retaliate, and forgive. The Hamlet fish system seems to fit the conditions for this model to work.

Section III. Answer of ONE the following (D or E, not both):

D. What is polyandry and why is this mating system so rare in birds and mammals? Give an example of a polyandrous mating system and explain why it is adaptive under these circumstances.

Answer D. Polyandry – Polyandry is a form of polygamy in which one female mates with more than one male. It is rare in birds and mammals due to a number of factors dealing with parental investment. Biparental care is usually necessary in birds which selects against polygamy, in general. Parental care in mammals is provided by females, in general, since males cannot produce milk. In both birds and mammals, fertilization is internal, which means that females are "stuck" with the offspring. For mammals, development is also internal. In general, eggs are costlier to produce than sperm. For all of these reasons, reproductive investment by females is generally greater than by males in birds and mammals, often selecting for more females to invest more in parental care than in mating opportunities. There are a number of examples of polyandry, even in birds, which do not have the same constraints as listed above. Among these are various fish, birds, and insects in which males provide care of offspring, rather than females. In describing these, you should have pointed out the differences in those systems compared to the normal bird and mammal systems.

E. Anders Moller showed that female tree swallows (Hirundo rustica) prefer to mate with males with long outer tail feathers. How did he show this? Explain his hypotheses, predictions, experimental design, and his conclusions. Then, offer one explanation for selection on exaggerated male traits.

Answer E. Moller's experiment – Moller hypothesized that male tail feather length had an effect on male reproductive success due to female preference for long tails. He predicted that male with long tail feathers (outer tail feathers) would be more successful than males with short tail feathers. To test this, he manipulated tail length in male swallows. He manipulated tail length by cutting a section from the middle of the feather of the outer tail feathers to change the natural tail length of the birds. He had four experimental groups. The first group had their tails shortened by removing the cut section from the feathers. The second group had their tails elongated by adding an extra length of feather to the middle of the feather. The third group was a control in which tail length was not manipulated. The fourth group was a "sham" operated, or positive control, in which the feathers were cut, but glued back together so that the length was unaltered. This last control group was necessary to test for an effect of the manipulation itself. To assess male reproductive success, he measured time to acquire a mate, number of fledglings produced, whether or not a second clutch was successfully produced. In all measures, the males with the artificially long feathers were more successful and the males with the shortest tails were least successful. He found that there was no effect of the manipulation, in that the positive controls and the controls were not different in any measured variable.

There are many possible explanations for the female preference for long tail feathers. Most have to do with honest signaling. Exaggerated traits are reliable indicators of male quality because they are traits that are expensive to produce and therefore can't be displayed unless the male is of high quality. Variations on this include the Handicap Principle and the Hamilton-Zuk hypothesis. The latter says that the exaggerated trait is negatively correlated with parasite load. Moller also did experiments to show that tail length is negatively correlated with parasite load and that parasite load mattered to reproductive success, which ought to be important to females when choosing mates. Although male quality may result in direct benefits to females (nuptual feeding, etc.), most of these arguments deal with females selecting for "good genes" for their offspring.