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Behavioral Ecology 8 Behavioral Ecology Baby Killers: A Case Study KEY CONCEPTS Lions are unique among cats in that they live in social groups called prides. CONCEPT 8.1 An evolutionary A typical lion pride contains anywhere from 2 to 18 adult females and their approach to the study of behavior cubs, along with a few adult males. The adult females form the core of the leads to testable predictions. pride, and they are closely related: they are mothers, daughters, aunts, and cousins. The adult males in a pride may be closely related as well (e.g., broth- CONCEPT 8.2 Animals make ers or cousins), or they may be a coalition of unrelated individuals that help behavioral choices that enhance their energy gain and reduce their one another. risk of becoming prey. The lions in a pride hunt cooperatively, and the females often feed, care for, and protect one another’s cubs. But life in a pride has a dark side as well. CONCEPT 8.3 Mating behaviors The male in Figure 8.1 is killing one of the cubs in his pride, a behavior that reflect the costs and benefits of seems both horrific and puzzling. Why do adult male lions do this? To shed parental investment and mate light on this murderous behavior, let’s consider some aspects of the life his- defense. tory of lions in more detail. CONCEPT 8.4 There are As young adults, male lions are driven from the pride into which they advantages and disadvantages to were born. A group of young males expelled from a pride may stay together living in groups. to form a “bachelor pride.” Bachelor prides may also consist of males from different prides that meet and begin to hunt together. By the time they are 4 or 5 years old, the young males in a bachelor pride are large and strong enough to challenge the adult males of an established pride. If their chal- lenge is successful, the new males drive off the “dethroned” males, and they typically try to kill any young cubs that were recently fathered by those males. Although the females fight back, the new males often succeed in killing cubs. If a female’s cubs are killed, she becomes sexually receptive soon thereafter. In contrast, it can take up to two years for a female with cubs to resume sexual cycling. This delay in sexual receptivity can help us to understand the behavior of the incoming males. On average, incoming males remain with a pride for just two years before they are defeated and displaced by a new group of younger males. By killing cubs when he enters a pride, a new male increases the chance that he will reproduce before he is displaced by a younger male. As a result, incoming males that commit infanticide should leave more offspring than do males that do not commit infanticide. This logic suggests that infanticidal behavior by males is favored by natural Figure 8.1 Killing the Cub The male African lion shown here is attempting to kill the juvenile offspring of another male; such attempts often succeed. Why might this behavior be evolutionarily adaptive for the murdering male? Behavioral Ecology 183 a young trout that remains close to a hiding place while feeding may increase its chance of escaping predators, but in so doing, it may forgo the oppor- tunity to forage in areas that are rich with food but lacking in protective cover. As these examples suggest, the behavioral deci- sions made by individuals have very real costs and benefits that affect their ability to survive and re- produce. These examples also highlight the fact that animal behaviors take place in an ecological setting: the behavioral decisions of the lion and the trout are made in the presence of competitors and predators. As we’ll see in this chapter, the facts that behaviors occur in an ecological setting and that they affect survival and reproduction are central themes in the field of behavioral ecology, the study of the ecologi- cal and evolutionary basis of animal behavior. Behavioral ecology is a dynamic field, broad in scope. We cannot provide a comprehensive survey of this field in a single chapter (for that, see the text- books on behavioral ecology listed in the Suggested Figure 8.2 Females That Fight to Mate with Choosy Males Red phalarope Readings on the website). Instead, following up on (Phalaropus fulicarius) females (the two birds on the left) are larger and more the topics highlighted by life in lion prides, through- colorful than the male of their species (at the right). In this species, the females out this chapter we’ll emphasize three aspects of fight over the right to mate with the males—and the males choose which female behavior: foraging behavior, mating behavior, and they will mate with. living in groups. We’ll begin by taking a closer look at the types of questions that behavioral ecologists address in their research. selection, leading us to expect that it would be common CONCEPT 8.1 in lion populations (which it is). An evolutionary approach to the study of Infanticide is just one of the seemingly odd behaviors behavior leads to testable predictions. we see in animals. Fruit flies, for example, sometimes lay their eggs in food sources that contain high concentra- Ecology 3E Cain tions of ethyl alcohol, a toxic substance. Why do they do An Evolutionary Approach Sinauer Associates this? And why is it that the females of many species are Morales Studio more “choosy” than the males in selecting a mate—and to Behavior Ecology3E_08.02.ai Date 12-24-13yet in some species (such as the birds in Figure 8.2) the Researchers studying animal behavior can seek to answer males are choosy and the females try to mate with as questions at several different levels of explanation. You many males as possible? For answers, we turn to the might ask, for example, why a robin hopping around your strange and wonderful world of animal behavior. yard periodically tilts its head to the side. It turns out that the bird is listening for worms moving through the soil. Robins can do this because their sensory and nervous sys- Introduction tems can detect the faint sounds of worms moving through In nature, many of an animal’s activities are centered on the soil. (You can hear those sounds in Web Extension 8.1.) obtaining food, finding mates, or avoiding predators. Thus, one explanation for the robin’s behavior might focus The behavioral decisions an animal makes often play a on how the required sensory equipment works. Further- key role in its ability to meet these three critical needs. more, hunting by listening might enable a robin to detect Consider the dilemma facing a young male lion decid- otherwise hard-to-find prey. Hence, a second explanation ing whether to challenge the adult males of a lion pride. of the robin’s head-tilting behavior might focus on wheth- An incorrect decision by the young male could lead to er listening for worms increases the efficiency of foraging, serious injury or death (if he is defeated in combat), or thus enhancing the bird’s survival and reproductive suc- it could lead to a missed opportunity to join a pride and cess. If so, then this behavior may have become common reproduce (if he delays combat unnecessarily). Likewise, over time because it was favored by natural selection. uncorrected page proofs © 2013 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. 184 Chapter 8 Notice that the first explanation we mentioned ad- dresses a “how” question about behavior: it looks within 1.0 Wild type an individual bird to explain how the head-tilting behav- 0.8 ior functions. By focusing on events that take place during Insecticide-selected an animal’s lifetime, this approach seeks to explain behav- 0.6 iors in terms of their immediate or proximate causes. In 0.4 Wild-type contrast, the second explanation addresses a “why” ques- 0.2 cockroaches tion about behavior: it examines the evolutionary and his- did not avoid torical reasons for a particular behavior. By addressing 0.0 glucose. previous events that influenced the features of an animal –0.2 Cockroaches from as we know it today, this approach seeks to explain be- Feeding index insecticide-selected –0.4 haviors in terms of their evolutionary or ultimate causes. populations avoided –0.6 eating agar that Although behavioral ecologists examine both proxi- contained glucose. mate and ultimate causes in their research, they are pri- –0.8 marily concerned with ultimate explanations of animal –1.0 behaviors. We will follow their lead in this chapter, focus- Corn syrup 1 M fructose 1 M glucose ing on selected ultimate explanations for why animals behave as they do. We’ll begin by examining how natural Figure 8.3 An Adaptive Behavioral Response Feeding selection affects behavior. behavior in two populations of the German cockroach (Blat- tella germanica), one of which (“Wild type”) had no prior Natural selection shapes animal behaviors exposure to insecticides, while the other had been exposed over time to insecticides. Cockroaches could choose to eat plain (un- As we’ve seen in earlier chapters of this book, an individ- sweetened) agar, agar that contained one of three sources of ual’s ability to survive and reproduce depends in part on sugar—fructose, glucose, or corn syrup (which contains both its behavior. Therefore, natural selection should favor indi- fructose and glucose)—or both.
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