Exam 2 - Example essays that were well done

SECTION I

A. (20 points) Explain Figures 2 and 3a from Cratsley and Lewis (2003) and use these figures to explain sexual selection in terms of Zahavi’s Handicap Principle.

Cratsley and Lewis (2003) investigated sexual selection and female choice in fireflies. Specifically, they looked at the value of male flashes as an honest signal females use in mate choice. Fig 2 relates spermataphore mass to flash duration. A spermataphores are protein masses males provide the female as a “nuptual gift”. Although in many systems females choose males based on “good genes” or other indirect benefits, a spermataphore is a direct benefit to females, since it is the primary source of nutrients available to females during which males can provide large spermataphores. Fig 3a demonstrates variation in female response to flash durations from an experimental manipulation of flash duration . Female response is positively correlated with flash duration. This supports the hypothesis that females choose males based on flash duration. If spermataphore size is related to flash duration and longer flash duration elicits stronger response from females, then it is likely females are using flash duration as one means by which to assess male quality in terms of direct benefits to the female. Zahavi would say that flash duration is a reliable signal of spermataphore size. Zahavi would also say that for any signal (such as flash duration) to be honest, it must be costly. Only costly signals provide reliable information. Thus, Zahavi would say that flash duration must be costly. In this case, the energetic demands of producing the spermataphore is correlated with the energetic demands of producing the flash (it is also possible that flash duration increases predation risk). Thus the correlation between flash duration and spermataphore mass can’t be faked and is therefore an honest and reliable signal that can be used by females to assess potential mates.

B. (20 points) What is the difference between optimality theory and an optimality model? Use the Polygyny Treshold Model to illustrate how an optimality model is developed. You can use this figure to illustrate your point, if that would be helpful.

Optimality is the body of theory at the heart of modern behavioral ecology. It assumes behavior is shaped by natural selection to maximize fitness. It explains behavior in terms of benefits and costs and the optimal behavior is that which provides the best benefit to cost ratio. Optimality models apply this approach to predict optimal behaviors under specific circumstances. These models specify variation in the behaviors under consideration, choose a measure of fitness, assess the costs and benefits of possible behaviors, and define the relationships between costs, benefits and the behaviors. Model predictions can then be scientifically tested. To construct an optimality model, a researcher should do the following:

define a range of possible behaviors for the animal in question (After all, a behavior cannot be optimal if the animal cannot perform it.). In the case of the Polygyny Threshold Model (PTM), the range of behaviors available to a female is to be the first or second female to be mated to any particular available male.
choose the “currency”, i.e., what to measure as an indication of costs and benefits. This “currency” must be somehow related to fitness, either directly, such as measuring number of young, or indirectly, such as measuring energy content of food (which would lead to increased survivorship and increased fitness). For the PTM, it is reproductive success directly, which could be offspring fledged, etc.
evaluate the costs and benefits of each of the possible behaviors, using as much quantification as possible via the chosen currency. For the PTM, the costs and benefits are defined by the shape of the two curves and the distance between them which indicates that there is variation in territory quality and that there is a distinct and constant difference between being the first and second female in any territory.
create the optimality model, by defining the relationships between the measured costs and benefits, and can make a prediction based on the point in the model where ratio of benefits over costs is maximized. This model predicts the order of territory occupancy by females by comparing any two choices faced. For example, if a female has the choice to be a first female in territory G or the second female in territory A, the female should choose the latter.

Section II

C. (10 points) Is sexual selection different from natural selection? Why or why not?

Sexual selection is not different from natural selection. It is a subset, or special case, of natural selection. Natural selection requires variable, heritable fitness. While survival can increase fitness under some circumstances, fitness is what matters. Sexual selection deals with increasing reproductive success only, generally through mate acquisition and can act on either sex to exaggerate traits in order to increase reproductive success relative to others in the population, i.e., fitness.

Sexual selection is a specific component of the theory of natural selection. Sexual selection depends upon the advantage one individual has over another individual of the same sex and species in relation to reproduction. Sexual selection operates under the same principles of natural selection and relies upon variation, heritability of that variation, and differential reproductive success. The theory of natural selection is broader and encompasses sexual selection as well as ecological or environmental selection.

D. (10 points) Under what general ecological conditions would learning be adaptive?

Learning is assumed to have some costs associated with it and so is put on a continuum with the genetic transmission of fixed behavioral information. In environments that are constantly changing or do not change at all, learning is not beneficial because it is either unnecessary or irrelevant to change behavior within one individual’s lifetime, and genetic transmission is favored. Learning is, therefore, adaptive when ecological conditions are changing, but not very often. Stephens’ model breaks this down and argues that learning is adaptive when environmental uncertainty between generations is high, but within an individual’s lifetime is low.

E. (10 points) A researcher presents data on the mating success of males in a population of a previously unknown bird species. In this population, 3 males account for 75% of the recorded matings. Fifty males account for 5% and the remaining 20% is divided among the remaining 20 males. What kind of mating system is this and briefly explain your answer.

This is clearly a polygynous species, in which a few males mate with many females. The high variance (75% of the matings by 3 males, etc.) suggests extreme sexual selection in which there is no male contribution to reproduction other than sperm. This is probably a lekking species.

This is an example of a polygyny lekking mating system. In these systems males set up and defend territories for mating where no apparent resources are present. Females then select mates out of all possible males based on their territories. This usually results in a few males with the best territories mating with many females, while some will have moderate success and other may not mate at all. Bowerbirds are a perfect example of this mating system.

F. (10 points) You observe two beetle species. In one species, the males provide only sperm to reproduction and male-male competition (fighting) determines mating success of males. In the other species, males provide females with spermatophores prior to mating. Make a prediction as to which species will have greater variance in male reproductive success and briefly explain why.

I predict that there will be a greater variance in male reproductive success in males where fighting determines mating success. In this scenario males who are able to win the most fights will be able to reproduce the most, while some males might not have mating success at all. Males that provide females with spermatophores prior to mating will have less variance in reproductive success because even males of lower quality may be able to mate at times. Some females may be willing to mate with lower quality males for the direct benefit of their spermatophores.

Greater variance will exist in the male dominance system, because male-male competition will lead to a few males that mate with females, while many losers will have no chance to reproduce in that mating season. In contrast, a system in which males provide females with spermatophore will give opportunities for more males to mate. These males are defending a resource that is costly to produce. Thus males producing spermataphores are limited in the number of females with which they can mate, allowing more males an opportunity to mate with females needing nutrients.

G. (10 points) If testosterone reduces average lifespan in male mammals, as it is suspected of doing, why hasn't natural selection led to the elimination of testosterone or the modification of its survival-threatening effects in mammals?

Testosterone has not been eliminated by natural selection because testosterone can be advantageous in male mammals. For example, higher levels of testosterone increase aggression and give advantages to males during fighting. Testosterone changes animal behavior and how animals respond to environmental stimuli. If changes in male behavior associated with elevated testosterone increased mating success, sexual selection would lead to an increase in males with higher testosterone levels. For example, if males with higher levels of testosterone were more successful in male-male competition, they would also be more successful in mating and their genes would be passed to the next generation.

Testosterone increases reproductive success, aggression, strength, resource defense, and “attractive” phenotypes. The hormone’s natural levels are also heritable. By the time a male reaches the age the negative impacts of high testosterone appear, it will have likely successfully reproduced. The high reproduction success derived from high testosterone makes up for the shortened lifespan. Males with lower testosterone might live longer, and thus be able to reproduce for longer, but they have lower fecundity. Natural selection can only eliminate or modify testosterone if the males’ life span is shortened to the point their reproductive success over their lifespan drops below that of males with lower levels.

H. (10 points) In the redback spider, the male inserts its copulatory organs, then does a back flip that places its body next to the female's mouthparts such that the female is able to consume the male during copulation. A) What evidence would support the hypothesis that this behavior is adaptive for these male spiders? B) What evidence would support the hypothesis that this male behavior is maladaptive?

[NOTE: this question is essentially asking you to make predictions based on the two hypotheses, not an explanation of why this happens – many answers are possible, here are two:]

A. If flipping behavior is adaptive, then we might expect that males that are eaten during copulation produce more offspring than males that are not eaten. Likewise, if the size of the eaten male is positively correlated with the number of offspring produced, then flipping behavior B. If flipping behavior is maladaptive, then we would expect to see that some males are not eaten during copulation and that they are able to mate with other females and produce more offspring from those subsequent matings.

If studies showed that males that place their bodies next to a female’s mouthparts had greater lifetime reproductive success than males that do not exhibit this behavior, we would have evidence that this behavior is adaptive because it results in greater male fitness. On the other hand, if spiders that did not exhibit this behavior had greater lifetime reproductive success than spiders that did, we would have evidence that this behavior is maladaptive because spiders that did not exhibit it had greater fitness.

I. (10 points) What is the key difference between classical conditioning and operant conditioning (trial and error learning)?

The key difference between the two forms of conditioning is that classical pairs stimuli to passively produce involuntary behaviors (inherent) while operant uses reinforcement (positive or negative) to actively pair a stimuli with a voluntary response (acted to gain reward or avoid punishment). The placement of control during the experiment is in the hands of the researcher during classical and in the hands of the subject during operant conditioning. In other words, operant conditioning strengthens a behavior while classical attaches an automatic response to a stimulus.

Classical conditioning takes advantage of an unconditional response to an unconditional stimulus. The unconditional stimulus is paired with a conditional stimulus, which initially fails to produce the desired response. Through this repeated association, the animal will produce the initially unconditioned response to the conditioned stimulus in the absence of the unconditional stimulus. Operant conditioning is essentially trial and error, where the animal learns to associate a behavior with a consequence. The main difference between these two is that operant conditioning requires the animal to actively produce a behavior to elicit a reward, whereas classical conditioning is passive.

J. (10 points) How does a change in behavior within a population through cultural transmission differ from change in behavior within a population through natural selection?

A change in behavior through natural selection results from a change in genetic information. It does not occur on the individual level but rather on the population level and it occurs over many generations. Genetic information is transferred from vertically, from parents to offspring. A change in behavior through cultural transmission is a result of teaching or social learning. The change is transmitted from individual to individual and it can occur within or between generations of animals. It can occur through vertical, horizontal, or lateral transmission. For these reason, changes in behavior due to cultural transmission occur much faster than changes in behavior due to natural selection.