Instructor’s Manual to accompany
Animal Behavior, Ninth Edition John Alcock
Discussion Questions Chapter 3: The Development of Behavior 3.1 In the context of discussing the causes of developmental differences between individuals, what significance do you attach to the fact that the queen bee behaves very differently from her workers even though she has essentially the same genome as her worker sisters and daughters? Develop at least one hypothesis on why the two categories of bees behave so differently after you find out about how worker bee and queen larvae are reared. The Internet will be helpful in this regard. Answer: Queen bees behave differently from their sisters not because of genetic differences between them but because queens are reared differently. They receive more food as larvae from nurse bees, which feed their charges with a special secretion from glands in their heads called royal jelly. Worker bees receive some royal jelly but are fed largely on stored nectar and pollen. The two diets activate different parts of the honey bee genome, so that the pattern of gene activity in future queens diverges from that in workers. The distinctive environment–gene interactions occurring in the developing queen affect the structure and operation of her nervous system, and thus ultimately the behavior of this kind of bee.
3.2 Honey bees possess a gene (called for) that contains information for the production of a particular enzyme called PKG. If this genetic information is important for foraging activity in worker honey bees, what prediction follows about the levels of PKG enzyme present in the heads of foragers versus nonforagers taken from a typical bee colony composed of workers of many different ages? Figure 3.4 presents data on the quantity of the messenger RNA coded by for that is present in the brains of nurses and foragers in three typical honey bee colonies; this RNA is required for the production of PKG. Are these results consistent with your prediction? But since foragers are older than nurses in typical colonies, perhaps the greater activity of for is simply an age-related change that has nothing to do with foraging. What additional prediction and experiment are required to reach a solid conclusion about the causal role of for in regulating foraging in the honey bee? Suggestion: Take advantage of the ability to create experimental colonies of sameage workers. If the for gene contributes to the switch to foraging by worker bees,103 how can this change be influenced by the workers’ social environment as well? Can you produce a hypothesis that integrates the genetic and environmental contributions to the switch in bee behavior?
Answer: The prediction is that the levels of PKG should be different in the brains of foragers versus nonforagers and the data in Figure 3.4 are consistent with that prediction, given that the levels of brain mRNA needed for the production of PKG itself differ between the two groups. But because this difference could be a simple function of an agerelated change that has nothing to do with foraging, we need to create experimental colonies of same-age individuals and then collect samples of workers that make the transition to foraging before their companions that remain nurses within the hive. The prediction is that individuals of the same age that differ in behavioral phenotype will also differ in their brain PKG levels. This difference could be environmentally triggered by a shortage of foragers (or some product associated with foraging), for example, activating the for gene in some individuals. This activation causes the gene’s information to be used in the production of PKG, which then acts as an environmental signal turning on other genes and altering the chemistry of brain neurons in such a way as to motivate a behavioral change in the worker bee.
3.3 The nature–nurture controversy involves those who believe that our nature (essentially our genes) dominates our behavioral development and those who argue just as forcefully that our nurture (especially our upbringing as children) is what shapes our personalities. Some have dismissed the controversy by saying that the two sides might as well be fighting about whether a rectangle’s area is primarily a matter of its height or mostly a function of its width. What’s the point of the rectangle analogy? Does the analogy have any weaknesses? Answer: Because a rectangle’s area can only be a product of both its height and width, it would be silly to say that one of the two dimensions was more important than the other. Likewise, since phenotypes can only be produced through an interaction of both genes and environment, it would be silly to say that a trait was the product of nature (or nurture) alone. The analogy does have a weakness in that height and width can be measured in identical units (say, centimeters) whereas the contributions that genes and the environment make to development are extremely different. Moreover, if the nature– nurture argument is about which factor is more important in causing children to differ with respect to a given trait, then one can make a strong case that the “argument” is legitimate, and not to be dismissed as silly or simple-minded.
3.4 Return to the chocolate cake analogy (see page 69), and use it to illustrate how a change in either genes or environment could lead to developmental differences between individuals. Answer: Because the development of an individual is the product of both genes and environment, two individuals that either possessed different genes or experienced different environments would undergo different gene–environment interactions, which could result in different developmental outcomes. By the same token, if we look at two chocolate cakes, a German chocolate cake and a mint chocolate one, we could potentially
attribute some of the differences between them to the different recipes used by the cooks, but we could also potentially point to differences in the ingredients that actually went into the cake dough or to differences in the oven temperature as “environmental” factors that generated differences in the cake phenotypes. 3.5 A good predictor of a young person’s vocabulary is the amount of time parents spent talking to their child when he or she was very young. Some have concluded that family environment is therefore the essential factor in determining a person’s language skills. What’s a logical problem with this conclusion? Answer: Families not only share similar social and intellectual environments, they share genes in common as well. This makes it difficult to separate similar nurture from similar nature in terms of the contribution to the similarities in the language skills exhibited by two generations of family members.
3.6 A few blackcaps live year-round in southern France, although 75 percent of the breeding population migrates from this area in winter. Perhaps the difference between the two behavioral phenotypes is environmentally induced and not hereditary. Make a prediction about the outcome of an artificial selection experiment in which the experimenter tries to select for both nonmigratory and migratory behavior in this species. Describe the procedure and present your predicted results graphically. Check your predictions against the actual results (see Berthold113). Answer: If an artificial selection experiment was done, perhaps focusing on migratory restlessness in cages in the early winter, then the offspring of relatively restless individuals should be no more likely to be restless themselves than the offspring of relatively sedentary birds (if the environmental differences hypothesis is correct and if the birds were held under identical conditions). No high or low lines would result.
3.7 The black redstart is a bird species that migrates a relatively short distance from Germany to the Mediterranean region of Europe, whereas the common redstart travels as much as 5000 kilometers from Germany to central Africa. The scale in Figure 3.17 shows the duration of migratory restlessness in three groups of captive birds all hand raised under identical conditions: black redstarts, hybrids created by crossing black and common redstarts, and common redstarts. Why do black redstarts exhibit migratory restlessness at night for fewer days than common redstarts? What does the behavior of the hybrids tell us about the genetic differences hypothesis for the difference in the duration of migratory restlessness in the two parental species? Answer: Given that environmental differences have been ruled out as a factor contributing to developmental differences between these birds, we can only conclude that
a genetic difference is responsible for the differences in their migratory restlessness. If this conclusion is correct, then hybrids, which will have a mix of the relevant genes, should exhibit a level of restlessness intermediate to both parental species (assuming that the relevant alleles contributed by one species are not uniformly dominant to those contributed by the other species). The actual data shown in Figure 3.17 match the expected results, confirming the genetic differences hypothesis.
3.8 Robert Plomin and his colleagues have compared the cognitive abilities of children with those of their parents (genetic or adoptive) and twin siblings.1156 What significance do you attach to these data (Figure 3.18) in the context of determining whether genetic or environmental differences are responsible for the differences between humans in their spatial and verbal abilities? If environmental differences are the key to understanding differences in these human phenotypes, what is the predicted relationship between the number of years a child has spent in an adoptive home and the degree of difference between the child’s spatial and verbal attributes and those of his or her genetic parents? Answer: Adopted children share a family environment with their adoptive parents but not with their genetic parents. The fact that adopted children are more similar to their genetic parents than their adoptive parents with respect to spatial and verbal abilities does not support the environmental differences hypothesis for why people differ behaviorally. If this hypothesis were correct, we would also predict that the number of years spent in an adoptive home would increase the difference between a child and its genetic parents, who live elsewhere. (In fact, the difference decreases.) This research strongly suggests that some of the differences in spatial and verbal abilities between people are caused by genetic differences between them.
3.9 Debi Fadool at Florida State University headed a research team that studied a strain of genetically modified mice that lacked the ability to make a protein called Kv1.3.452 In unaltered mice, this protein is found in regions of the brain that process olfactory information, leading Fadool and her team to predict that the two kinds of mice should differ in their ability to smell things. In fact, the genetically modified mice were able to smell scents at much lower concentrations than mice that possessed the protein; the mutant mice found odorous foods, such as peanut butter crackers, much faster than their wild-type cousins. What evolutionary question is raised by these findings? What ultimate explanation do you have for the fact that mice with Kv1.3 protein are actually less sensitive to food odors than mice without that protein? Answer: You might think that a greater sensitivity to food odors would be selectively advantageous since it would enable mice to find food faster. So why hasn’t the mutation for hypersensitivity spread through the species of wild mouse from which lab mice are derived? Perhaps the answer is that the mutation simply hasn’t happened yet. But another explanation is that mutations for higher olfactory sensitivity have occurred in the past and
have been selected against. Why? Because mice that were “too sensitive” to odors would waste time and energy tracking down food sources that were very small and unproductive. Having a higher threshold for detecting could actually help an individual ignore economically worthless items in favor of those that were more profitable.
3.10 Does the fosB knockout experiment demonstrate that a single gene determines the maternal behavior of a female mouse? You should know by now that the answer is no. Use this example to illustrate the difference between claiming that maternal behavior is genetically determined and claiming that certain differences among individuals in their maternal behavior phenotypes are genetically determined. How is the idea that genes are responsive to particular kinds of environmental inputs illustrated by the fact that when a female mouse inspects her pups after birth, she receives olfactory stimulation, which affects the mouse’s brain and triggers fosB gene activity in a mouse with the typical genotype? How might this gene’s activity initiate additional changes in other genes, leading to a specific pattern of biochemical events? Answer: Although knockout experiments have shown that single genes contribute important information to the development of particular behaviors, this conclusion is not the same as saying that environmental inputs are unimportant. Olfactory stimuli activate neurons, causing chemical changes that trigger fosB gene activity; when the gene’s information is expressed, a chemical product is made, which then is available as an environmental trigger to affect the activity of other genes and thus, the biochemistry of cells and the development of the individual.
3.11 Infant humans learn languages by listening to the speech of other persons. Given the obvious importance of this environmental factor on language acquisition, what do you make of the finding that certain alleles of two genes (ASPM and microcephalin) are much more likely to be found in people who speak a so-called tonal language (like Mandarin Chinese) than in those of us who speak a nontonal language (like English)?378 (In tonal languages, the meaning of a word depends not just on its consonants and vowels but also on the tone or pitch, higher or lower, that the speaker imparts to a given syllable.) Explain this genetic finding in the context of the interactive theory of development, and link it to the evolution of language learning in our species. Answer: It is possible that the ability to learn a tonal language is enhanced by the possession of certain kinds of neurons in the appropriate parts of the brain involved in language acquisition. Perhaps the alleles in question have been selected for in environments in which children learn tonal languages because these individuals were more adept at acquiring and using language than other children with a slightly different genetic makeup. If this hereditary difference translated into a reproductive advantage, no matter how small, over time the populations of people using tonal languages for communication may have diverged genetically from other human populations.
3.12 Identify the probable adaptive basis for the flexible development of body size in the redback spider. Predict what effect large body size must have on female choice in this species versus the effect of large body size on the ability of male redbacks to compete physically with rival males. Check your answer with Kasumovic and Andrade.750 Answer: Being able to develop into larger or smaller adult individuals enables the spiders to deal with the reality that their physical and social environments will vary in different places. By becoming an adult male more quickly in the absence of male rivals, a redback has the chance of being first to mate with an available female. In this species, being relatively small apparently does not handicap the male in his interactions with a female. But if there are other males around, then it pays a male to slow his maturation in order to become larger because relatively large males are more able to defeat rivals in contests for access to females.
3.13 Some marine fishes exhibit a spectacular polyphenism in that individuals can, under special circumstances, change their sex from female to male (in other species, the switch goes from male to female). This developmental change involves reproductive organs, hormones, and mating behavior.1514 A key social cue for the switch in some species is a change in the makeup of the social unit in which the sexaltering individual lives; the removal of a dominant, breeding male from a cluster of females triggers a sex change in the largest female present. Here we have a case of socially induced polyphenism. Identify the apparent restrictions imposed on this system, starting with the most obvious one, namely, the ability to be transformed into a member of the opposite sex rather than some sort of intermediate sex. Speculate on the benefits associated with each restriction. Answer: The restricted nature of sex-change polyphenism is evident not just in that the only option is to go from female to male and not, say, to hermaphroditism, but also in that only specific environmental cues can act as triggers for the sex change. Thus, the social status of the individual female and the presence or absence of a male associate are the key factors in determining whether a female will exercise her capacity for a sex change. A host of other variables are irrelevant. The benefits of having such a carefully channeled capacity for sex change might include the following: females that switched sex while a large dominant male was present might well be ousted from the group and lose fitness as a result; small females that switched sex after the loss of the resident male would probably face competition from larger males and would be ousted with little chance of reproducing whereas by remaining a female in a harem, a small individual can secure at least some reproductive success.
3.14 In a study in which men and women were asked to sit at a computer and navigate through a virtual maze (Figure 3.41), the men were able to complete the
task more quickly and with fewer errors over five trials than the women. The conclusion that men do better at location learning than women has been supported by other research as well (e.g., Jones and Healy736). (Note, however, that in other tests, involving language skills, women score higher on average than men.) What possible proximate developmental mechanisms might be responsible for this sex difference in navigational ability? Keeping in mind the evolutionary explanation for sex differences in spatial learning ability in voles, what prediction can you make about the nature of human mating systems over evolutionary time? Answer: Men might be inherently different from females on average with respect to such brain components as the hippocampus, which is thought to be important in navigation. Alternatively men may have had more experience in computer game playing than women and this factor may have enabled them to perform more skillfully at the mazemaneuvering task. In other words, some differences in hippocampal functioning may stem either from differences between males and females in their genes or their experiential environment, either one of which could have affected the gene–environment interplay that affects the development of the brain. To the extent that there have been consistent sex differences in spatial learning ability between the sexes, this would provide support for the hypothesis that men have routinely covered more ground than women, which is a trait characteristic of polygynous species (namely those in which at least some males acquire several mating partners).
