Antipredator Behaviour Flashcards
Avoiding Predators
interactions between predators and their prey are so critical for understanding the process of natural selection. There are two basic types of antipredator behaviors: those that help prey avoid detection by predators, and those that function once a prey encounters a predator.
Examples of defense compounds used against predators:
In 2009, for example, Karen Osborn and her colleagues discovered seven new species of annelid and wondered how these species, living in a world with almost no light, defend themselves from the dangers around them worms. Five of the seven species that Osborn and colleagues discovered produce small bioluminescent sacs (globules)—what the researchers
termed bombs—that they likely release when encountering predators. These bombs light up for a few seconds after the worms release them and may startle a predator long enough to allow the worm to escape the danger.
Larval Zygaena filipendulae (Hymenoptera) secrete a viscous compound through segmentally arranged cavities on their cuticle when they encounter a
potential predator (Pentzold et al., 2016). These droplets, composed of a mélange of proteases, protease inhibitors, oxidases, and other chemicals, essentially glue together the appendages of predators,
sometimes killing them.
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If prey can avoid being detected by their predators, they decrease not only the probability of being captured and eaten but also the costs associated with fleeing or fighting back (Brilot et al., 2012; Ruxton et al., 2004; Stanford, 2002). We will examine three ways
that animals can avoid their predators: (1) blending into the environment, (2) being quiet, and (3) choosing safe habitats.
Avoiding Predators
ways that animals avoid detection by predators:
Blending into the Environment
One way for animals to avoid predators is through cryptic matching to the environment (blending into environment), making detection by predators less likely.
Francis Sumner, for example, found that populations of the mouse Peromyscus polionotus had fur coloration that matched the background of the beaches (or inlands) on which they lived (F. B. Sumner, 1929a,b). Mice were not behaving in any particular way that increased their crypsis; “beach mice” displayed light coat pigmentation that better matched the sand on which they lived, and coat pigmentation became darker in populations farther from the beach and closer to the inland areas (Figure 12.4). Subsequent work has found that a single mutation in the melanocortin-1 receptor (Mc1r) plays a role in this example of crypsis (Hoekstra, 2006).
Cephalopods—which include octopuses, squids, and cuttlefish—are especially adept at changing color quickly to blend into their background, decreasing their
chances of being attacked by a predator (Hanlon and Messenger, 1988; Packard, 1972). While most work on camouflage from predators (fur seals, bottlenose dolphins, and some species of fish; Hanlon et al., 2007, 2013; Buresch et al., 2015) has been conducted during the day or at sunset, Roger Hanlon and his team have found that predation on cephalopods also occurs at night, as many predators of cephalopods have sharp night vision.
The researchers found that cuttlefish were camouflaged in seventy-one of eighty-three (86 percent) nocturnal observations, which is likely an underestimate, as some cuttlefish may have
matched the environment so well that the underwater camera employed simply missed them. Hanlon found that cuttlefish could match their background in one of three ways, and they could change their color and pattern to match their background in a matter of
seconds.
- the first kind of crypsis involved a “uniform” camouflage pattern in which cuttlefish takes on a single skin color—a color that matched their background. Although this form of camouflage was rare, when it occurred it often involved a cuttlefish mimicking the rocks around them.
- the second kind of crypsis involved “mottled camouflage” patterns, where cuttlefish changed their appearance. The size and color of the splotches often mimicked the cuttlefish’s background.
- the third is a “disruptive” camouflage pattern, with large light and dark areas that enable it to blend in with
the background.
All of these different forms of camouflage techniques strongly suggest that for large, soft-bodied creatures such as the giant cuttlefish, blending into the environment is an important antipredator behavior (Hanlon and Messenger, 1988).
Avoiding Predators
Being Quiet: an acoustic crypsis
When predators use sounds made by their prey to detect them one thing that an animal can do to escape danger is to be quiet (M. Ryan, 1985).
Luke Remage-Healey and his colleagues examined the
role of sound suppression in the antipredator repertoire of the Gulf toadfish, as well as whether they can distinguish between dolphin whistles (social, not hunting) and clicks/pops which are used when hunting (Opsanus beta; Remage-Healey et al., 2006). Gulf toadfish are preyed on by adult bottlenose dolphins (Tursiops truncatus), and make u to 13% of their diet (Barros, 1993). Dolphins orient toward the “boat-whistle” sound produced by male toadfish during the breeding season.
After experimentally exposing the male toadfish to snapping shrimp pops, dolphin whistles alone, dolphin pops alone, and dolphin whistles and pops, they drew blood from the males and measured their cortisol levels. Males exposed to pops not only responded to the pops by reducing their own boat-whistle calls, but they also showed higher levels of cortisol than males exposed to the sound of snapping shrimp.
Avoiding Predators
Choosing Safe Habitats
Another way that prey can avoid predators is by living in habitats that are relatively predator free
From a phylogenetic perspective, there are bird taxa that contain both species that nest in tree cavities (TC nesters) and species that nest in other sorts of cavities (OC “other cavity” nesters).
Donald Brightsmith addressed these questions in parrot species from both Australia and Amazonia, where many populations are at risk from manmade (anthropogenic) factors (Olah et al., 2016).
He then mapped the nesting data onto the phylogenies to examine ancestral and derived nesting behaviors. This analysis suggests that the ancestral state was tree cavity nesting, and that nesting in other cavities had evolved independently many times in both Australian
and Amazonian parrot species.
Brightsmith next examined what selective forces, if any, were responsible for the evolutionary shift from tree cavity nesting to OC nesting.
With respect to the Amazonian parrots, Brightsmith estimates that almost all of the species that rely on cavities other than trees arose in the late Oligocene–early Miocene geological period, 20 to 30 million years ago (Miyaki et al., 1998). South American mammal
communities were undergoing a large change at that time, with rapid increases in the number of nest predators, including both tree rats and primates from Africa (Poirier et al., 1994). These nest predators
may have been responsible, in part, for the Amazonian parrots’ evolutionary shift away from tree cavity nesting.
Avoiding Predators
Conservation Connection: co-evolution, naive prey and introduction programs
Conservation biologists sometimes use translocation programs—moving individuals from one natural habitat to another—to protect threatened or endangered species (Ewen et al., 2012, 2014; Germano et al., 2014).
Researchers involved in such translocations are starting to take into account that the species they are moving originally evolved in an environment with a particular set of predators, and that co-evolution may
have been occurring between these predators and prey.
Co-evolution occurs when changes to traits in species 1 lead to changes to traits in species 2, which in turn feed back to affect traits in species 1, and so on.
Predator-prey co-evolution can lead to an evolutionary arms race between predator and prey: The prey evolve behaviors that help them protect themselves against the predators, and the predators evolve detection systems that help them find prey. This co-evolutionary dynamic is important to understand for translocation programs, as such programs may inadvertently introduce the translocated species to predators with
which they have had no evolutionary history and to which they are especially susceptible.
To understand how exposure to new predators in a new environment affects prey, Isabel Barrio and her colleagues studied the antipredator behavior of the wild rabbit (Oryctolagus cunuculus), which was introduced into Australia by European settlers (Barrio et al., 2010). Rabbits use odor cues to detect many of their predators. In Australia, rabbits are preyed upon by foxes, cats, and ferrets, which have also been introduced there. But rabbits and these other species have a long-shared evolutionary history, primarily on the Iberian peninsula (Jaksic and Soriguer, 1981).
Wild rabbits are also preyed upon by predators that are native to Australia, such as the spotted-tail quoll (Dasyurus maculatus; Glen and Dickman, 2006).
When Barrio and her colleagues exposed the wild rabbits in Australia to the odor of foxes, cats, and ferrets, the rabbits responded by reducing their use of the area with the predator odor. No such response occurred when the rabbits were exposed to the odor of the spotted-tail quoll, with whom they shared no evolutionary history (Figure 12.9). The rabbits’ usual
first line of defense against predators, odor detection, was ineffective for this new predator, leaving the rabbits vulnerable to the quolls.
This work suggests that to maximize success rates, introduction and relocation program managers need to consider whether the species they are trying to protect shares an evolutionary history with the predators in
the new environment. The species may not possess evolved antipredator adaptations in either a specific or a general sense. In some cases, translocated species may be at serious risk because they have not evolved antipredator behaviors to a specific predator in their new environment, as in the case of the wild rabbits in Australia (though, fortunately, some recent work suggests that rabbits are starting to display an evolved recognition of the odors of two quoll species; Tortosa et al., 2015).
In other cases, the translocated species may not have evolved antipredator behaviors to certain types of predators in their new environment (for example, ambush hunters, predators that detect prey by
odor, and others). Detailed knowledge of these sorts of issues can help conservation biologists and managers design better programs.
What Prey Do When They Encounter Predators
Here we shall examine five behaviors that prey use once they encounter a predator: (1) fleeing, (2) approaching a predator to obtain information, (3) feigning death, (4) signaling to the predator, and (5) fighting back.
how animals respond when they encounter predators
Work by David Smith and his colleagues on the proximate effect of predator odor on mice has shown that the frontal cortex area of the brain regulates the
effect of stressors on behavior in rodents and humans, and that this area of the brain may alter neurological and endocrinological responses to stressors such as predators (Amat et al., 2005; Drevets, 2000; Osuch et al., 2000; D. G. Smith et al., 2006; Spencer
et al., 2005).
To examine this in more detail, Smith and his team
exposed mice to two different stressors. One group of mice were exposed to the odor of a predator. A second group of mice was exposed to a physical stress—these individuals were immobilized in a device with the bizarre name of a Universal Mouse Restrainer
(UMR). A third group of control animals was exposed to neither predator odor nor the UMR.
Smith and his group found that both predator odor
and the UMR increased the circulation of the neurotransmitters acetylcholine, serotonin, and dopamine within the frontal cortex, but that the increase was greater in response to predator odor.
Smith and his team did find that when chlordiazepoxide, a drug that reduces anxiety in humans, was administered to mice before exposure to the odor of predators, the increases in acetylcholine, serotonin, and dopamine described above disappeared. This finding suggests that predators may indeed cause anxiety in nonhumans.
What Prey Do When They Encounter Predators
Fleeing
The most common response of prey that have spotted a predator is to flee for safety (Blumstein, 2003; Camp et al., 2012; Cooper and Blumstein, 2104; Lima, 1998; Lima and Dill, 1990; Stankowich and Blumstein, 2005; Ydenberg and Dill, 1986)
A Meta-Analysis of Flight Initiation Behavior:
Animal behaviorists have measured flight initiation distance—how close a predator can approach before prey flee—in many species. Stankowich and Blumstein
gathered published data from sixty-one studies of flight initiation in mammals, fish, birds, and reptiles.
Factors they found inhibiting flight initiation behaviour:
- When animals that were far from a refuge
(their territory, for example) so, distance to safety is crucial.
- animals involved in foraging, mating, or
fighting were slower to flee from predators
- predator’s size, speed and the directness of its approach affected the prey’s perception of risk.
- Morphological traits of the prey affected its
behavioral decision of when to flee from a predator.
- The prey’s ability to camouflage itself affected its flight decision as well.
- the quality of the habitat and the physical condition of the prey (how hungry it was, its size and age, whether it
was pregnant, and whether it was defending young offspring).
- experience with predators and learning
all in all, the meta-analysis found that many variables,
including the degree of crypsis, the distance to a refuge, and the prey’s experience with a predator influenced decisions about flight initiation distance.
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Schooling fish tend to flee from predators en masse (though there are exceptions), and so an understanding of schooling behavior can shed light on fleeing behavior. found that variation in schooling and variation in the body armor of sticklebacks were linked to a region of chromosome.
Using transgenetic tools, the promoter for a gene hypothesized to be associated with schooling behavior in sticklebacks living in an open water habitat was
inserted into embryonic sticklebacks from the benthic population. When they matured, these individuals were then mated to wild fish from the benthic population and the schooling behavior of their offspring was recorded.
They discovered that offspring from benthic population parents with the inserted promoter schooled in a manner more similar to pelagic than wild-type benthic fish.
The results mean that transgenic tools may speed the rate at which animal behaviorists can map genotype-phenotype associations.
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Though we think of the antipredator options available to embryos as limited, some research suggests embryos have evolved adaptations to flee from predators. Karen Warkentin’s studied red-eyed treefrogs how embryos have adapted ways on fleeing from predators.
Red-eyed treefrogs attach their eggs to the vegetation that hangs over water. When eggs hatch, the tadpoles that emerge drop down into their aquatic habitat. Both the terrestrial habitat of the egg/embryo and the aquatic habitat of the tadpole have a set of predators. But the predators are different in each of the habitats. If
terrestrial predation from snakes and wasps is weak, eggs hatch late in the season (Warkentin, 1995, 1999; Figure 12.15).
This serves two functions: First, it lengthens the time that the eggs/embryos are in a low-predator terrestrial habitat, and second, such late hatching allows the embryos to grow to a size that lowers the levels of fish
predation once the eggs finally hatch and the tadpoles fall into the water.
Warkentin predicted that treefrog eggs would hatch sooner if predation in the terrestrial environment increased, because natural selection should favor embryos that avoid terrestrial predators when such predators are at high frequencies (compared with the aquatic predators that feed on treefrogs; Warkentin, 2000).
When predation from snakes and wasps is high, it might increase survival rates to mature early and drop into the water, away from heavy terrestrial predation.
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Clare Fitzgibbon studied Thomson’s gazelles in the Serengeti National Park, Tanzania (Fitzgibbon, 1994). the flight distance is initiated differently for the different predators:
- for cheetahs its a short 101-330 yards (Walther, 1969)
- for lions its longer 101-500 yards (more stamina, hunt in groups)
- much further for wild dogs 501-1000 yards (have hunting groups and exceptional stamina)
What Prey Do When They Encounter Predators
Cognitive Connection: Heritability of Conditioned Fear Responses
For natural selection to act on a cognitive trait associated with antipredator behavior, there must be variation, fitness correlates, and heritability associated with the cognitive trait in question (Croston et al., 2015; Rowe and Healy, 2014; Thorton et al., 2014). In this box we will focus on the small, but growing, work on the heritability of cognitive traits associated with conditioned fear responses (Croston et al., 2015).
In rodents, and in mammals in general, conditioned fear responses include increased heart rate, defecation, and freezing behavior. In rodents (and mammals in general), there is evidence to suggest that one area of the brain that plays a key role in the conditioned fear response is the amygdala. Activity in this area of the brain increases as fear conditioning occurs, and experimental stimulation of the amygdala produces many of the same behaviors seen
during conditioned fear responses. In addition, damage to the amygdala causes a reduction in the behaviors typically produced by conditioned fear responses (Applegate et al., 1983; Blanchard and Blanchard, 1969, 1972; Cohen, 1975; Quirk et al., 1995; Rogan et al., 1997).
If natural selection has shaped conditioned fear responses (mediated by the amygdala), then this suite of behaviors should be heritable. While little work has been done on this in natural populations, laboratory
experiments, primarily using rodents, have found evidence for the heritability of conditioned fear responses (Figure 12.14) (Shumake et al.,
2014). In addition, many artificial selection experiments on fear responses, including conditioned fear responses, have found strong responses to the
selection pressure applied, suggesting an underlying heritability to these antipredator behaviors.
More work on the heritability of fear, particularly in natural populations, combined with work on variation and the fitness consequences of fear, will help us better understand this most basic of antipredator behaviors.
What Prey Do When They Encounter Predators
Approaching Predators
Animals sometimes approach predators when they encounter them. This may allow prey to gather important information about putative predators, and reduce their chances of mortality.
Clare Fitzgibbon studied costs and benefits of approach behavior in natural populations of Thomson’s gazelles in the Serengeti National Park, Tanzania (Fitzgibbon, 1994). Approach cheetah and lion (hunt by surprise and speed), but not wild dogs (hunt by stamina).
Fitzgibbon examined three, non–mutually exclusive benefits of approaching a predator. Approach behavior might
1. decrease the current risk of predation.
In particular, she found that cheetahs responded to gazelle inspection behavior, which is most common and most pronounced in large gazelle groups, by moving farther between rest periods and between hunting periods. This could cause cheetahs to leave an area sooner than normal as a result of gazelle approach behavior, leading to decreased rates of mortality among potential prey.
2. allow gazelles to gather information about a potential threat.
3. serve to warn other group members of the potential danger associated with predators.
What Prey Do When They Encounter Predators
Feigning Death
Faking, or feigning, death is an antipredator behavior seen in many species. Death feigning occurs in insects when, in response to a predator, an insect falls and then remains frozen, absolutely still. This is sometimes referred to as tonic immobility (Miyatake et al., 2004; Ruxton et al., 2004).
Tatsunori Ohno and Takahisa Miyatake have studied death feigning in the adzuki bean beetle (Callosobruchus chinensis; Ohno and Miyatake, 2007). When a beetle is on a branch and a predator
approaches, beetles can either fly away or feign death, but they cannot do both at the same time. Ohno and Miyatake hypothesized that those beetles that feigned death for a long period of time would be poor flyers, and those beetles that feigned death for shorter time periods would be especially good flyers.
The researchers established two behavioral assays—one for feigning death and one for flying ability. For the former, they exposed beetles to danger and measured how long the beetles remained frozen, feigning death.
Eight generations of artificial selection on death feigning produced dramatic differences between the selected lines. Individuals in the long-duration line showed death-feigning times that were about forty times as long as those in the short-duration line. In
addition, Ohno and Miyatake found a negative genetic correlation between death feigning and flying abilities. Individuals in the long duration line were very poor flyers, and, conversely, individuals in the short-duration line were adept flyers.
Ohno and Miyatake ran a second artificial selection experiment. They found that beetles in populations selected for long bouts of death feigning had higher
brain concentrations of dopamine than beetles from populations selected for short bouts of death feigning (Nakayama et al., 2012).
What Prey Do When They Encounter Predators
Signaling to Predators
Prey sometimes transmit information to a predator to deter an attack, warning the predator of the dangers of contact, or that it has been sighted and may not succeed in capturing a prey. These signals are often visual but can also be auditory.
Warning Coloration in Monarch Butterflies:
During their caterpillar stage, monarch butterflies (Danaus plexippus) ingest milkweed plants, which contain cardiac glycosides. These chemicals are toxic to birds, but do not harm the monarchs, indeed,
there is growing evidence they aid monarch butterfly resistance to infection: Lefevre et al., 2012; Gowler et al., 2015. These chemicals are sequestered and stored by the butterflies in their own tissue. If a bird predator eats a monarch, the toxins in the monarch make the
predator violently ill (Figure 12.23). From that point forward, the color patterns of monarchs act as warning coloration for that predator that now avoids feeding on monarchs: birds learn to associate monarch color with illness.
How could natural selection act on prey to produce the sort of warning coloration that we see in monarchs? If a predator must eat a monarch to learn how dangerous monarchs are, how could selection ever favor the monarch ingesting milkweeds and possessing a
distinctive color pattern? How could natural selection favor a trait in which the individual in possession of the trait must die for the predator to learn about the danger? There are a number of ways that this could occur. R. A. Fisher suggested that if prey live in groups,
such warning coloration could preferentially aid genetic relatives and so be favored by natural selection (R. A. Fisher, 1930). But the most likely explanation for the evolution of this sort of coloration is that the predator does not always kill the monarch before it senses the toxin, as touching the monarch may be enough to alert the predator to the presence of the toxin. The presence of the toxin and warning coloration may save the life even of a prey that is the victim of a predator’s first encounter with a monarch.
Tail Flagging as a Signal:
Signals can serve to warn a predator that it has been spotted. When the predator is an ambush hunter that relies on surprise, such a signal often causes it to move on and leave the area. Even when predators aren’t strictly ambush hunters, prey may still benefit by
signaling predators if signals reduce the probability of capture. Consider tail flagging in ungulates, where individuals “flag” their tails after a predator has been sighted (Figure 12.24). Such flagging occurs as part of a sequence of antipredator behaviors, and often it
involves an individual lifting its tail and “flashing” a conspicuous white rump patch. Flagging often, but not always, occurs when a predator is at a relatively safe distance from its potential prey (Hirth and McCullough, 1977).
Tim Caro’s work on white-tailed deer, as well as the work of other researchers, provides some evidence of pursuit deterrence (Bildstein, 1983; Caro, 1994b; Caro et al., 2004; Woodland et al., 1980). Caro found that white-tailed deer that run fast flag their tails and are using this signal to communicate to the predator that an attack is unlikely to succeed because the fleeing
deer will escape a pursuing predator. This sort of pursuit-deterrence signal is not only found in white-tailed deer nor is it restricted to tail flagging. For example, in a phylogenetic study involving 200 species
and seventeen antipredator behaviors, Caro and his colleagues found that, in ungulates, snorting also serves as a signal to predators (Caro et al., 2004). This signal deters attack, perhaps because snorting indicates the health and vigor of the signaler.
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alarm calls
predator knows prey are aware; also alerts nearby group members. seen in Impalas
the more vocal they are, the more likely there is a predator around.
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stotting
e.g. in Thomson’s gazelle, also other antepoles: springbok.
a jumping motion with all four legs straight.
signals good condition, likely to be a poor choice of individual for a predator to choose.
Fitzgibbon and Fanshawe (1988)
What Prey Do When They Encounter Predators
Fighting Back
Chemical Defense in Beetles:
Thomas Eisner and his colleagues have studied how bombardier beetles use chemical weapons to defend themselves against predators, including swarming ants, orb-weaving spiders, and toads (Dean, 1980a,b; Eisner and Dean, 1976). In the bombardier beetle, Stenaptinus insignis, individuals blast potential predators with a
highly noxious spray (Eisner and Aneshansley, 1982, 1999; Eisner et al., 2000, 2006).
A beetle can discharge its acidic spray twenty times before depleting its supply of chemicals. The heat produced by this chemical reaction causes an audible pop, and the spray shoots out at a temperature of 100°C. The beetles themselves are not injured by their own noxious sprays, but predators are.
S. insignis does more than just release an acidic spray when a predator attacks. Using high-speed photography, Eisner and Aneshansley have shown that the beetles selectively aim this spray at predators. When they are attacked from the front, they fire the
spray forward; when attacked from the rear, they fire the spray backward (Eisner and Aneshansley, 1999; Figure 12.26).
How did such a complicated antipredator mechanism evolve? Researchers have found a clue in the spray mechanism of Metrius contractus, which is the oldest of all extant species of bombardier beetles (Eisner et al., 2000).
M. contractus emits its spray in a unique manner. When it is attacked from the rear, it produces a froth secretion, which builds up on the body of the beetle and wards off predators. Although it is not clear why, these mechanisms of discharging chemical weapons appear to lower the temperature of the disseminating chemicals from 100°C to approximately 55°C. This work on M. contractus hints that spraying an extremely hot chemical secretion may be a derived trait, but that
frothing and using the forewing tracks to disseminate a somewhat reduced heat spray represents something similar to the ancestral version of chemical defense in bombardiers.
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Social Learning and Mobbing in Blackbirds:
Blackbirds (Turdus merula) sometimes mob their predators (Altmann, 1956; Sordahl, 1990). Once a flock of blackbirds spots a predator, they join together, fly toward the danger, and aggressively attempt to chase it away. Such group attacks often work well enough to force predators to leave the blackbirds’ area.
Eberhard Curio and his colleagues examined whether young, predator-naive blackbirds learn what constitutes a predator by watching which species is mobbed and classifying such a species as predators (Curio et al., 1978).
In each trial of Curio’s experiment, he and his team had a “model” and a “naive” bird, each in its own aviary.
Curio’s team found that, once naive blackbirds had seen a model apparently mobbing a friarbird, the naive blackbirds themselves were much more likely to mob this odd new creature than if they had not been exposed to the model: information about what constitutes a danger was transmitted culturally. The researchers next asked whether the (now not so naive) blackbird subject would act as a model for a new naive blackbird. And if that worked, how many times could they get a former naive blackbird to successfully act as a model?
The longer cultural transmission chains are, the more
powerful cultural transmission may be in spreading antipredator behaviors through a population. Though their sample was small, they found that the blackbird cultural transmission chain can be six birds long, and then the chain of information transfer was broken.
Predation and Foraging Trade-Offs
When animals spend time engaged in antipredator activity, they could potentially be doing something else—foraging, mating, resting, playing, and so forth (Lima and Dill, 1990; Figure 12.27). Or, rather than totally curtailing alternative behavior, antipredator behavior
could create pressure to perform other behaviors in a different manner—for example, to forage in the vicinity of a refuge, to mate at times when predation is minimal, and so on. In either case, trade-offs between some other behavior and antipredator tactics are often
common.
Predation pressure affects virtually every aspect of foraging— including when a forager begins feeding (Clarke, 1983; Lima, 1988a,b), when it resumes feeding after an interruption (De Laet, 1985; Hegner, 1985), where it feeds (Dill, 1983; Ekman and Askenmo, 1984; Lima, 1985; Schneider, 1984), what it eats (Dill and
Fraser, 1984; Hay and Fuller, 1981; Lima and Valone, 1986), and how it handles its prey (Krebs, 1980; Valone and Lima, 1987).
Consider Steven Lima and Thomas Valone’s work on predation and foraging in the gray squirrel (Sciurus carolinensis; Lima and Valone, 1986; Figure 12.28). Early work by Lima had demonstrated that squirrels alter their foraging choices as a result of predation pressure
from redtailed hawks (Buteo jamaicensis; Lima, 1985).
Squirrels that could either eat their food items where they found them or carry the food to cover were more likely to carry items to an area of safe cover, particularly as the distance to safe cover decreased. The closer the refuge from predation, the more likely squirrels were to
use such a shelter when foraging; when it was a quick run to reach safety, squirrels generally chose to do so.
Squirrels were also much more likely to carry larger (rather than smaller) items to safe areas before continuing to forage.
Lima and Valone followed up the above study with one in which they presented squirrels with two types of food—a large chunk of cookie (associated with long handling times) or a small chunk of cookie (associated with short handling times; see chapter 11). Cookie chunks, rather than nuts, were used to avoid the confounding variable of food storage, as nuts are often buried, but cookies are always eaten. A combination of large and small items was placed either close (8 m) to an area of cover or farther from safety (16 m).
In order to make sense of Lima and Valone’s results, we need two critical pieces of information. First, the profitability of small food items was greater than the profitability of large food items. In the absence of predation, we would expect that squirrels would always take any small food item that they encountered. Second, the total handling time associated with larger food items was great enough that optimal foraging models predicted that larger items should be brought to cover, where it is safe, before being eaten, particularly when the distance to cover was not great. This was not the case for smaller items. Lima and Valone hypothesized that, if faced with predation, squirrels might sometimes pass up the smaller, more profitable food items and continue to search for larger morsels to bring back to cover. This, in fact, is what the squirrels did. Smaller items were rejected in favor of larger items that were brought back to cover.
Anti-predator benefits of living in groups
group living disadavantages:
- increased conspicuousness (more likely to be detected)
- increased susceptibility to disease
- feeding and resource competition
- cannibalism and infanticide
- problems of locating own offspring
believe the costs are outweighed by the benefits
- reducing ones individual risk of predation as a result of living in a group.
seen in Guppies, Poecilia Reticulata in Trinidad
predation risk drives the tendency to live in groups.
The 5 D’s:
-detection
“Many eyes” hypothesis can potentially spot an incoming predator. Study done by Christopher Boland on emu’s which was to test how quickly a predator is spotted in groups of different sizes, but the disadvantage of working on these free living emu’s is that Boland could have spent all day watching the emu’s waiting for a predator to arrive. so his solution was that he would become the predator, he would run at a steady pace towards the emu’s and look to see how long it took the group to detect his attack.
he found that as the group size increases, there is an increase in the mean number of individuals who are likely to be vigilant at any one time in the group. the benefit of this is that there is a decrease in the detection time. Pulliam suggests a double benefit of grouping; not only do group-living animals respond more quickly to the “predator” but they can also spend more time doing other things, especially feeding.
-dilution
an individuals chance of predation is a function of group size (other things being equal). the larger the group, the less likely it is that you are going to end up being the prey item. Reproductive synchrony; dilution in time.
the selfish herd effect; individual risk of predation also be reduced by positioning within larger groups.
-defence
active predator defence - sufficient individuals acting en masse may be sufficient to deter a predator. e.g. seagull colony nesting site defence, mobbing by prey birds of predatory birds.
-D’ confusion Detection
complex and random movement can disrupt predators attention on individual targets, leading to less chance of being preyed on
