Behavioral Ecology
advertisement
Behavioral Ecology What is Behavior? Behaviors describe how organisms interact with their environment, with members of their own species, and with members of other species. The Hows & Whys of Behavior Biologists can ask 4 different types of questions about animal behavior. Questions about Mechanism: What internal and external factors trigger a given behavior? Questions about Ontogeny: How does this behavior develop over the lifespan of an individual? The Hows & Whys of Behavior Questions about Adaptive value: Why is this behavior adaptive for the individual? This is the focus of behavioral ecology. Questions about Phylogeny: How has this behavior evolved over time and why does it differ among species? To Forage is to Choose Few behaviors affect an individual's fitness as much as the foraging "decisions" it makes. The American Pika needs to gather food because, she will not hibernate; instead, she will feed on the stored food during the long, dark winter. To Forage is to Choose Foraging decisions include: How far to travel to get food? How much food to collect on each trip? What type of food should be collected? To Forage is to Choose Behavioral ecologists rely heavily on models to provide insight into why creatures behave in certain ways and not others. These models are formal hypotheses that attempt to explain why organisms act as they do. When such a model predicts a behavior accurately, that suggests the model has captured some of the essential features of the behavior. Harvesting Hay: How Far to Travel? Behavioral ecologists expect evolution to favor pikas that forage efficiently. Maximizing the net rate at which they store energy while haying. Optimal Foraging Theory Some foraging strategies work better than others. Assuming that pikas with a better strategy are more likely to survive and reproduce (i.e., have higher fitness)—and assuming that their foraging strategy is partly under genetic control—the average foraging efficiency of a pika population should increase over time as a result of evolution by natural selection. Optimal Foraging Theory Behavioral ecologists hypothesize that, given enough time, pikas should come to use the best foraging strategy possible, one that maximizes each pika's fitness. This is known as optimal foraging. Surrogate Measures of Fitness Measuring fitness directly can be difficult. Instead, researchers typically evaluate behaviors according to how they affect some other variable— some currency—that is related to fitness, such as average net haying rate for pikas. Surrogate Measures of Fitness When measuring fitness costs must be considered as well as benefits. Benefits – food gathered Costs – time & energy spent running back and forth. Testing the Optimal Foraging Model You can evaluate a model by testing the predictions it generates. How well does the simple graphical costbenefit model predict pika behavior? Estimate actual energy values for the cost and benefit curves. Compare the predicted foraging distance to what pikas actually do. Our model predicts that the optimal distance to maximize benefits is about 8 m from the haypile. Net energy gained Benefits – costs 29 cal – 13 cal = 16 Predation Risk Affects Foraging Decisions Any time an individual searches for food, looks for a mate, or otherwise "behaves", there is some risk that a predator will capture and kill it. Once killed, all future reproductive opportunities are lost, which is why evolution favors wary behavior for prey. Predation Risk Affects Foraging Decisions Benefits remain the same, but with the addition of predators, costs will increase! According to this revised model, if perceived predation risk is high, how should an optimally-foraging pika alter its behavior? By foraging farther from the rocks and storing hay more quickly. By foraging farther from the rocks and storing hay more slowly. By foraging closer to the rocks and storing hay more quickly. By foraging closer to the rocks and storing hay more slowly. Behavior in the Marketplace Behavior in the Marketplace A pika must answer a variety of questions as she forages, including: How far should she travel to forage? How long should she spend foraging in a given patch? Which species should she harvest? Behavior in the Marketplace Hypothesis: Natural selection will favor pikas that forage as optimally as possible. To test this hypothesis: Compare predictions based on optimality theory to observed results. So, what is optimal? models What to Eat? Foragers usually have a variety of food items to choose from. Handling time Nutritional value What to Eat? Our pika prefers yellow flowers to blue. She should always pick yellow flowers. When should she go for the more common, but less preferred blue ones? What to Eat? When yellow flower density is low, she has to choose blue flowers. As yellows increase, she chooses those & fewer blues. When yellow flowers are common, she hardly ever chooses blue, no matter how common they are. Prey Profitability Why does the pika prefer yellow flowers? Lower energy (so not that) Shorter handling time Prey Profitability Prey profitability accounts for both handling time and energy. Profitability = E/h Choosing What to Eat Add in search time. Blue has been found, so s2=0. How long would it take to find a yellow flower if you skip the blue? Choosing What to Eat The rule involving critical search time says that an optimal forager should include a second item in its diet if it can gain energy more quickly by pausing to handle this less profitable prey (i.e., the blue flower), whenever found, than it can by continuing to search for the relatively hard-to-find, more profitable prey (i.e., the yellow flower). Choosing What to Eat Charnov's model predicts that a forager should switch from a specialist (focusing on only yellow) to a generalist (eating a mix of flowers) when: S1 > (E1h2 / E2) – h2 Choosing What to Eat When S1 > 1 s the pika should eat both flowers as they are encountered, but if S1 ≤ 1 s the pika should only eat yellow flowers. Testing Predictions with Real Foragers The researchers offered the birds two types of prey: a large mealworm and a smaller mealworm to which a piece of plastic tape had been affixed. The large mealworm was more profitable because its energy content was higher and its handling time was smaller. During the experiment, birds perched in a chamber and fed on the two prey items as they passed by on a conveyor belt. Using the conveyor belt, researchers manipulated search times for both prey. Testing Predictions with Real Foragers They predicted that when large mealworms were rare, birds would feed on both prey types, but when the large mealworms were common, the birds would specialize on the more profitable prey. Testing Predictions with Real Foragers The bird did not forage exactly as predicted. The bird was a bit slower to switch from a broad diet to a more specialized one, and even when it did specialize on the larger mealworms, it always included some of the smaller, less profitable prey items in its diet. Optimal Loads Because the patch's contents are finite, and because the pika's mouth size limits what she can collect, she faces diminishing returns as the patch becomes depleted and/or her mouth gets full. Optimality Models Optimality models develop clear, testable predictions about how an organism should behave. Critical assumption of optimal foraging theory that rarely holds: that foragers have perfect knowledge of their environment. Models are useful despite limitations! Playing Games Playing Games Animals continually compete for resources like food, mates, and territories. Fight? Display? Run? Playing Games In 1982, John Maynard Smith turned to an interesting branch of mathematics known as game theory to help answer questions about conflict. He suggested that contests over resources should be viewed as games in which each player attempts to pick the best strategy. Evolution favors players whose choices maximize their fitness. Playing Games Pikas must defend their haypile from other pikas. When should a pika fight? When fighting enables it to survive and reproduce better than its opponent. Hawk vs. Dove Hawk: Pikas playing hawk fight for the territory, risking grievous injury when their opponent also plays hawk. Hawks always beat Doves, but win just half of their contests with other Hawks. Dove: Pikas playing dove do not fight, and therefore do not get injured. They flee from Hawks but win half their contests with other Doves, where they compete by showing off to each other—displaying—at low cost. Hawk vs. Dove Game Theory Ecologists find game theory useful because it makes clear, testable predictions. For example, it predicts that fights should be more common and more intense— that animals should be more willing to play hawk—when the ratio of benefits to costs is high. Game Theory Male elephant seals engage in ferocious battles over the right to breed with a harem of females. Because only the dominant males breed and most males die without reproducing, the benefit of winning, evolutionarily speaking, is huge, and despite frequent injuries, the benefit-to-cost ratio is high. Game Theory Male toads also frequently fight over mates. Toads are poor fighters without any serious weapons and the loser of these contests is rarely injured. Toads fight not because the potential benefits are so great, but rather because the costs are so low, which also results in a high benefit-to-cost ratio Hunters vs. Pirates Bald eagles adopt either a hunter or pirate strategy. Hunters search for unclaimed fish, while Pirates are kleptoparasites, stealing fish from other birds. Some birds employ pure strategies where they either hunt or pirate exclusively, while others employ a mix of both. Hunters vs. Pirates The payoffs to the two strategies were approximately equal, as both Hunters and Pirates fed at approximately the same rate and neither suffered injuries. Hawk-Dove-Bourgeois Contests are rarely symmetrical and players may use different rules when deciding whether or not to fight. Because vacant pika territories are relatively rare, most such contests occur between a territory owner and an intruder. These two individuals are unlikely to value the territory equally. Hawk-Dove-Bourgeois Consequently, territory owners are more likely to escalate fights than invaders. a Challenger adopting bourgeois plays hawk whenever he owns the territory and plays dove whenever he attempts to invade another's territory, regardless of what the Opponent does. Bower-Wrecking Bowerbirds Fights and other interactions are not always face-to-face confrontations. Males build elaborate bowers, and females are very picky, mating only with males who build the most beautiful bowers. To gain an advantage in this intense competition, male bowerbirds often handicap their neighbors by destroying the neighbors' bowers, a behavior known as marauding. Bower-Wrecking Bowerbirds The problem with sabotaging neighbors' bowers is that the marauder is not at his own bower if a potential mate or marauding competitor shows up. There is a clear cost to marauding. Maraud or Guard? To calculate costs and benefits, the Pruett-Jones team measured how long each male spent at his own bower, how long it took to repair a destroyed bower, travel time to and from a neighbor's bower, etc. Maraud or Guard? Marauding is the Evolutionarily Stable Strategy, but the average male would do best if everyone guarded their own nest. An evolutionarily stable strategy (ESS) is one that cannot be invaded by an alternative strategy. Family Matters Bowerbirds are best described as a polygynous speci es, one where each successful male mates with multiple females. All effort is spent attracting females – males do not care for offspring. Family Matters Many other species are monogamous, with males and females forming pairs for the entire breeding season or longer. For example, blue tit (Parus caeruleus) couples work together to raise their young. Questions Why are males of some species so much showier than females? What do females gain by mating with sexier males? Why are some species more monogamous than others? Why do some females accept mates who do not provide parental care? Why do some "monogamous" individuals mate with individuals other than their partners? Anisogamy Answers to many of these questions begin with the difference between male and female gametes. Males produce sperm that are small, plentiful, mobile, and cheap to make. In contrast, females produce eggs that are large, few in number, immobile and energetically expensive to produce. Anisogamy Anisogamy Because of this difference in investment in gametes, the two sexes experience different selective pressures that favor distinct reproductive strategies. Sexual Selection In general, males tend to mate with many females to produce as many offspring as possible. Because of the higher investment in eggs and often in parental care, females have a different strategy. Mate choice Sexual Selection Charles Darwin recognized that these differences in fitness drive a distinct form of evolution by natural selection that he termed sexual selection. Sexual selection explains not only why males and females tend to adopt different strategies, but also why they are so often sexually dimorphic, with the two sexes looking quite distinct. Intrasexual Selection Competition over mates can take a variety of different forms, including physical battles in which males compete directly for mates. Contests between rhinoceros beetles are Intrasexual Selection: battles of strength, and Contests between males the bigger male for access to mates. typically wins. https://youtu.be/_VBz0FaXN1c Intrasexual Selection Males of many species including elk, sea lions, frogs, and fiddler crabs engage in contests over mates. Wielding weapons such as antlers, horns, tusks and claws. These weapons can serve to reduce injuries. Honest signals of an individual's condition often reduce the likelihood of escalated battles. Intersexual Selection Intersexual selection occurs when one sex— usually females— chooses among members of the opposite sex. Female choice Example – long-tailed widowbird Males mate with multiple females Polygynous Widowbird Experiment H1: Female widowbirds prefer extravagantly ornamented males. H2: Female widowbirds choose males based on territory quality. Widowbird Experiment Predictions – What results would you expect for each of our alternate hypotheses? Widowbird Experiment Females did prefer males with longer tails. Did not discriminate between males with shorter tails and normal length tails. What Females Want Female choice has led to the evolution of many of the extravagant, showy ornaments some males sport. But why? What do females gain from mating with sexier males? What Females Want Good Genes. Showy males offer genes that increase the fitness of a female's offspring by ensuring heterozygosity or conferring advantages such as disease resistance. Assumption is that showiness is an honest signal of good genes. Ex maintain long tail without getting eaten What Females Want Sexy Sons. Female preference creates an evolutionary feedback loop that selects for more and more extreme male ornaments. Females who prefer males with extreme traits (e.g., long tails) produce sons that inherit their father's long tail and daughters that inherit her preference for long tails. Indian Peafowl Research Male reproductive success is correlated with number of eyespots and mass of tail. Reduce eyespots = reduced success Males with many eyespots have stronger immune systems and are less likely to fall prey to foxes. Peacocks with larger eyespots father larger young that survive better. Tail morphology is partially heritable. Sneaky Males In species for which intrasexual selection is important, males that are larger and betterarmed win most contests for females. What should a small male do? Smaller males adopt alternative strategies that allow them to fertilize females without risking a fight. Sneaky males Ex. Atlantic Salmon Migratory males vs precocious parr Raising the Kids Some species require significant parental investment. Parents provide food, grooming, protection, training, etc. This care is costly, and each parent might do better (produce more offspring) if the other parent shouldered the burden. Raising the Kids Life history patterns play an important role. In mammals ,for example, internal gestation and la ctation mean that females provide all of the initial care. As a result, female-only care is the rule in 91% of mammalian species. Raising the Kids Male and female birds are equally capable of incubating eggs and feeding chicks, and two parents often are more successful than one. This helps explain why biparental care occurs in 81% of bird species Raising the Kids Trade-off: Parents should only invest resources in their current offspring if they can't do better by investing those resources in their own survival or in future offspring. Bluegill Sunfish Experiments Parentals are large, territorial males that compete for nesting sites, guard potential mates and, most importantly, provide care for offspring. Cuckolders do none of these things. When small, they sneak fertilizations by darting in while parentals are spawning. Bluegill Sunfish Experiments H: A male is more likely to provide parental care if he is sure he is the father. If no other males are present during spawning, a parental male can be relatively confident that he fertilized the eggs, but if another male is present, his paternity certainty is likely to decline. Males can’t tell if eggs are theirs, but they CAN tell if fry are theirs! Bluegill Sunfish Experiments Make a prediction: If seeing a sneaker male reduces a parental male's paternity certainty, how should that affect the care he provides to the eggs in his nest? To the young fry once they've hatched? Bluegill Sunfish Experiments Results: The male provided less parental care to the eggs, but once they hatched he know they were his & parental care increased. Mating Systems Monogamy: one female and one male pair up for a breeding season or lifetime. Polygyny: a male will mate with many females. Polyandry: a female will mate with many males. Polygynandry: both males and females mate with multiple partners. Cooperation Vampire Bats Vampire bats rely on stealth, attempting to feed without disturbing their victims, if they are noticed they will be shaken off before they can feed. On nights when a bat's attempts are thwarted or it simply cannot find a suitable victim, it will return to the roost hungry. Worse, if a bat fails to feed for several days in a row, it will likely starve to death. Vampire Bats Upon returning to the roost, hungry bats beg food from other members of the colony. Sometimes hungry beggars receive a regurgitated blood meal from a female neighbor. Optimality models and game theory predict selfish behavior. So why do bats help each other? Social Interaction Four types of social interaction +/+ Mutual benefit +/- Selfishness -/+ Altruism -/- Spite Vampire Bats Consider costs and benefits. In order for selection to favor sharing, the donor bat most give up less than the recipient gains. Vampire Bats Using game theory we would predict that it is always better to withhold a meal rather than share. Selfish behavior is often predicted because it will maximize fitness. Vampire Bats If Withholders always win, why do real vampire bats share meals at all? Real vampire bats don't play the game just once with each roostmate, nor do they forget about past games. Instead, they play the game every night for years, mostly with the same roostmates (bat colonies are relatively small and stable). Tit-for-Tat Strategy Research on iterated games illuminate a clear path toward the evolution of altruism. A simple strategy known as tit-fortat did exceptionally well. Tit-for-Tat Strategy If individuals repeatedly interact, tit-for-tat and other such strategies can have higher fitness than simply defecting all the time. Reciprocity Reciprocity is favored when The benefit for the recipient is greater than the cost to the actor. There are frequent opportunities for repayment. Individuals can recognize each other and remember past behavior. Reciprocity The sharing of blood meals by vampire bats appears to meet these criteria. Why Sound the Alarm? Belding's ground squirrels (Urocitellus beldingi) are colonial, herbivorous rodents that inhabit the meadows of many mountains in the western United States. Why Sound the Alarm? When a squirrel spots a predator such as a hawk flying in the sky or a coyote prowling on the ground, they often make a whistling alarm call to warn their neighbors. They use a short high-pitched call for aerial predators and a longer, multiplenote trill for terrestrial predators. Why?? Why Sound the Alarm? Benefits Caller: Calling directly benefits the caller by helping it avoid being caught. Predictions: Calling reduces predation risk for caller. Males and females are equally likely to call. Callers do not discriminate against noncallers. Why Sound the Alarm? Reciprocity: Calling directly benefits the caller because the warning is reciprocated at a later time by the squirrels who are warned by the caller. Predictions: Males and females are equally likely to call. Callers know each other and discriminate against non-callers. Why Sound the Alarm? Alerts Relatives: Calling indirectly benefits the caller by increasing the survival of close relatives, who share some portion of the caller's genes. Even if calling increases the predation risk for the caller, the caller will ultimately benefit if kin hear the alarm and avoid predation. Predictions: Females are more likely to call. Callers do not discriminate against non-callers. Why Sound the Alarm? Results Females call more often than expected. Call of the test using ground predators: more with relatives near. Males call less often than expected. Calling is dangerous! Kin selection Why Sound the Alarm? Results using aerial predators: Caller is more likely to escape. Males and females equally likely to call. Callers do not discriminate against noncallers. Benefits caller Relatedness, Kin Selection & Hamilton’s Rule How much help should an individual provide to relatives? Depends on how closely related they are. Relatedness, Kin Selection & Hamilton’s Rule Relatedness simply describes how many alleles, on average, two individuals have in common. Quantified using the coefficient of relatedness (r). Parent-child r=0.5 Full siblings r=0.5 Half siblings r=0.25 Cousins r=0.125 Inclusive Fitness Direct fitness – raising own offspring Indirect fitness – helping to raise a relative’s offspring Inclusive fitness = direct + indirect Hamilton’s Rule Evolution favors behaviors where the evolutionary benefit—the fitness gain— exceeds the cost. Hamilton’s Rule rB-C > 0 r = coefficient of relatedness Because indirect fitness benefits are greatest when helping a close relative, ecologists call selection for these behaviors kin selection. Turkeys’ Unusual Partnerships As with other lekking birds, competition for females is fierce and a few successful males gain most of the mating opportunities. Coalition of 2-4 males Dominant male mates with all females. Turkeys’ Unusual Partnerships Why do subordinate males help? Two Hypotheses: Kin Selection Males fitness are closely related – maximize inclusive Patient Males Males are “apprenticing” – practice until a territory becomes available. Turkeys’ Unusual Partnerships Careful observations of turkey coalitions to distinguish between the 2 hypotheses. Coalitions tend to form early in life. When dominant males die, subordinate males do not acquire new coalition partners and never achieve dominance, attract females, or mate. Rules out Patient Males hypothesis. Turkeys’ Unusual Partnerships To evaluate the Kin selection hypothesis: Determine coefficient of relatedness Determine Benefit to dominant male Determine Cost to subordinate male Eusocial Systems In all eusocial species, only a few select individuals breed, while the rest of the colony help raise the offspring. Queen honeybee – only reproductive female Workers – all non-reproductive females Drones – males (only job is to mate with queen) Eusocial Systems Kin selection may help explain the complex and apparently altruistic behaviors of bees and other eusocial species. This is especially likely because some eusocial species have a strange genetic system called haplodiploidy. Haplodiploidy Females are diploid – receive half their genes from mom half from dad. Males are haploid – all of their genes come from mom. Sisters have r=0.75 Eusocial Systems Some eusocial species are diploid. Colonies tend to be physically isolated centered around a defensible nest members of each colony have very high relatedness with each other, either from haplodiploidy or from inbreeding. Helping Kin Means Helping Oneself (Almost) Behaviors that improve an individual's fitness— its inclusive fitness—will be favored by evolution. By carefully analyzing the benefits and costs of any behavior, from foraging to fighting to choosing mates, behavioral ecologists can gain insight into the adaptive value of the behavior.
