Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
大學部 生態學與保育生物學學程 (必選) 2010 年 秋冬 抗掠食行為(Antipredator behavior) ─動物行為學 (Ethology) 鄭先祐(Ayo) 國立 臺南大學 環境與生態學院 生態科學與技術學系 教授 Ayo NUTN Web: http://myweb.nutn.edu.tw/~hycheng/ 12 抗掠食行為 (Antipredator behavior) Camouflage (偽裝) Polymorphism (多型性) Warning coloration (警告色) Batesian mimicry (擬態) Diverting coloration, structures, and behavior Intimidation (恫嚇 ) and fighting back Pronouncement of vigilance (警戒) Group defense Maintenance of antipredator behavior Ayo 教材 (動物行為學 2010) 2 Butterflies are eaten by many predators They have developed ways to outsmart their enemies Using a combination of color pattern and behavior to avoid being eaten Warning potential predators that they taste bad Caterpillars eat milkweed and incorporate toxins in their tissues Birds that eat butterflies vomit and avoid them in the future Poisons, stolen from plants, may deter some predators No defense system works all the time Its effectiveness varies with season, species of predator, and context of the predator-prey encounter Ayo 教材 (動物行為學 2010) 3 Animals have antipredator abilities In summer and autumn, monarch butterflies migrate to Mexico Black-backed orioles and black-headed grosbeaks eat monarchs Despite their chemical defenses Butterflies converge in enormous numbers at their overwintering colonies Tens of millions of individuals dilute the predation risk to any one individual Ayo 教材 (動物行為學 2010) 4 Camouflage: “I am not here” Animals have several forms of camouflage Coloration matching the visual background Disruptive coloration (斷裂顏色) Countershading (反差陰影) Transparency (透明) Masquerade (化裝舞會) : prey animals appear inedible to predators (i.e. a leaf, twig, or bird dropping) Ayo 教材 (動物行為學 2010) 5 Animals can match their background Reduces the risk of detection by visually hunting predators Animals blend with the background upon which they are resting Animals, such as this grouse(松雞), that blend with their background are often described as cryptic. Ayo 教材 (動物行為學 2010) 6 Coloration matching the visual background Concealment(隱藏) that results from an animal’s resemblance to a random sample of the visual background Background matching is maximized when coloration visually matches a random sample of the background Great tits had to search longer for the prey that was more difficult to detect Ayo 教材 (動物行為學 2010) 7 Animals select “correct backgrounds” Prey coloration matching a random visual sample of the background may only maximize background matching on simple backgrounds There is more to background matching than color and pattern Once on the “correct” background, animals exhibit behavior that maximizes their camouflage California yellow-legged frogs lies motionless against a background to which they are perfectly matched Boulders(卵石) covered by a yellow-brown layer of algae Ayo 教材 (動物行為學 2010) 8 California yellow-legged frogs Ayo 教材 (動物行為學 2010) 9 Matching and selecting the correct background is adaptive Stoneflies (石蠅) select dark brown resting spots during daylight hours No selection was observed in the dark They are vulnerable to predation by rainbow trout (紅 鱒魚) when resting on light substrates rainbow trout Stoneflies Ayo 教材 (動物行為學 2010) 10 Selecting the correct background Resting on dark resting spots by stoneflies has been naturally selected It reduces the risk of being found and eaten by visually hunting fish Animals need to be cryptic only when they are most vulnerable to predation by visual hunters Substrate choice by stoneflies may also conceal them from their own prey Ayo 教材 (動物行為學 2010) 11 Animals are camouflaged in certain areas Animals using background matching occur in areas where they are best concealed Species can get around this restriction by changing color as they change backgrounds A cuttlefish (烏賊) swiftly and dramatically changes color and pattern When resting on the bottom: it adjusts its color to the substrate Within seconds, the dorsal color can change from gray to brown Bold elements are part of disruptive coloration. Ayo 教材 (動物行為學 2010) 12 cuttlefish (烏賊) Ayo 教材 (動物行為學 2010) 13 Cuttlefish can rapidly change color Cuttlefish change color to match their background They can even display bold contrasting elements Ayo 教材 (動物行為學 2010) 14 Disruptive coloration Animals avoid being seen by matching their background But visual predators may recognize prey by their body outline Animals break up their body outline by through bizarre projections or bold contrasting markings Bold patches prevent or delay visual recognition of the prey Drawing attention away from the prey’s body Patches at the periphery of the prey’s body break up the body’s outline Disruptive coloration: prevents perception of a prey animal’s form Ayo 教材 (動物行為學 2010) 15 Disruptive coloration enhances survival Patterns at the periphery enhance survival Ayo 教材 (動物行為學 2010) 16 Disruptive coloration enhances survival High contrast patterns enhance survival Ayo 教材 (動物行為學 2010) 17 Disruptive coloration vs. habitat specialization There is a relationship (tradeoff) between the types of coloration and habitat specialization Species that rely on background matching evade detection by predators as long as they are living in a particular habitat They are habitat specialists Disruptive(斷裂的) coloration works on a greater variety of visual backgrounds It may be a strategy employed by habitat generalists Ayo 教材 (動物行為學 2010) 18 Countershading: self-shadow concealment Countershading: animals with dark backs and light bellies Makes animals hard to detect They obscure their own shadows Light usually comes from above Being darker dorsally and paler ventrally obscures the body outline Self-shadow concealment: the mechanism by which countershading achieves camouflage Ayo 教材 (動物行為學 2010) 19 Countershading: background matching Countershading makes animals difficult to detect Common in aquatic animals With light coming from above and a light belly: the animal matches the bright background when viewed from below The dark back matches the deep waters when viewed from above Ayo 教材 (動物行為學 2010) 20 Countershading in the naked mole-rat Naked mole-rats live in underground colonies Containing a breeding female (the queen), breeding males, and non-breeding workers Colony members display countershading A darker dorsal coloration A lighter (pink) ventral coloration Exceptions to the countershaded pattern Queens, newborns, breeding males, and very old individuals are pink Ayo 教材 (動物行為學 2010) 21 Why do naked mole-rats have countershading? Hypotheses for countershading in this species Protects individuals from ultraviolet light Facilitates thermoregulation The pigment melanin protects skin from abrasions (擦傷) Provides camouflage for individuals dispersing above ground It has no current function Data were most consistent with the camouflage hypothesis The age of dispersal coincides with the time when naked mole-rats have well-developed countershading Ayo 教材 (動物行為學 2010) 22 Transparency (透明) Some animals are camouflaged by being transparent Cnidarians (刺胞動物門) (hydroids (水螅) and jellyfish(水母)), ctenophores (櫛水母) (comb jellies), and the pelagic (open ocean) larval stages of many fish are almost transparent The high water content of tissues Small size Reduced number of light-absorbing molecules or pigments Transparency is the dominant form of camouflage in aquatic environments In pelagic(遠洋的) habitats where organisms have no surfaces to match or places to hide Ayo 教材 (動物行為學 2010) 23 Transparency is rare in terrestrial organisms In aquatic organisms the refractive index (angle of light traveling from the water into the watery tissues of an aquatic animal) is unchanged The animal appears to be transparent In a terrestrial environment, light passes from air into the water-filled tissues of an animal Creating an obvious body outline and reducing transparency To protect terrestrial animals from harmful ultraviolet radiation Animals need protective pigments While ultraviolet radiation is filtered in aquatic habitats Ayo 教材 (動物行為學 2010) 24 Masquerade (化裝舞會) Prey may be detected but deemed (視為) inedible by predators Leaf resemblance is a common disguise (偽裝) Insects have green or brown coloration, leaf-like patterns of venation, and flattened shapes They also behave like leaves: remaining still or swaying Leaf resemblance also occurs in some small vertebrates Ayo 教材 (動物行為學 2010) 25 Masquerade in vertebrates Leaf resemblance also occurs in amphibians and fish Amphibians that inhabit the leaf litter of the forest floor A nocturnal catfish (鯰魚) Ayo 教材 (動物行為學 2010) 26 Animal color and pattern may act together or in opposition Evading(迴避) predators is not the only function of color pattern Color affects heat balance and plays a role in thermoregulation It is also important in communication, mate recognition, courtship, male-male competition, and territorial defense How can animals communicate and yet be inconspicuous? Animals evolve auditory or olfactory signals to communicate But many still rely on visual cues Color patterns may be a compromise between camouflage and conspicuousness (顯而易見) Ayo 教材 (動物行為學 2010) 27 Mate acquisition vs. camouflage in wild guppies (孔雀魚) Females favor brighter colored, more visible patterned males Selection by predators favors less colorful and conspicuous patterns As predation increases, males have Less conspicuous colors Fewer number and size of spots Reduced diversity of patterns As predation decreases Colors and patterns become more conspicuous to attract mates Ayo 教材 (動物行為學 2010) 28 Polymorphism Background matching is not foolproof Predators may develop a search image for a species And systematically search out and consume individuals Predators that rarely encounter individuals forget the search image Species that occur at widely spaced locations use background matching Species avoid the problem of search images by exhibiting polymorphism Species occurring in several different shapes and/or color forms Ayo 教材 (動物行為學 2010) 29 Melanistic polymorphism in fox squirrels Fox squirrel color varies among and within populations Dorsal coloration ranges from gray or tan to black Melanistic (black) and nonmelanistic young occur in a litter Melanism is correlated with the frequency of wildfires Both are more common in the southeastern United States Variable coat color in fox squirrels Matches the background of an environment that periodically burns and regenerates Ayo 教材 (動物行為學 2010) 30 Squirrel polymorphism is maintained by fire Wildfires blacken the ground and tree trunks Dark squirrels are less conspicuous to hawks New plant growth causes fox squirrels with variable amounts and patterns of black to be more difficult to see When the forest has regrown, the advantage shifts to light squirrels Ayo 教材 (動物行為學 2010) 31 Being different can save lives Some polymorphic species do not match their background They rely on their diverse appearance to evade detection Predators search for individuals with a specific appearance Species that occur at high densities exhibit extreme polymorphism Different appearances prevent formation of search images Ayo 教材 (動物行為學 2010) 32 Apostatic selection (變節的選擇) Prey switching One form of frequency-dependent selection Members of a population look as different as possible When two morphs are equally camouflaged The rare morph has a selective advantage Predators develop a search image for the common morph Predators do not need to hunt by search image to cause apostatic selection in prey Some predators may have an aversion to rare or unfamiliar prey Ayo 教材 (動物行為學 2010) 33 Being different pays off Painted mussel shells with pieces of meat were offered to carrion crows Some shells were monomorphic (all the same color) Others were trimorphic (red, yellow and black) Crows took fewer trimorphic prey When prey populations occur at the same density Individuals in polymorphic populations experience less predation than those in monomorphic populations Ayo 教材 (動物行為學 2010) 34 Warning coloration (aposmatism)(警告色) Aposematism (warning coloration): a conspicuous appearance that advertises dangerous or unpleasant attributes Discourages a predator’s attack Bright colors and contrasting patterns Bold (black, white, red, or yellow) markings A skunk’s black and white color warns predators A wasp’s yellow and black body warns of its painful sting Ayo 教材 (動物行為學 2010) 35 Warning coloration in dendrobatid frogs Dendrobatid frogs (箭毒蛙) have bright coloration and toxic skin secretions Species are red, yellow, blue, or some combination The colors may contrast with black markings The most notorious is Phyllobates terribilis It has enough toxin in its skin to kill 100 humans The bright coloration of these frogs is an example of aposematic coloration The more toxic species were the most colorful Ayo 教材 (動物行為學 2010) 36 Toxicity and brightness can be decoupled In some dendrobatid frogs the most toxic species was not the most conspicuous And the most conspicuous was only moderately toxic The relationship between conspicuousness and toxicity varies under different ecological conditions A negative relationship might occur if the costs of conspicuousness are so high That prey are better off decreasing their investment in bright colors And increasing their investment in toxins Ayo 教材 (動物行為學 2010) 37 Learned predator responses to aposematism Predators learn to avoid unpalatable prey if the prey are conspicuously colored They sample the prey Discover its unpleasantness And avoid prey of similar appearance Predators learn more quickly to avoid distasteful prey that are conspicuous. Ayo 教材 (動物行為學 2010) 38 Garter snake were first offered pieces of fish on aposematic (yellow and black) or on nonaposematic (green) forceps. The snakes were then made ill by an injection of lithium chloride. The post-treatment attack latencies to (a) pieces of fish or (b) earthworms indicate that snakes in the aposematic treatment group had a stronger aversion to fish than did snakes in the nonaposematic treatment group. Ayo 教材 (動物行為學 2010) 39 Mullerian mimicry Two warningly colored species can look alike The two noxious species benefit from a shared pattern Predators consume fewer of each species while learning to avoid all animals of that general appearance Ayo 教材 (動物行為學 2010) 40 Innate predator responses to aposematism Some predators display innate avoidance of aposematic prey An innate response to warning coloration might be favored over a learned response When prey can be fatal to the predator Learning at the moment of death is of little value Ayo 教材 (動物行為學 2010) 41 Sometimes warning coloration is ignored A predator that is starving Wolves will attack skunks (臭鼬) and porcupines (豪豬) when other prey is scarce Some predators are specialists They can eat certain animals As long as an antipredator device confers a net advantage in survival and reproduction It will continue in the population Ayo 教材 (動物行為學 2010) 42 Batesian mimicry A palatable species adopts the warning characteristics of a noxious or harmful species The harmless species is called the mimic The noxious one is the model By resembling a noxious species, the mimic gains protection from predators The mimic does better when it is rare and less likely to be detected by the predator than the noxious model The more distasteful the model, the better the mimic fares Ayo 教材 (動物行為學 2010) 43 Mimics experience various degrees of protection Other factors play a role in protection of mimics depending on The memory of predators Availability of alternate prey If mimics and models are encountered together or separately Animals associated unpleasantness not just with the model But also with the place where it was experienced A mimic gains the most if its habits and activity overlap the model species Ayo 教材 (動物行為學 2010) 44 Mimics do not have to match models perfectly Hoverflies (食蚜蠅) mimic wasps (黃蜂) Pigeons rank hoverflies according to their similarity to wasps The two most common types of hoverflies least resemble wasps Yet pigeons rank them as being very similar to wasp The wasps have some key feature used by pigeons in pattern recognition When studying the defenses of prey animals, consider the cognitive and perceptual abilities of their predators Ayo 教材 (動物行為學 2010) 45 Hoverflies (食蚜蠅)是食蚜蠅科 Wasp (黃蜂),又稱為胡蜂 或螞蜂,分布廣泛、種類 繁多、飛翔迅速的昆蟲。 雌蜂身上有一根長螫針, 在遇到攻擊或不友善干擾 時,會群起攻擊,可以致 人出現過敏反應和毒性反 應,嚴重者可導致死亡。 (Syrphidae)的昆蟲。牠們經常會 停留在花朵上,成年的食蚜蠅主要 吃花蜜及花粉,而幼蟲則吃不同種 類的食物。一些物種的幼蟲是腐生 生物,吃腐化的植物及動物。另外 一些物種的幼蟲是食蟲動物,獵食 蚜蟲、牧草蟲及其他吮食植物的昆 蟲。已知有約6000種共200屬的食 蚜蠅。牠們在世界各地都很普遍, 除了南極洲外,各大洲都可以找到 牠們的蹤跡。 Ayo 教材 (動物行為學 2010) 46 Mimics gain protection from their enemies A system of Batesian mimicry involves the juvenile stage of a snail (the mimic) And the tubes of a polychaete worm(多毛綱 蟲)(the model) White-spiral phase juveniles are indistinguishable from worms Fewer were eaten by fish Ayo 教材 (動物行為學 2010) 47 Diverting a predator’s attention: false heads Many predators attack the head of the prey Some prey species have evolved false heads at their posterior end Misdirecting predators Lycaenid butterflies (小灰蝶) deflect predator attacks toward a false head Complete with dummy antennae Ayo 教材 (動物行為學 2010) 48 Diverting a predator’s attention: autotomy Some prey hand over a “disposable” body part to their attacker Autotomy (自割): the ability to break off a body part when attacked Has evolved in both vertebrates and invertebrates Tail autotomy in lizards, salamanders The prey breaks away from its attacker The detached tail moves, distracting the attacker When attacked, sea cucumbers (海蔘) expel their guts The predator feeds on the sea cucumber’s offering In most autotomy cases, the body part is regenerated Ayo 教材 (動物行為學 2010) 49 Costs of tail autotomy May lead to reduced speed, balance, swimming, climbing, or mating ability When the tail is used as a display Declines in social status occur Regeneration of the tail entails costs in energy and materials Lizards have fat deposits in their tails that are lost with the tail Once used, autotomy cannot be employed again, at least for a while Ayo 教材 (動物行為學 2010) 50 Diverting a predator’s attention: feigning injury or death Ground-nesting birds (i.e. killdeer) may feign(假裝) injury to divert a predator’s attention away from its nest and young An adult drags its wing as it flutters away from its nest The predator follows The killdeer suddenly recovers and flies away The predator wanders off May cause a predator to lose interest i.e. opossums (負子鼠) and juvenile caimans (美洲鱷) Ayo 教材 (動物行為學 2010) 51 An opossum playing dead. Ayo 教材 (動物行為學 2010) 52 Hognose snakes (豬鼻蛇) have a complex antipredator repertoire When first disturbed, the hognose bluffs (嚇唬) It flattens and expands the front third of its body and head to look larger It then coils and hisses(發出嘶嘶聲) , making false strikes When further provoked it writhes(扭動) violently and defecates (排便) Then it rolls over, belly up, with its mouth open and tongue lolling If the predator loses interest and moves away, the snake rights itself and crawls off Ayo 教材 (動物行為學 2010) 53 Hognose snakes (豬鼻蛇) The hognose snake is a type of colubrid snake (無毒蛇類) characterized by an upturned snout. They are notorious for playing dead(裝死) when threatened. Ayo 教材 (動物行為學 2010) 54 Young hognose snakes Young snakes use cues from predators to adjust their antipredator behavior Assessing the degree of threat Recovery from feigning death (裝死) of newly hatched snakes under various conditions An owl and a direct human gaze(凝視 ) caused longer recovery times Ayo 教材 (動物行為學 2010) 55 Death feigning(裝死) involves immobility Prey immobility may function to mimic death Immobility may also enlarge a prey’s functional body size When grasped by a frog, a pygmy grasshopper (菱蝗) assumes a rigid T-shape The posture is an adaptation for avoiding frog predation The size-enhancing rigid posture of grasshoppers is ineffective against praying mantises (螳螂) These predators gnaw (啃咬) on prey rather than swallowing it whole (整個吞) Ayo 教材 (動物行為學 2010) 56 Prey can intimidate (威嚇) predators or fight back The size-maximization principle A cat hunches its back and erects its fur Toads and fishes inflate themselves Intimidation (威嚇) does not need to be visual Calls, hisses, or growls deter a predator Animals display weapons Ungulates display their horns and paw at the ground Porcupines (豪豬) erect their spines Cats display their teeth Ayo 教材 (動物行為學 2010) 57 Intimidation displays in several species of animals. Ayo 教材 (動物行為學 2010) 58 Eyespots Some animals have spots that resemble eyes Small eyespots misdirect a predator Similar to false heads Located on nonvital portions of the body Prey escape with less than fatal damage Suddenly flashed spots can startle a predator A disturbed peacock butterfly suddenly opens its wings Exposing bright colors and large eyespots And emitting a hissing sound Ayo 教材 (動物行為學 2010) 59 peacock butterfly (美眼蛺蝶),鱗翅目、錘角亞目, 蛺蝶科。是香港一種常見的蝴蝶。 Ayo 教材 (動物行為學 2010) 60 Eyespots increase butterfly survival Researchers presented wild blue tits with palatable peacock butterflies Butterflies survived better when they had eyespots than when the eyespots were colored over Or when they had eyespots and hissed Eyespot displays can be very effective in deterring a predator Hissing might deter other predators Ayo 教材 (動物行為學 2010) 61 Ayo 教材 (動物行為學 2010) 62 Chemical repellents Many insects discharge noxious chemicals when they are captured Powerful toxins or irritants In some species they can be shot with accuracy The assassin (刺客) bug spits fluid in the direction of the attacker The saliva causes intense local pain Ayo 教材 (動物行為學 2010) 63 Bombardier beetles deter predators by emitting a hot, irritating spray Ayo 教材 (動物行為學 2010) 64 Vertebrates also use chemical deterrents One of the most famous examples: skunks When disturbed by canid (犬科的) predators such as kit foxes and coyotes the Texas horned lizard spatters (濺) its attacker with a stream of blood Ejected from the sinus surrounding its eyes The ejected blood contains noxious (有毒的) components Ayo 教材 (動物行為學 2010) 65 coyotes kit foxes Texas horned lizard Ayo 教材 (動物行為學 2010) 66 Pronouncement of vigilance (警戒) Vigilant prey (警戒的獵物) scan their surroundings for potential predators Prey reactions communicate to the predator the prey is alert and aware Sit-and-wait predators might leave Timber rattlesnakes spend hours or days at an ambush site Chipmunks (花栗鼠) , gray squirrels and wood thrush (畫眉鳥) harass(持續騷擾) them Harassment displays had visual (tail-flagging) and auditory components The snakes abandoned their site Ayo 教材 (動物行為學 2010) 67 gray squirrels Chipmunks (花栗鼠) wood thrush (畫眉鳥) Timber Ayo 教材 (動物行為學 2010) 68 Detected predators abandon the hunt Stotting: a stiff-legged bounding display performed by pronghorn, deer and antelope Signals to a predator that it has been detected Cheetahs (印度豹) abandoned hunts when their prey stotted Two other functions of stotting To distract a predator from offspring Fawns inform their mother that they have been disturbed at their hiding place Ayo 教材 (動物行為學 2010) 69 Stotting is when a quadruped jumps into the air, lifting all four feet off the ground simultaneously. Stotting (also pronking or pronging) is a gait of quadrupeds, particularly gazelles (e.g. Thomson's Gazelles), involving jumping high into the air by lifting all four feet off the ground simultaneously. This may occur during pursuit by a predator. It might also occur during play. Ayo 教材 (動物行為學 2010) 70 Stotting The type of predator When hunted by predators that rely on stamina(耐力), gazelles (瞪羚) use stotting to signal their ability to outrun predators Coursing (奔馳) predators chase stotting individuals at lower rates The antiambush hypothesis: stotting is not a signal but allows the animal to gain a better view of its surroundings When approached by a human, black-tailed deer stotted more often in taller vegetation Stotting also occurs during intraspecific encounters Ayo 教材 (動物行為學 2010) 71 驚嚇 Ayo 教材 (動物行為學 2010) 72 Group Defense Membership in a group Alarm signals: given when a predator approaches a group of prey One or more individuals give a signal that alerts other members to the predator’s presence May be visual, auditory, or chemical Inspire retreat by prey to a safe location The alarm may aid the signaler or its relatives Or everyone in the area, including other species Ayo 教材 (動物行為學 2010) 73 The western toad’s chemical alarm system Injured western toad tadpoles produce an alarm substance Functions as an effective antipredator device Tadpoles exposed to the alarm substance from an injured conspecific were less vulnerable to predation Ayo 教材 (動物行為學 2010) 74 Improved detection Early detection of a predator translates into escape for prey Groups are superior to lone animals in spotting predators Increases in the number of group members (and eyes, ears, noses, etc.) Allow earlier detection of predators The escape response of a vigilant (警戒的) individual can alert others to approaching danger Ayo 教材 (動物行為學 2010) 75 Florida scrub jays coordinate vigilance If a predator is spotted, the sentinel哨兵 sounds the alarm Family members mob a ground predator Or monitor the movements of an aerial attacker Sentinel哨兵 systems have also been reported for mammals Dwarf mongooses (侏獴) and meerkats (狐獴) Members of temporary groupings at foraging locations benefit from improved detection of predators Members of mixed-species groups also benefit from improved detection Communicating predator detection to other group members Ayo 教材 (動物行為學 2010) 76 meerkats(狐獴) Dwarf mongooses (侏獴) 獴科(學名 Herpestidae),哺乳綱食肉目的一科,包括各種 獴,外形較象貓。最新的分類方法只包括獴亞科一個亞科。 Ayo 教材 (動物行為學 2010) 77 Dilution effect (稀釋效應) Individuals in groups are safer because they detect predators But also because of the dilution effect: each individual has a smaller chance of becoming the next victim As group size increases, the dilution effect becomes more effective Predators aggregate in areas where their prey are abundant Some grouped prey may suffer higher predation rates Ayo 教材 (動物行為學 2010) 78 The dilution effect vs. aggregating predators Aphids(牙蟲) benefit by forming groups in the presence of predators Ladybird beetles(瓢蟲) gather at aphid groupings They also increase their feeding rate Grouping reduces predation risk for aphids The dilution effect occurs despite aggregation of predators If parasitoids and pathogens increase rapidly in aphid groups It may eliminate the antipredator advantages of the dilution effect Ayo 教材 (動物行為學 2010) 79 The tendency for prey individuals to form large groups and thereby dilute their chances of becoming the next victim by be countered by the tendency by their predators . Predatory lady bird beetles gather in larger number at aphid colonies that contain the most individuals. Ayo 教材 (動物行為學 2010) 80 Selfish herd (自私群) Centrally located animals are safer than those at the edges Lower chances of being attacked Increased probability that a peripheral animal will be eaten Selfish herd: a group is composed of selfish individuals Each trying to position as many others as possible between itself and the predator Differs from the dilution effect and improved detection because it considers the spatial arrangement of individuals within a group Ayo 教材 (動物行為學 2010) 81 The center of a group is not always best A location’s safety depends on the predator’s method of attack Schools of fish cope with a number of predators Each uses a different attack strategy The center is sometimes the most dangerous place to be Silverside fish at the center of a school suffered the most attacks from seabass Seabass split the school, striking the tail end of group Which contains individuals that were in the center Ayo 教材 (動物行為學 2010) 82 Optimal positions Optimal positions within a school are also affected by Foraging efficiency: those in the front see the food first Energetics of locomotion: fish in the front have more “drag” Silverside fish seabass Ayo 教材 (動物行為學 2010) 83 Confusion effect (困惑效應) Predators that direct their attacks at a single animal in a group May hesitate or become confused when confronted with several potential meals at once Any delay operates in favor of the prey Confusion effect: predators are less successful in attacking prey because they are unable to single out and attack individuals Ayo 教材 (動物行為學 2010) 84 The confusion effect in schooling fish One of the primary antipredator advantages of schooling When fish scatter, it’s hard for visual predators to focus on a single one Squid, cuttlefish, and pike are ambush predators Perch chase their intended victims To succeed, predators restrict their attacks to individuals That have strayed (走散) from the school or are conspicuous Ayo 教材 (動物行為學 2010) 85 For all four predators, attack success per encounter decreased as the size of the school of prey increased Ayo 教材 (動物行為學 2010) 86 Mobbing (暴衝) Sometimes prey attack predators Mobbing: approaching, gathering around, and harassing one’s enemies This strategy involves visual and vocal displays and even direct hits Initiated by an individual, conspecifics, or members of another species Ayo 教材 (動物行為學 2010) 87 The functions of mobbing The functions of mobbing include Confusing the predator Discouraging the predator through harassment (煩擾) or announcing that it has been spotted Alerting others Ayo 教材 (動物行為學 2010) 88 Mobbing is a selfish act Mobbing is not performed to protect the group It is the selfish act of individuals trying to protect themselves and their mates, offspring, and relatives Mobbing is costly It takes time and energy away from other activities Mobbers have a greater chance of being preyed on As more individuals join the mob, the risk to any one of them is reduced through dilution or confusion effects Ayo 教材 (動物行為學 2010) 89 Maintenance of antipredator behavior If an animal freezes or flees when it detects a predator It’s not doing something else: foraging, looking for mates, or resting Responding to everything as if it were dangerous decreases an animal’s fitness Costly antipredator behavior should be lost when it is no longer needed Ayo 教材 (動物行為學 2010) 90 Maintenance of antipredator behavior Animals on islands are free of predators Marine iguanas on the Galapagos Islands have been free of predators for the last 5-15 million years Until feral cats and dogs were introduced 150 years ago After exposure, they became more fearful, but not fearful enough Marsupials (kangaroos, wallabies, and their relatives) also lost some (but not all) of their antipredator behaviors after they were isolated on islands Animals can lose their fear of predators Ayo 教材 (動物行為學 2010) 91 Marine iguanas wallabies kangaroos Ayo 教材 (動物行為學 2010) 92 Summary Antipredator mechanisms decrease the probability of an encounter with a potential predator Prey may go undetected if it matches the visual background Or through disruptive coloration Prey exhibit polymorphism to prevent formation of search images by predators Prey are recognized as inedible through warning coloration Or be unrecognized through masquerade and Batesian mimicry Ayo 教材 (動物行為學 2010) 93 Summary Other defenses operate during an encounter with a predator Prey divert the predator’s attention, inform the predator that it has been spotted or fight back Group membership has antipredator advantages Prey can detect, confuse, and discourage predators An individual has a lower probability of being selected (dilution effect) Other group members are used as a shield (selfish herd) Antipredator behaviors have costs Such behaviors are lost when its predators disappear Ayo 教材 (動物行為學 2010) 94 問題與討論 [email protected] Ayo 台南 NUTN 站 http://myweb.nutn.edu.tw/~hycheng/ Ayo 教材 (動物行為學 2010) 95