Instructor’s Manual
to accompany
Animal Behavior, Tenth Edition
John Alcock
Chapter 4: The Evolution of Communication
DISCUSSION QUESTIONS
4.1Females of many African cichlid fish lay their eggs on lake bottoms in depressions made by males. The females brood their eggs and young fry in their mouths. A female picks up her orange eggs almost as quickly as she lays them. As this happens, the male cichlid that made the “nest” may move in front of her and spread his anal fin (Figure 4.5), which in many species is decorated with a line of large orange spots. The female may try to pick up the objects on the fin. As she does, the male releases his sperm, some of which swim into the female’s mouth, where they fertilize her eggs. If sensory exploitation explains the evolutionary origin of the female’s behavior, what prediction can you make about how female fish of a related species will respond to the normally unspotted anal fins of males that have been painted with colorful egg-like spots? Check your prediction against Egger et al. Was the first male to use this signal exploiting his mate in the sense of reducing her fitness to benefit himself?
Answer: If females of this species had evolved a perceptual sensitivity to the color orange because of the usefulness of this sensory bias in identifying eggs, then males that happened to mimic the color (and shape) of the objects that elicit the egg-pickup behavior would have better drawn females to the site of sperm release.This would result in a higher fertilization rate for these males. Because females also benefit from brooding fertilized eggs rather than sterile, unfertilized ones, the first male to possessthe egg-mimicking anal fin would not have been taking advantage of his mate in the sense of lowering her fitness.
4.2 The body of the giant wood spider is extremely colorful (Figure 4.6). When the bright patches are painted black, the rate at which moths and other nocturnal prey fly into the spiders’ webs declines sharply, especially at night. What relevance does this research have for persons interested in the sensory exploitation hypothesis for the evolution of courtship signals in animals?
Answer: The research provides an example of animals responding innately to signals in ways that reduce, rather than increase, their reproductive success. In effect, the giant wood spider is engaging in sensory exploitation when it lures moths into its web by offering them conspicuous color patches that for some reason elicit an approach. This kind of example makes it more likely that in some animals, females (or males) may engage in sensory exploitation of the opposite sex by triggering particular responses in partners via courtship signals that may benefit only the signaler, not the respondent.
4.3 In studying the courtship behavior of the empid flies, E. L. Kessel was amazed to find a species, Hilara sartor, in which males gather together to hover in swarms, carrying empty silken balloons, which females accept prior to mating with balloon-carrying males. Use the concept of a co-opted, preexisting sensory system to explain how this sort of behavior could have originated. You should know that many empid flies are predatory (Figure 4.14) and that males often offer their mates a prey item as an inducement to mate. In one species of this sort, some males supply their mates with an inedible dandelion-like tufted seed.
Answer:If females empid flies evolved a sensitivity to the visual signals provided by prey offered by courting males, then this sensory system may have caused females of some species to react positively when they encountered males carrying silken balloons just as females in at least one other species sometimes accept a nuptial gift in the form of an inedible tufted seed.If, however, other females in these species secured an edible gift from their partners, one would think that balloon- and seed-providing males would generate mating resistance from females with whom they interacted.(In fact, in the balloon-offering species, females and males are no longer predatory but appear instead to have retained some signaling elements of courtship from their predatory predecessors.)
4.4 To test adaptationist hypotheses on the spotted hyena’s greeting ceremony, how would you take advantage of information on the behavior of the other species of hyenas (see Mills, Owens and Owens, and Watts and Holekamp116). In addition, could you make use of information on another highly social mammal, the naked mole rat (see Figure 3.22 and pages 61–62)? Then there are some mammals in which females have an enlarged clitoris. What might these species tell us about the adaptive value of the spotted hyena’s behavior?
Answer:The various hypotheses on the spotted hyena’s greeting ceremony differ but some, for example, are based on the proposition that the highly cooperative yet competitive, female-dominated society of the spotted hyena provides the selection pressures that have resulted in the evolution of this mammal’s unusual behavior and morphology.If true, then we can predict that in other species of hyenas, which lack the enlarged clitoris and elaborate greeting ceremony, other key ecological features will prevail, such as an absence of females at the top of group dominance hierarchies.(Divergent selection pressures are predicted for the close relatives of the spotted hyena.) In other unrelated mammalian species however, especially those in which females exhibit an enlarged clitoris, our expectation is that these species would share certain key ecological elements with spotted hyenas, such as a female-dominated social hierarchy.In these cases, identifying the selection pressures that have produced convergent evolution in structure and behavior could be the key to testing ideas about the evolution of the spotted hyena’s pseudopenis and greeting displays.
4.5 One sometimes hears that the reason why so many species resolve their contests via mostly harmless threat signals is to reduce the number of injuries and thereby protect the breeding adults needed to produce the next generation of offspring. What’s the problem with this hypothesis?
Answer:The problem with this hypothesis is that it is a naïve group-benefit explanation that fails to consider why group-benefiting individuals would not be replaced over time by those that acted strictly in their own self-interest.Imagine that there were some adult animals that essentially deferred to others in order to protect the breeding stock of their species.In a population of this sort, if other hereditarily different individualsthat fought with others, even to the point of injuring rivals, appeared by mutation, and if these fighters on average had more surviving offspring than those that honored threat displays, then the “selfish” fighters would spread by natural selection at the expense of the alternative good-for-the-group types.
4.6 Baby songbirds usually produce fairly loud vocalizations in response to the arrival of a parent bird with food at the nest. This begging behavior could be an honest signal of the need for food by each nestling. Alternatively, the vocalizations could be an honest signal of the nestling’s “quality,” its likelihood of achieving high fitness, which could provide a parent with information needed to invest more in offspring with the potential for higher fitness. If the first hypothesis is correct, what prediction follows about the begging intensity of well-fed nestling songbirds when placed in a nest with food-deprived youngsters of the same age?
Answer:If the need-for-food hypothesis is correct, well-fed nestlings should vocalize less intensely than their food-deprived companions in an experimental brood.If food is not really the key to baby bird signaling, then well-fed youngsters might beg as loudly and effectively as hungry companions in the nest.One paper on this subject is by Smith and Montgomerie (1991) Behavioral Ecology and Sociobiology 29: 307–312.
4.7 Birds are not the only animals in which offspring have special signals that appear to provide information to a parent. Develop an evolutionary analysis of crying by human infants in which you consider the two hypotheses outlined in Discussion Question 4.6 for begging by baby birds. Then employ the following data in evaluating your hypotheses: (1) young infants expend considerable energy when crying; (2) the growth rate for typical infants is highest during the first 3 months of life, with smaller and smaller portions of the energy budget thereafter going to growth as opposed to maintenance; (3) consumption of breast milk peaks at 3 to 4 months of age and then declines; (4) crying peaks at about 6 weeks of age and occurs progressively less often after 3 months of age, except when the child is being weaned; (5) babies who are carried everywhere and nursed on demand (as in traditional societies) cry far less than babies in Western societies; and (6) the high-pitched cries of unhealthy babies are considered especially unpleasant by adult listeners.
Answer:(1) The fact that crying is costly for young infants suggests that their behavior could provide honest signals of body condition, thereby providing information parents can use to adjust their care in favor of relatively strong babies likely to repay an additional investment of parental assistance.(2, 3, and 4) Crying occurs most often at the time when growth rate and milk consumption are highest—in other words, when the need for parental feedings is greatest. (5) Babies that feed on demand cry less, which again suggests that crying is a signal of need, with babies whose needs have been met having nothing to gain from superfluous crying. (6) The inability of physically disadvantaged infants to produce costly signals of need and the unwillingness of their parents to invest in them indicate that normal crying is an honest signal of need and that parents who attend to these particular signals are likely to be rewarded by having infants that survive the difficult early months of life.
4.8 When males of the Australian slender crayfish compete aggressively with one another (Figure 4.22), they begin by displaying their enlarged front claws. The larger the claw, the more likely the male is to dominate his rival, which may leave without grappling with the larger-clawed crayfish. However, the muscles in the claws of males generate only half the force of the claw muscles of females. In addition, the actual strength of the claw has no bearing on which male is dominant. Much the same applies to male fiddler crabs in which males with a regenerated large claw quickly defeat rivals, despite the fact that replacement claws are relatively weak. Are males with large but weak claws dishonest signalers? But how could a mindless crab be dishonest?
Answer:It is likely that the large but weak-clawed males are employing a dishonest signal that works perhaps because if smaller males were to fight larger ones, they would sometimes meet a large-clawed male with truly powerful muscles and suffer a devastating loss.
Mindless crabs are capable of “dishonest signaling” because they need not be aware of the false information in their behavior. In fact, they almost certainly are not aware of their dishonesty.Persons who worry about the intentions or motivation of animals other than humans can be trapped by their anthropomorphic outlook, in which a term used to describe human intentions leads the person to assume that the term must mean the same thing when applied to other species. In behavioral ecology, an honest or dishonest signal simply means an action that conveys accurate or inaccurate information to a signal receiver without implying that the signaler intends to be accurate (or deceptive) in its dealing with the other individual.
4.9 Honey bee queens produce a complex olfactory signal, the queen pheromone, which they use either to control the workers, forcing them to keep working for the queen, or to provide accurate information about the reproductive state of the queen, making it adaptive for workers to be altruists (by helping them determine that the queen is capable of producing many siblings of the workers). Which of these hypotheses is more congruent with the logic of natural selection theory? Which hypothesis is supported by the discovery that components of the queen pheromone are related to the volume of sperm stored in the queen and her ovarian function? For a full and fair evaluation of the two competing hypotheses, see Kocher and Grozinger.
Answer: The control hypothesis is founded on the premise that queens are attempting to maximize their reproductive success by keeping their workers on task, (i.e., helping the queens reproduce, as this would be favored by natural selection).The hypothesis does, however, suggest that workers can be forced to act in ways that reduce their inclusive fitness even though those actions raise their mother’s direct fitness.The honest information hypothesis is based on inclusive fitness or kin selection theory.This hypothesis suggests that queens induce workers to behave in ways that are to the advantage of both queens and workers; healthy queens gain by making it adaptive for their worker daughters to produce future queens with a strong allied work force, while workers gain indirect fitness by increasing the odds of producing a new queen with a good chance of founding a viable colony.The second hypothesis gains support from the finding that queen pheromone could convey information about the degree to which the queen has the sperm needed to fertilize many eggs and the ovaries needed produce those eggs.
4.10 An assassin bug that captures and kills orb-weaving spiders sometimes approaches a web and plucks at the silken strands it encounters. The resident spider sometimes responds by moving across the web to the place where the assassin bug is at work. The spider’s behavior can lead to its death. Use net benefit theory to explain why spiders respond to web vibrations in this way. Use your hypothesis to make predictions about the response of spiders to web movements caused by the predatory bug, prey struggling in a web, leaves falling into the web, and male spiders that have moved onto the web to court the female web builder. Check your predictions against Wignall and Taylor.
Answer:The spider’s response to the signals provided by the predatory assassin bug is clearly disadvantageous. Under these circumstances, we might explain the behavior of the spider in the same way that many other cases of deception have been explained, namely as responses to signals that have some negative effects on fitness but on average generate fitness gains.If true, spiders will respond to web movements produced by assassin bugs and by struggling prey in much the same way, whereas leaves falling into webs should largely be ignored.This pattern is consistent with the hypothesis that the predatory bugs are exploiting a generally adaptive response of their prey.Male spiders may also try to mimic struggling prey in order to attract females (sensory exploitation) but since the males do not benefit from dying without mating, one would predict that their signals would have some added elements that enable female spider to recognize them for what they are, namely potential mates.
4.11 Charles Darwin loved both orchids and sundews, the latter because of their carnivory. As is true for certain orchids, sundews engage in deceptive signaling (Figure 4.29). The fluids exuded by the plants attract insects that become trapped when they alight upon the sticky, glue-like droplets, a prelude to their death and digestion by the plant. Analyze the evolutionary basis for this case of deception, using the approach we have outlined above..
Answer:This is another case of deception that can be explained in terms of the net benefits on-average for insects that can identify and respond to sugary fluids in their environment.Sundews exploit this response and reduce (but do not eliminate) the benefit their prey gains from reacting to droplets that look edible.
4.12 The fork-tailed drongo, an African bird that often perches in trees, sometimes gives alarm calls that warn of terrestrial predators when it is accompanying flocks of the pied babbler, a bird that forages on the ground. Upon hearing the drongo’s alarm call, pied babblers dash to cover, sometimes leaving behind recently captured insect prey.100 If we hypothesize that these alarm calls are often deceptive, what prediction can we make about the kind of alarm call produced by drongos vocalizing in the absence of babblers? Why might it be adaptive for babblers to react to drongo warning calls if some, or even most, are false alarms? Does the same argument apply to the case of male topi antelopes that produce false alarm signals when associating with groups of females on their mating territories?
Answer:If drongos produce deceptive alarm calls to fool babblers into fleeing and leaving behind edible items that the drongos can consume, then in the absence of the babblers, drongos should give somewhat different alarm calls that have evolved to communicate alarm to their fellow drongos.Even though babblers lose by fleeing when they hear deceptive drongo calls, they may well be safer by dashing off rather than waiting to see if a hawk really is about to catch them. Likewise, female topi antelopes that hear an alarm snort may be safer if they stay put with other females because if they ignore an alarm signal that happens to be real, then they put themselves in greater danger of being killed by a stalking lion or hyena clan.