Behavioral Ecology

Chapter 8 Behavioral Ecology CONCEPT 8.1 An evolutionary approach to the study of behavior leads to testable predictions. CONCEPT 8.2 Animals make behavioral choices that enhance their energy gain and reduce their risk of becoming prey.

Chapter 8 Behavioral Ecology CONCEPT 8.3 Mating behaviors reflect the costs and benefits of parental investment and mate defense. CONCEPT 8.4 There are advantages and disadvantages to living in groups.

Baby Killers: A Case Study Lions are the only cats that live in social groups called prides. Adult females in a pride are closely related. A pride hunts cooperatively, and females often feed and care for each other’s cubs. 4

Baby Killers: A Case Study Adult male lions often kill the cubs of another male in the pride. Why would this behavior be adaptive?

Baby Killers: A Case Study Young adult male lions are driven from the pride and may form “bachelor prides” that hunt together. At 4 or 5 years, a male can challenge adult males in an established pride. If successful, the new male may kill cubs recently sired by the vanquished male. 6

Baby Killers: A Case Study A female lion will become sexually receptive soon after her cubs are killed, as opposed to 2 years if she has cubs. The new male is increasing the chances that he will sire cubs before he is replaced by another, younger male. 7

Baby Killers: A Case Study Many seemingly odd behaviors exist in the animal world. In many species, females are more “choosy” than males in selecting a mate; but in some species males are choosy, and females try to mate with as many males as possible. 8

Figure 8.2 Females That Fight to Mate with Choosy Males Ecology3e-Fig-08-02-0.jpg

Introduction An animal’s behavioral decisions play a critical role in activities such as obtaining food, finding mates, avoiding predators. These decisions have costs and benefits that affect an individual’s ability to survive and reproduce.

Introduction Behavioral ecology is the study of the ecological and evolutionary basis of animal behavior. 11

CONCEPT 8.1 An evolutionary approach to the study of behavior leads to testable predictions.

Concept 8.1 An Evolutionary Approach to Behavior Animal behaviors can be explained at different levels: Proximate causes (immediate)—or how the behavior occurs. Ultimate causes—why the behavior occurs; the evolutionary and historical reasons. Behavioral ecologists mostly focus on ultimate causes.

Concept 8.1 An Evolutionary Approach to Behavior Because an individual’s ability to survive and reproduce depends in part on its behavior, natural selection should favor individuals whose behaviors make them efficient at foraging, obtaining mates, and avoiding predators. Animal behaviors are often consistent with this prediction.

Concept 8.1 An Evolutionary Approach to Behavior If the traits that confer advantage are heritable, natural selection can result in adaptive evolution: Traits that confer survival or reproductive advantages tend to increase in frequency over time.

Concept 8.1 An Evolutionary Approach to Behavior Many studies have documented adaptive behavioral change. Silverman and Bieman (1993) showed that cockroaches exposed to traps with a bait containing an insecticide plus glucose evolved glucose aversion, which is controlled by a single gene. 16

Figure 8.3 An Adaptive Behavioral Response Ecology3e-Fig-08-03-0R.jpg

Concept 8.1 An Evolutionary Approach to Behavior Most aspects of animal behavior are controlled by both genes and environmental conditions. Weber et al. (2013) studied burrow construction in two mice species. 18

Concept 8.1 An Evolutionary Approach to Behavior Oldfield mice build a long entrance tunnel and an escape tunnel, possibly an adaptation to living in open habitats that provide little protective cover. Deer mice construct a simpler burrow, with a short entrance tunnel and no escape tunnel. 19

Figure 8.4 Distinctive Mouse Burrows Ecology3e-Fig-08-04-0.jpg

Concept 8.1 An Evolutionary Approach to Behavior The two mice species can interbreed and produce fertile offspring. All of the F1 hybrid offspring built burrows with escape tunnels, as did about 50% of backcross mice (F1 hybrids mated with deer mice). This indicates that building escape tunnels is controlled by one genetic locus. 21

Figure 8.5 The Genetics of Escape Tunnel Construction Ecology3e-Fig-08-05-0R.jpg

Concept 8.1 An Evolutionary Approach to Behavior Genetic mapping (quantitative trait locus analysis, or QTL) also showed that entrance tunnel length was controlled by three genetic loci. Although few studies have identified the genes, many behaviors are known to be heritable, and most are influenced by multiple genes. 23

Concept 8.1 An Evolutionary Approach to Behavior Individuals with an allele for a certain behavior may not always perform that behavior, and may change behavior when in different environments. But by assuming that genes affect behaviors, and natural selection has molded them over time, we can make specific predictions about how animals will behave. 24

CONCEPT 8.2 Animals make behavioral choices that enhance their energy gain and reduce their risk of becoming prey.

Concept 8.2 Foraging Behavior Food availability can vary greatly over time and space. If energy is in short supply, animals should invest in obtaining the highest- quality food that is the shortest distance away.

Concept 8.2 Foraging Behavior Optimal foraging theory: Animals will maximize the amount of energy gained per unit of feeding time, and minimize the risks involved. The theory assumes that natural selection acts on the foraging behavior of animals to maximize their energy gain. 27

Concept 8.2 Foraging Behavior Profitability of a food item (P) depends on how much energy (E) the animal gets from the food relative to amount of time (t) it spends obtaining the food: 28

Figure 8.6 Conceptual Model of Optimal Foraging Ecology3e-Fig-08-06-0R.jpg

Concept 8.2 Foraging Behavior An animal’s success in acquiring food increases with the effort it invests; but at some point, more effort results in no more benefit, and the net energy obtained begins to decrease. 30

Concept 8.2 Foraging Behavior Tests of the model: Benefits may incorporate net energy gained, time spent feeding, or risk of predation. If optimal foraging is an adaptation to limited food supplies, then we must be able to relate the benefit to survival and reproduction of the animal. 31

Concept 8.2 Foraging Behavior In a study of great tits, proportions of prey types and encounter rates were varied. The time it took birds to subdue and consume the prey (handling time) was measured. The model correctly predicted consumption rates of large mealworms as profitability of prey items varied. 32

Figure 8.7 Effect of Profitability on Food Selection Ecology3e-Fig-08-07-0R.jpg

Concept 8.2 Foraging Behavior A field study of Eurasian oystercatchers (Meire and Ervynck 1986) showed that the birds select prey items in a specific size range. Small bivalves do not provide enough energy to offset the energy needed to find and open them. Largest bivalves are too difficult to open. 34

Concept 8.2 Foraging Behavior Marginal value theorem (Charnov 1976): An animal should stay in a patch until the rate of energy gain has declined to match the average rate for the whole habitat (giving up time). Giving up time is also influenced by distance between patches. 35

Figure 8.8 The Marginal Value Theorem Ecology3e-Fig-08-08-0R.jpg

Concept 8.2 Foraging Behavior The longer the travel time between food patches, the longer an animal should spend in a patch. Cowie (1977) tested this in lab experiments with great tits. A “forest” of wooden dowels contained food “patches” of plastic cups containing mealworms. 37

Concept 8.2 Foraging Behavior “Travel time” was manipulated by covering food cups, and adjusting ease of mealworm removal. Results matched predictions made by the theorem very well. 38

Figure 8.9 Effect of Travel Time between Patches Ecology3e-Fig-08-09-0R.jpg

Concept 8.2 Foraging Behavior Optimal foraging theory does not apply as well to animals that feed on mobile prey. The assumption that energy is in short supply, and that this dictates foraging behavior, may not always hold. Resources other than energy can be important, such as nitrogen or sodium content of food. 40

Concept 8.2 Foraging Behavior For foragers, risk of exposure to their own predators is also important. Trade-offs that affect foraging decisions may be related to predators, environmental conditions, or physiological conditions. 41

Concept 8.2 Foraging Behavior Presence of wolves affected foraging behavior of elk in the Yellowstone ecosystem (Creel et al. 2005). Radio collars were used to track elk movements. When wolves were present, elk moved into forests that had more protection but less food. 42

Figure 8.10 Elk Change Where They Feed in Response to Wolves Ecology3e-Fig-08-10-0R.jpg

Figure 8.11 Movement Responses of Male and Female Elk Ecology3e-Fig-08-11-0.jpg

Concept 8.2 Foraging Behavior Small bluegill sunfish were found to spend more time foraging in vegetation if a predator was present, which provided only one-third the food of more open habitats. Larger sunfish (too large to be eaten by the bass) foraged in ways predicted by optimal foraging theory (Werner et al. 1983). 45

Concept 8.2 Foraging Behavior Even a perceived risk of predation can alter foraging patterns. Song sparrows exposed to recordings of predators fed their young fewer times per hour than did sparrows that heard recordings of nonpredators (Zanette et al. 2011). 46

Figure 8.12 Young Receive Less Food When Parents Fear Predators Ecology3e-Fig-08-12-0R.jpg

Concept 8.2 Foraging Behavior Prey species have evolved a broad range of defenses against their predators. Antipredator behaviors include those that help prey avoid being seen, detect predators, prevent attack, or escape once attacked. 48

Figure 8.13 Examples of Antipredator Behaviors Ecology3e-Fig-08-13-0.jpg

CONCEPT 8.3 Mating behaviors reflect the costs and benefits of parental investment and mate defense.

Concept 8.3 Mating Behavior Males and females often differ in physical appearance; males often posses weapons such as horns or gaudy ornaments. The sexes may also differ in behavior. Many males fight, sing loudly, or perform strange antics to gain access to females.

Figure 8.14 A Male Shows Off Ecology3e-Fig-08-14-0.jpg

Figure 8.15 A Male Courtship Dance Ecology3e-Fig-08-15-0.jpg

Concept 8.3 Mating Behavior Darwin proposed that the extravagant features of some males resulted from sexual selection: Individuals with certain characteristics gain an advantage over others of the same sex solely with respect to mating success. 54

Concept 8.3 Mating Behavior Example: Bighorn sheep with large horns defeat other males to win the right to mate with females; their genes are passed to the offspring, and large horn size becomes common. 55

Concept 8.3 Mating Behavior A test of sexual selection hypothesis: Male long-tailed widowbirds have extremely long tail feathers. They establish territories—areas that they defend against intruders. Andersson (1982) captured males and altered the length of their tail feathers. 56

Concept 8.3 Mating Behavior Males with lengthened tails had higher mating success than control males or males with shortened tails. This supported the hypothesis that female mating preferences affect male mating success. Many other studies since have found similar results. 57

Figure 8.16 Males with Long Tails Get the Most Mates Ecology3e-Fig-08-16-0.jpg

Concept 8.3 Mating Behavior In some species, males provide females with a direct benefit for mating—gifts of food, help in rearing young, access to a territory with good nesting sites, food. etc. In other species, males provide nothing—instead, females may receive indirect genetic benefits. 59

Concept 8.3 Mating Behavior The handicap hypothesis: a male that can support a costly and unwieldy ornament is likely to be a vigorous individual whose overall genetic quality is high. 60

Concept 8.3 Mating Behavior The sexy son hypothesis: the female receives indirect genetic benefits through her sons, who will themselves be attractive to females and produce many grandchildren. 61

Concept 8.3 Mating Behavior These hypotheses were tested in a study of the stalk-eyed fly. Eyestalk length is heritable, and females prefer to mate with males with the longest eyestalks. 62

Concept 8.3 Mating Behavior By selecting for long and short stalks over 13 generations, Wilkinson and Reillo (1994) showed that extreme male eyestalk lengths are maintained by sexual selection. Female mating selection also evolved differently in the two populations. 63

Figure 8.17 Mating Preferences of Female Stalk-Eyed Flies Ecology3e-Fig-08-17-0R.jpg

Concept 8.3 Mating Behavior Females may benefit from selecting males with long eyestalks because their male offspring will be attractive to the next generation of females, which supports the sexy son hypothesis. But, eyestalk length in male flies is correlated with overall health and vigor (David et al.1998), supporting the handicap hypothesis. 65

Concept 8.3 Mating Behavior Females and males often differ in the amount of energy and resources they invest in their offspring. Females are usually more choosy than males in mate selection. In anisogamous species, the female invests a lot more to produce a large egg than the male does to produce sperm. 66

Concept 8.3 Mating Behavior Females often continue to invest more in the offspring, (e.g., incubating eggs, caring for young, etc.) Because of these costs, males often produce more offspring during their lifetimes than females. 67

Table 8.1 Ecology3e-Table-08-01-0.jpg

Concept 8.3 Mating Behavior Selection should favor different mating behaviors: It should be advantageous for a male to mate with as many females as possible. A female should “protect” her investment by choosing males that provide ample resources or appear to be of high genetic quality. 69

Concept 8.3 Mating Behavior There are exceptions: in some species females compete for males. In these cases, we would expect that males would provide more parental care than females, leading to competition among females for the right to mate with choosy males. 70

Concept 8.3 Mating Behavior Females in these types of species have higher reproductive potential than males do. Red phalarope females abandon their nests once eggs are laid, and search for other males. The males incubate the eggs. 71

Concept 8.3 Mating Behavior Pipefish males have a special pouch in which they protect and nourish the fertilized eggs while the female mates with other males. Males select the largest, most highly ornamented females, who produce the most eggs. 72

Concept 8.3 Mating Behavior Ecological factors also affect mating behavior. Mate choice can be altered by factors such as number and locations of potential mates, mate quality, food availability, and presence of predators or competitors. 73

Concept 8.3 Mating Behavior Ecological factors can also influence mating systems: number of mating partners and patterns of parental care. Diverse mating systems result from the behaviors of individuals striving to maximize their reproductive success or fitness (Emlen and Oring 1977). 74

Table 8.2 Ecology3e-Table-08-02-0.jpg

Concept 8.3 Mating Behavior Polygyny can occur if females show a clumped distribution; a male can monopolize them. The brushtail possum is monogamous in habitats where food and nest sites (and hence females) are widely separated, but polygynous in habitats where food and nest sites (and hence females) are clumped. 76

Figure 8.18 Ecological Factors Can Affect the Potential for Polygyny Ecology3e-Fig-08-18-0R.jpg

Concept 8.3 Mating Behavior Monogamy usually occurs in mammalian species where it is difficult for males to defend access to more than one breeding female. 78

Concept 8.4 Living in Groups There are advantages and disadvantages to living in groups.

Concept 8.4 Living in Groups Benefits of group living: Higher reproductive success—especially when males hold high-quality territories. Group members may share feeding and care of young. Reduced risk of predation—individuals can band together to prevent attacks; predators may be detected earlier.

Figure 8.19 A Formidable Defense Ecology3e-Fig-08-19-0.jpg

Concept 8.4 Living in Groups Dilution effect: as the number of individuals in a group increases, the chance of being the one attacked by a predator decreases. Group members may respond to a predator by scattering in different directions, making it difficult for the predator to select a target. 82

Concept 8.4 Living in Groups Group members may have better foraging success. Lions, killer whales, wolves, and many other predators may coordinate their attacks, such that actions of one predator drive prey into the waiting jaws of another. Herbivores may also forage more effectively when in groups. 83

Concept 8.4 Living in Groups Costs of group living: As group size increases, the members deplete the available food more rapidly; more time may be spent in moving between feeding sites. 84

Figure 8.20 Safety in Numbers Ecology3e-Fig-08-20-0R.jpg

Concept 8.4 Living in Groups Competition for food can become more intense. In groups with a dominance hierarchy, subordinate members can spend much time and energy on interacting with group members. 86

Concept 8.4 Living in Groups Members of a large group may live closer together or come into contact with one another more often than in a small group. As a result, parasites and diseases often spread more easily. 87

Concept 8.4 Living in Groups Group size may reflect a balance between costs and benefits. Optimal size should be the size at which net benefits to the members are maximized. But unless group members can prevent other individuals from joining once optimal size is reached, observed group size may be larger than the optimal. 88

Figure 8.21 Traveling in a Group Ecology3e-Fig-08-21-0.jpg

Concept 8.4 Living in Groups It may be advantageous for individuals to belong to groups that are larger than optimal, but not so large that a new arrival would do better on its own. An intermediate-sized group might be large enough to reduce risk of predation, but small enough to avoid running out of food. 90

Figure 8.22 Should a New Arrival Join the Group? Ecology3e-Fig-08-22-0R.jpg

A Case Study Revisited: Baby Killers The males of many species kill the young of their potential mates—examples include langur monkeys, horses, chimpanzees, bears, and marmots. DNA analysis showed that male langurs were not related to the infants they killed, but were related to the females’ subsequent offspring (Borries et al. 1999).

A Case Study Revisited: Baby Killers In some species, females commit infanticide, such as giant water bugs and wattled jacanas. In these species, the males provide most or all of the parental care, and the females have higher reproductive potential. 93

A Case Study Revisited: Baby Killers Female fruit flies sometimes lay eggs in foods with high alcohol content. Exposure to alcohol kills wasps that lay their eggs on fruit fly larvae, thereby increasing the overall chance that the larva will survive. 94

A Case Study Revisited: Baby Killers Kacsoh et al. (2013) showed that adult female fruit flies altered their egg-laying behavior in response to the presence of wasps. When female wasps were present, fruit flies laid over 90% of their eggs in high- alcohol foods. This behavior increased survival of fruit fly larvae exposed to wasps. 95

Figure 8.23 Fruit Flies Medicate Their Offspring Ecology3e-Fig-08-23-0R.jpg

Individuals often change their behavior in response to predators. Connections in Nature: Behavioral Responses to Predators Have Broad Ecological Effects Individuals often change their behavior in response to predators. When exposed to recordings of predators, song sparrows fed their young less often, built nests in less desirable areas, and spent less time incubating eggs (Zanette et al. 2011).

Fear of predation can alter behavior and result in reduced fitness. Connections in Nature: Behavioral Responses to Predators Have Broad Ecological Effects The sparrow offspring lost body heat more rapidly and weighed less than did offspring of sparrows exposed to recordings of nonpredators, and number of offspring produced per year declined. Fear of predation can alter behavior and result in reduced fitness. 98

Behavioral responses to predators can also affect ecosystem processes. Connections in Nature: Behavioral Responses to Predators Have Broad Ecological Effects Behavioral responses to predators can also affect ecosystem processes. Hawlena et al. (2012) found that presence of spider predators initiated a series of events in their grasshopper prey that ultimately slowed the decomposition of plant litter in the soil. 99

Connections in Nature: Behavioral Responses to Predators Have Broad Ecological Effects When spiders were present, grasshoppers were physically stressed and required more energy for maintenance. This altered foraging behavior, leading to consumption of high-carbohydrate foods that were low in nitrogen. 100

Connections in Nature: Behavioral Responses to Predators Have Broad Ecological Effects The altered carbon:nitrogen ratio in decomposing bodies of grasshoppers influenced the ratio in the soil, which affected the community of soil microorganisms that decompose leaves and other plant matter. 101