Here are the final answers for the multiple choice questions on the first midterm. First I want to explain the answers to a couple of the questions that clearly baffled a lot of people and second I want to give a report from the BAC.
Question 6 (on both form 1 and form 2) was my favorite question on the test. Because it talks about Hamilton's Rule we know that we need to determine B, C, r and n and then plug them into Hamilton's Rule equation. The question states that we need to assume that "the probability of survival in wild dogs is directly correlated with the amount of food they eat". This statement should allow us to calculate both the benefit received by the recipients and the costs to the actor. First, the actor regurgitates 1 pound of food which she should have been able to eat herself. Thus, by sharing food she reduces her probability of survival one pound's worth. Thus, C = 1 unit worth of survival.
Next we can calculate the benefit received that the dogs that share her food. Because the food is divided evenly among all individuals that are sharing, the benefit of receiving food depends on the number of sisters sharing the food. When( the actor is sharing with only one sister that sister receives 1 pound of meat which increases her survival by 1 pound worth (B = 1 survival unit). Now we can plug into Hamilton's Rule Equation. Does
(1 survival unit)(1/2) > 1 survival unit?
because the answer to this question is no, the the actor should be selfish.
When two sisters share the regurgitated food, then both sisters receive 1/2 extra pound of meat which increases their chance of survival by 1/2 survival units (B = 1/2). Plug into Hamilton's Rule
2(1/2 survival units)(1/2) > 1 survival unit
Again this is not true so the actor should be selfish.
When three sisters share the regurgitated food, each sister getw 1/3 pound of extra meet for a benefit of 1/3 survival unit. Plug into Hamilton's Rule
3(1/3 survival units)(1/2) > 1 survival unit
Again this is not true so the actor should be selfish.
Becasue the benefit received by each helped sister declines as the food is spread among more sisters, then the actor will NEVER be slected to be altruistic.
Question 23 (both form 1 and form 2)asks you determine if and why the population size of lions should change if the populstion size of hyenas was increased. Because they are scavengers, hyenas have an interesting relationship with lions. Becuase hyenas don't kill their own food (they only eat food killed by other species) they are not exploitative competitors with lions. Because they chase lions away from kills they are interference competitors with lions. Thus, increasing the population size of hyenas should decrease the population size of lions, but only by a direct interaction (interference competition).
BAC Report
The members of the BAC met with me on Wednesday afternoon to discuss test questions. They went through a large number of your comments and brought a number of questions to me for further discussion. I agreed with them on two questions.
Question 11 (both form 1 and form 2) dealt with mate choice in frigatebirds (one of my favorite birds). Frigatebirds, which were featured in the website I listed on the blog, have a bright red throat sac (skin!!!) that they use to attract females. Becuse we discussed how skin was used to indicate resistance of disease to females I chose answer (e) as being the best answer so this answer will receive credit. The BAC suggested that it was not clear whether or not the sac was make out of skin or feathers (although I suggest they called the sac a "ballon" should have been a good hint) that students thought that because females chose males with the "biggest and shiniest" throat sac that they might be choosing as a result of female choice. Because I did not clearly use the term "skin" in the discussion and because the word "biggest" suggested the sexy sons hypothesis, I have decided to award credit for answer (a) as well.
Question 18 (on both form 1 and form 2) asked why the population growth rate is initally very negative when the initialy population size is much larger than the carrying capacity in logistic growth. Because dN/dt = rN, dN/dt will be the most negative when you multiply a large negative value of r times a large N (answer (c) gets credit). Several students argued that
b - d = r and that r = (dN/dt)/N and that if you set these two equations equal to each other and did some algebra that it would be clear that answer (b) was equally correct. Although I think my answer is the best answer, coming up with answer (b) showed not only an understanding of the topic, but some clever creativity, so I will give credit for both answers.
You BAC representatives spent a considerable amount of time discussing these questions. I appreciate their efforts and I hope that you let them know how much you appreciate their hard work. You should discuss with them how to write more effective comments to the BAC on future exams.
Final Answers
1. b
2. c
3. d
4. d
5. e
6. d
7. d
8. d
9. e
10. c
11. a or e
12. b
13. c
14. c
15. e
16. a
17. a
18. b or c
19. d
20. c
21. d
22. d
23. b
24. b
25. b
26. d
27. a
28. d
29. e
30. a
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