University of Louisiana at Lafayette
Popular in Evolutionary Biology
Popular in Science
Joseph Merritt Ramsey
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This 11 page Class Notes was uploaded by Lauren Notetaker on Sunday April 24, 2016. The Class Notes belongs to EBIO 1010 - 02 at Tulane University taught by Bruce Fleury in Spring 2016. Since its upload, it has received 36 views. For similar materials see Evolutionary Biology in Science at Tulane University.
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Date Created: 04/24/16
Predators and their prey are caught up in an evolutionary arms race Predators coevolve with their prey - thicker the armor, the sharper the claw - the faster the prey, the faster the predator must be Predation is an extrinsic density-dependent limiting factor - Why?? Higher density of prey attracts more predators Higher prey density lowers predator search time Higher prey density supports larger population of predators - predators adjust their population level to that of their prey 4 or 5 questions, decide which of two terms goes in spot density dependent or independent scramble or contest competition Do predators regulate the size of prey populations? Yes No Maybe… C.B. Huffaker, Univ. of California - ingenious series of experiments on mites Eotetranychus - herbivorous mite Typhlodromus - predatory mite Huffaker put 40 spheres on a large tray Some were rubber balls - no food Some were oranges - food Food scattered in patches (natural) Without predators, mite populations fluctuated around their carrying capacity Introduce predatory mites, they hunted down the herbivorous mites, ate them all, then went extinct Huffaker introduced a maze-like environment (heterogenous, more natural) Barriers of vaseline, blocked movements of predatory mites Upright pencils - herbivorous mites could use them for ballooning (like baby spiders) In this more complex and natural environment, mites could often escape predators when cornered Populations of predators and prey settled down into a regular series of cycles Predator-prey cycles were known to the natural theologians The lynx and the hare are the classic example of a predator/prey cycle Data based on pelt records from the Hudson’s Bay Company - my ancestors! There are a few populations of hares on islands that have no predators On these islands hares go through exactly the same population cycles (!?) Who is regulating who?? Many other mammals show similar 10-year cycles of population growth Cycles linked to long-term cycle of sunspots (!) Sunspots affect weather Weather affects vegetation Vegetation affects prey population Prey population affects predator population Ladybugs love to eat aphids Temperature constant - populations are in balance with one another Ladybugs love to eat aphids Increase temperature - increase ladybug’s metabolism - eats all the aphids, then starves Decrease temperature - ladybugs slow down, can’t keep up - aphid populations grow rapidly Sometimes predators regulate populations of their prey (mites) Sometimes prey regulate populations of their predators (lynx and hare) Sometimes the balance depends on environmental conditions (ladybugs, Tribolium beetles) Without predators, many populations would explode out of control Prickly pear cactus (Opuntia) was introduced toAustralia by a careless gardener In the absence of any natural enemies, the cactus began to devourAustralia At its peak, cactus covered 43,320 square miles ofAustralia!! Was gobbling up the continent at the rate of 1,544 square miles per year Opuntia is held in check in its native SouthAmerican habitat by a moth that eats it Introduced cactus-eating moth, cactus quickly brought under control Moth introduced in the Caribbean in 1956, soon acquired a taste for other native cacti Moth invaded Louisiana in 2009 Could become a serious pest in Texas and esp. Mexico, where 50,000 farmers raise prickly pear cactus for food (it’s even on the flag!)l Sometimes predators are so important that the proper functioning of the ecosystem depends upon them Remove the predator, the system crumbles, like removing the keystone from an arch Off the coast of California are vast underwater forests of kelp Sea urchins graze on the kelp Sea otters feed on the sea urchins Sea otters were decimated by hunting Sea urchin populations exploded, ate up all the kelp - otters were keystone predators Kelp ecosystem collapsed, coastal erosion Otters were protected, back to normal What does this prove?? It’s not nice to fool Mother Nature! Behavioral Ecology Biological communities are shaped by the interactions of the animals that compose them Much animal behavior is instinctive, hard-wired, genetically determined More complex the animal, the greater the amount of learned behaviors Behavioral ecology examines the ways in which behavior is adaptive, how behavior varies, how it evolves Behavior of any animal in the community affects the behavior of many other animals Competitors, predator/prey, parasite/host, many examples of coevolved behaviors Take bats for example…. Bats belong to the Order Chiroptera - 880 sp. Order Chiroptera Suborder Megachiroptera - 155 sp., 40 genera, eat mainly flowers, fruit, nectar Suborder Microchiroptera - 725 sp., 140 genera, smaller bats that feed on insects, most of which are captured in flight Why are bats so scary?? Some microchiropterans eat lizards, the Desmontidae or vampire bats suck the blood of living mammals (mostly cows) Why are bats so ugly?? Faces are modified for sending and receiving sound signals About 700 species of bats have evolved the ability to hunt by sound - echolocation Most echolocating bats are the insect-eating microchiropterans Like sonar, sound waves tell bats where the prey is, how large it is, how fast it’s moving Bats hunt with FM sound (frequency modulation) Most sounds are ultrasonic, a few echolocate with signals that are barely audible, extreme upper end of human sound Why use high frequency sound to hunt?? High frequency waves are closer together, more accurate for small objects like bugs How did this behavior evolve? Sounds used for communication Sounds used for navigating in dark caves Sounds used to locate prey Other animals that echolocate? Dolphins, porpoises Oilbirds Cave swiftlets (birds nest soup) Ben Underwood and Daniel Kish All live in dark environments, strong selective pressure to navigate in the dark Small evolutionary step from using reflected sound to avoid bumping into things in the dark to using reflected sound to locate small targets in the dark Many insects can detect the sonic frequencies of hunting bats - lacewings, crickets, moths Damage a moths ears, they can no longer hear the bat, get captured 60% more often Moths in different parts of the world are sensitive to the frequency ranges used by local species of bats What can you do if a bat is closing in?? Lie perfectly still Make a desperate break for freedom Hide, run away, startle Fold your wings, plummet to the ground Turn sharply, steep vertical climb, heavier bats can’t change course as quickly *** Some species of tiger moths, FamilyArctiidae, have coevolved a unique strategy Send out sound bursts of their own, acts as a jamming signal Just like a fighter pilot punching out a cloud of metallic chaff to fool the enemy radar How do bats respond? Bats are good fliers, but the night sky is full of invisible and deadly obstacles - tree branches, vines, wires etc… Bats are frequently killed in collisions If sonic return is ambiguous, a sensible bat will break pursuit, veer off Moth 2 cm away can send a signal that appears to be 50 cm away One species sends a complex signal that may look like several moths to the bat Bats in lab tests, toss mealworms into the air, bat will catch them every time Toss mealworm and broadcast moth defensive signals, bats usually veer away So tiger moths use sound to foil the sonic hunters Bats are very common in the neotropics, much more common and numerous Trachops cirrhosus, fringe-lipped bat, a Panama species that loves to eat frogs Extensively studied by M.D. Tuttle and M.J. Ryan Fringe-lipped bats home in on the sounds made by frogs So sensitive, they can discriminate between edible and poisonous, large or small frogs They ignore the calls of the bullfrog (too big), but home in on the tungara frog (just the right size, and tasty too) But this tale has a different twist from that of the bat and the tiger moth Bats can hear the frogs Frogs can’t hear the bats! Frogs can see the bats when they fly over When they see a bat they stop singing If you keep singing, you croak…stay silent, less likely to be eaten Tuttle and Ryan used bat silhouettes, sent them over the ponds on a wire Frogs stopped calling faster on clear, moonlit nights, when bats were easily seen Slower response on cloudy or moonless nights when it was harder to see the bats Tuttle and Ryan used different sizes and shapes of fake bats Frogs stopped singing more slowly, and resumed singing more quickly, if the fake bat was a smaller insectivorous bat Many frog species have evolved anti-predator strategies against bats Chorus frogs - sing in a large group (herding) Sing from a hiding place, synchronize your calls (Panamanian tree frog Smilisca sila) Call near a waterfall, overlaps song frequency (Panamanian tree frog) Why go to all that trouble to sing, if singing gets you killed?? Frog song is sexual advertisement Male frog’s song announces his species, his desire to mate, and whether he’s a big hunk or a little pipsqueak Fundamental frequency of the song is correlated with size and age Female frogs prefer to mate with older males (can you blame them?) Tungara Frogs add a hot riff, more complex than the fundamental song Female frogs prefer the more complex song Unfortunately, bats can home in much easier on the more complex song Talk about a dilemma… If you don’t sing, you don’t mate If you sing, you might get eaten by a bat Male frogs go right on singing, the urge to mate overwhelms the fear of death Conflicting selective pressures where reproduction is concerned Darwin first described this unusual type of natural selection “Selection in Relation to Sex”, in 1871 Sexual selection Male-male competition Female choice Darwin came up with sexual selection to foil critics of his theory Any theory of evolution could explain why adaptive behaviors would be passed on Only Darwin’s theory could explain cases where natural selection seemed to be counter- productive Sometimes natural selection favors traits that are not useful for day to day survival, traits that can even get you killed Bright plumage, beautiful song, elaborate courtship displays, all attract predators Consider the common firefly... The male firefly must seek out the female in a dark meadow He homes in by exchanging pulses with the female, lands, scrambles through the grass May find a female, may find a horrible death… Female Photuris fireflies imitate signals of female Photinus, lures Photinus males to their doom! grass The gaudy tail of the peacock, the bright plumage of the Birds of Paradise, the incredible antlers of the Irish Elk all have the same purpose - attract a mate The rest of the year they are a major drag!! These traits evolved because they gave some males an advantage in attracting mates Darwin stressed sexual selection as a parallel process to natural selection Now regarded as a special case of natural selection Ironically, in trying to quiet critics of his theory, he created a major scandal Darwin claimed that sexual selection had two components Male/male competition - males competed with one another to attract females Female choice - females actively selected their desired mates Victorian society dismissed the notion that females chose their mates Even Darwin’s strongest supporters disagreed with Darwin’s theory of sexual selection Female choice not widely accepted until the 1970’s (Gordon Orians, Robert Trivers) What basis do females use to discriminate between potential mates? Several possible explanations Good genes model Handicap model Aesthetic preference Good genes model - (Borgia) - male’s appearance, his ability to thrive, suggest that he has good genes, great for making your babies Handicap model - (Zahavi) - if male can carry around huge tails or antlers and still avoid predators, feed himself, etc.. he must be really great Aesthetic preference - (Darwin) - maybe females are simply making an aesthetic choice - they like the way you look… Appearances are important, posture and grooming are important - my God, your parents were right (don’t tell them) Regardless of the basis of choice, once made the choice can become self-reinforcing Result can be runaway selection (R.A. Fisher) - trait becomes reinforced generation after generation until it is greatly exaggerated, can be a dangerous burden The Irish Elk is a great example of runaway selection Actually not an elk, but the largest deer that ever lived (now extinct) Probably vanished because of the burden of its enormous antlers Competition between males, courtship displays, give females a basis for choosing mates Females can judge from the quality of fur or plumage if the male has parasites or other health problems Intensity of the male’s display indicates his general physical fitness Well groomed males have time left over from feeding, avoiding predators etc… Quality of the male’s territory provides a very important basis for female choice Territories only make sense if resources can be monopolized If resources are evenly distributed, or superabundant, they can’t be monopolized If you can’t hold a territory, how do you sort out who is top dog?? Cows are not territorial - grass is everywhere Parakeets are not territorial - the ground is covered with seeds If resources cannot be monopolized, territoriality is not an option Males of non-territorial species must find other ways to compete for mates One solution is to fight one another to establish a dominance hierarchy within the pack or herd - a pecking order Linear sequence of dominant and sub-dominant males (alpha male, beta male…) In general, males compete with one another for females - females rarely compete Males typically court females, not vice-versa Females are much more selective than males in choosing mates - males will attempt to mate with just about anything Male competition often takes the form of a courtship display Courtship displays are elaborate ritualized behaviors that include songs, dances Birds have especially elaborate courtship displays - bright plumage, complex songs In many species of birds, a male’s repertoire, the number of songs he can sing, is directly correlated with his reproductive success This was also true of the Rolling Stones and the Beatles Tidbitting is a curious courtship behavior shared by many animals (including humans) Offer a potential mate a juicy morsel, like a ripe berry or juicy grub Shows her you are interested, more importantly shows you know how to find groceries in the wild Some spiders do this, and the praying mantis Not done out of desire, but done out of mortal terror Larger females will devour their mates, so toss a big fat grasshopper in front of her and hope you finish before she does Many types of birds compete by gathering together in one spot, called an arena, and performing for groups of females These courtship arenas are called leks, from the Swedish “leka”, meaning sex play! Grouse, prairie chickens, some cranes, parrots, hummingbirds, and bowerbirds While exploring WesternAustralia in the 1840’s, Captain J. Lort Stokes found a most extraordinary thing Found a tiny little house in the forest, made of upright sticks, with a little cleared courtyard decorated with carefully arranged tiny white shells Thought it was a little dollhouse built by an aborigine mom for her daughter Found another little playhouse later on, in NorthernAustralia - but this one was occupied by a large gray bird, about the size of a pigeon, one of eighteen species of bowerbirds Fourteen species of bowerbirds build bowers, little structures whose only purpose is to attract a mate Courtship begins in early November, continues through late December (Spring in southern hemisphere) Four different types of bowers Cleared area with decorations Mat of lichens with decorations Avenue bowers Maypole bowers Maypole bower construction can be very elaborate Includes interior rooms, lots of decorations, courtyard surrounded by a picket fence! Some species even paint the bower walls with berry juice, or charcoal, or chewed vegetation! Bowerbirds are polygynous - one male mates with several females Males perch near the bower and call frequently When a female appears, male hops down to the bower and starts his display - a prancing dance, with synchronized calls, displaying choice decorations in his beak If female approves, she enters the bower to show her willingness to mate They close the curtains, unplug the phone, and put out the cat… When mating is over, female builds her nest far away from the bower – all that prancing and dancing also attracts predators Eggs are timed to hatch at the emergence of the cicadas which will be the baby birds primary food Male will not see the female again until next year Only contribution he makes is his sperm, and the energetic cost of his display Large component of learned behavior in bower construction Like us, they learn by hanging out with older males and females Females have to learn how to recognize a well built bower, good male Juvenile females hang out with the older females as they travel through the forest to scope out the males Bowers are scattered throughout the forest Bower locations are fairly permanent Females know where to go to check out the competition Juvenile males build practice bowers Takes 5-7 years before they can build a bower good enough to attract a mate Adult males will lend a hand, share interior design tips As soon as mating season starts in earnest, adult males trash all the practice bowers Males will also try to trash the bowers of their adult rivals, and steal their best decorations Risky business… Certain ornaments are highly prized - species specific Great bowerbirds prefer grey or white objects - stones, bone, shells, bits of metal Satin bowerbirds prefer blue, esp. blue feathers - but the right shade of blue is relatively rare (two local parrot species) The scarcer the object, the more likely it is to impress the female Diamonds are a girls best friend… Easy to mark feathers and track them from bower to bower High payoff for collecting the most feathers Borgia (1985) - number of blue feathers and scarce snail shells on a bower was highly correlated with a male’s mating success Also found females prefer birds with well-constructed and highly decorated bowers Male success was correlated with the symmetry of the bower, the size and density of the sticks, and overall bower quality How did bower building evolve? The protection hypothesis suggests that predator protection was the driving factor The bower makes the female feel more protected from predator attack while she focuses on the male’s display This seems unlikely because predators are relatively scarce in the bowerbirds’range Also it is doubtful that the few sticks typical of a hypothetical early stage of bower evolution would provide much protection from predators Borgia (1995) now thinks that bowers evolved to protect females from forced copulations, common in ducks etc. She must agree to enter bower to copulate and she can leave at any time Bowers could be equivalent to the sexual contracts humans evolved in the 1990’s! Females prefer males with well-constructed and highly decorated bowers Females prefer males with glossy, healthy plumage and vigorous display behaviors These qualities indicate an older, more experienced and healthy male - a male with good genes for her offspring Are we reading too much into this? Do female bowerbirds really appreciate some bower styles over others? Darwin thought so, and he reminds us that in this respect we are no different from the other animals
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