Animal Behavior/Evolution

Evolutionary Biology

Nothing in biology makes sense except in the light of evolution --Theodosius Dobzhansky (1900-1975)

Biological Species Concept

Scientists have long sought to view the great diversity of organisms as a collection of distinct units. The species, as the atomic unit of diversity, represents a group of interbreeding natural populations that are reproductively isolated from other such groups. When individuals breed offspring, the genes of individuals are shuffled within a common gene pool representing the species' identity. The identity of species is based on the ability to breed, rather than on physical similarity. Limited transfer of genes between species causes different species to take on specific appearances and characteristcis.

Selection

Artificial Selection and Breeding

Consider the work of a professional dog breeder. Lets consider a person trying to obtain a breed that is best suited for hunting of water fowl. The breeder specifically chooses individuals which most closely match the desired traits and selectively breeds them to each other in the hope of obtaining progeny with a combination of useful traits. Such plans can only succeed if:

• phenotypic heritability in morphological and behavioral features is coded by <genes> (e.g. webbed toes, willingness to swim at least partially controlled by genes)
• phenotypic variability is present (some have a particular characteristic, some don't)
• differential survival/reproductive success of phenotypic variants is produced (you pick the ones who do)

Repeated, selective breeding events will alter the proportion of different genes over time. Genes which were present in those individuals that the breeder selected for reproduction will become overrepresented while those that occurred predominantly in discarded individuals will decrease in proportion. Animal breeding is a slow process, however, a combination of strong selection and a high degree of heritability can change the relative abundance of genes in a population up to 10% per generation.

Not all of the selections that we make are done intentionally. Penicillin became widely available during the second world war for the treatment of infected wounds. Just four years after drug companies began mass-producing penicillin in 1943, microbes began appearing that could resist it. With frequent and indiscriminant use of antibiotics we have fostered the emergence of antibiotic resistance in a variety of microorganism. The ability to withstand the effects of an antibiotic occurs when a rare mutation renders a small subset of individuals with lowered sensitivity to the effects of the drug. During the course of antibiotic treatment the wildtype individuals are killed first as intended while the mutants are able to survive a bit longer. If the treatment is stopped before the drug had an opportunity to kill all pathogens regardless of slight differences in sensitivity to it, only resistant individuals will survive and be able to infect new hosts. With each antibiotic treatment that ends prematurely we unintentionally select for those individuals that exhibit a higher ability to tolerate the drug.

Natural Selection

Natural selection is the process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms, while the proportion of less successful traits shrinks. "Favorable" and "successful" are defined in a purely functional sense as environmental conditions allow some individuals to leave more offspring than others. Selection cannot act on the genes per se but rather works on their expression into observable characteristics. Individuals with favorable phenotypes are more likely to survive and reproduce then the genotypes associated with favorable phenotyps will increase in frequency with each generation. Over time, this process can result in the emergence of adaptations that specialize a group of organisms for a particular ecological set of conditions (i.e., microevolution). If different subsets of the population adapt to excell in one of multiple, distinct niches, the emergence of any reproductive barriers between the groups may allow the population's split into separate species (i.e., macroevolution).

More offspring are produced than finite resources can support. Individuals thus can be viewed as in a constant struggle for existence. Individuals within a population are rarely clones, they commonly show variation in phenotypes as well as genotypes. Some of this variation in behavioral phenotypes is heritable. As successful variants are more likely to survive and reproduce, their genotypes will be become overrepresented in the next generation

Density-independent and density-dependent growth models: Exponential Model: a species can potentially increase in numbers according to a geometric series -- Thomas Robert Malthus (1766-1834) Logistic Model: the rate of population increase may be limited, i.e. it may depend on population density -- Pierre Verhulst (1838). Carrying Capacity (K): an environment's maximum persistently supportable load (Catton 1986).

Natural selection is the process by which environmental effects lead to varying degrees of reproductive success among individuals of a population of organisms with different hereditary characters, or traits. The characters that inhibit reproductive success decrease in frequency from generation to generation. It is the process whereby certain genes (alleles) gain greater representation in the following generations compared to other alleles. Adaptations are the complement of traits that increases the fitness of the owner. An individual's Fitness or Reproductive Success is the relative probability that an animal of a particular genotype and phenotype will manage to contribute its genes to the next generation

Aside from Natural Selection, changes in gene frequencies within a population may also arise from a variety of other sources including:

Mutations

changes to individual nucleotide bases along the DNA to largescale rearrangements of chromosomes Template:Animal Behavior stub

Gene flow

Emmigration, Immigration Template:Animal Behavior stub

Genetic drift

Changes in the genetics of the population as a result of chance. Bottlenecks and Founder Effect: Ellis-van Creveld Syndrome is an autosomal recessive skeletal dysplasia that results in short-limbed disproportionate dwarfism. The syndrome most commonly occurs in the Amish population of Lancaster, Pa. The incidence is approximately 5 per 1000 live births and 2 per 1000 living persons. No case had been described in the Amish of Ohio, Indiana, or other Amish areas at the time of an extensive search performed by McKusick, et al. The genealogy of the disorder in the Amish of Lancaster, Pa. can be traced back to the immigrants Samuel King and his wife

• Example: Kettlewell's work on Industrial Melanism of Moths

Sexual Selection

Sexual selection, a subcategory of natural selection, was first recognized by Charles Darwin and "occurs when individuals differ in their ability to compete with others for mates or to attract members of the opposite sex" (Alcock 493). By heavy courtship, fighting, or large territorial possession, males heavily compete for females. Eventhough a male may win a fierce competition for the mate of his choice, it is ultimately the female who decides on a partner that she wants. The female is often successful in her attempts to control reproduction by being choosy and having particular preferences for a male mate.

Females choose mates based on many factors. One important factor is male adornments, or ornaments. For example, Marion Petrie and Tim Halliday concluded that the removal of eyespots from a peacock's tail significantly reduces his attractiveness to females. "After 20 eyespots had been cut from their tails, males averaged two fewer mates in the following breeding season compared with their performance in the previous year" (Alcock 348). Thus, females are extremely conscious of the visual stimuli provided by males. Elaborate sensory cues alert the female that the male is reproductively superior to others. "Male adornments may more readily elicit the mating responses of some females who will thus mate preferentially in favour of the adorned males" (Bateson 53). Other factors involved in female preference in mate selection include body coloration or the "gift" that the male may present to the female before copulation.

Four theories are used to explain mate choice in females. The good parent theory suggests that "choosy individuals select partners on the basis of how well they will care for their offspring" (Alcock 491). This theory focuses on the female's search of a paternal male. The healthy mate theory occurs when females prefer "males healthy enough to produce and maintain elaborate ornaments" (Alcock 491). A good example of this is in female house finches, who choose male mates based on their bright coloration. Bright coloration tells the female that the male is more resistant to pathogens and parasites. The good genes theory "argues that females exhibit mate choice in order to provide their offspring with a partner's genes that will advance their offspring's chances of survival or reproductive success" (Alcock 491). A mother always wants the best for her child, even if it is a future child. Females select mates with certain traits, because they want their children to be healthy, viable, and reproductively successful. "Females that mate with attractive males are compensated for reduced fecundity by bearing "sexy sons" with higher than average mating success" (Andersson 45). The least obvious theory is the runaway selection theory. It states that by being choosy, females "create a positive feedback loop favoring both males with these attributes and females that prefer them" (Alcock 493).

A direct benefit of mate choice by females is the assurance of bearing offspring that will survive well and display high general fitness. This is a heritable benefit. Nongenetic benefits of mate choice include fecundity advantages, food, parental care, or a good territory. All of the benefits, both heritable and nonheritable, ultimately lead to the greater survival of a female's offspring.

Evolution of Diploidy

Most eucaryotic groups exist as diploids (i.e., those carrying 2 sets of genetic information), some occur primarily in haploid forms with a single set of genes, and others alternate between haploid and diploid phases. Different strains of yeast, which may be haploid or diploid, show advantages depending on the specific envrironmental conditions. In a favorable setting, when nutrients are present in excess, the two strains show little difference in reproductive rates. However, when growth was limited by the concentration of a single nutrient, haploid strains of yeast were able to out-compete diploids. This is consistent with the idea that haploid cells are more efficient as simple replicators. The main advantage of diploid cells emerges with a greatly improved resistance to damage. Sexual haploids may combine the advantages of both: spending much of the life cycle in the haploid state, then temporarily fusing to become diploid, followed by splitting to the haploid state.

Diploid organisms appreciate several key advantages:

• A second set of genetic information provides a backup in case of somatic mutations. With two copies of every gene, a diploid organism is able to mask recessive deleterious mutations. DNA damage can be repaired in a diploid state, robinsince there are two copies of the gene in the cell and one copy is presumed to be undamaged.
• A second set provides one copy as a substrate that is free to be altered by mutation, while a second, trusted copy remains available for basic cell functionality
• Increases genetic diversity that may or may not be expressed
• Provides conditions that allow for sexual reproduction.

Sexual Reproduction

As best we can tell, life originated without sex. Although asexual reproduction would arguably be considerably less wasteful than sexual reproduction in many respects, the great majority of species have prospered with reliance on latter. in sexually repoducing organisms half of the population is lost as a reproducing unit (i.e., the cost of males). In addition, both individuals contribute only half of their genetic material to the offspring, while the other half is lost. In addition, there is the danger associated with combining your genome with a largely unknown and untrusted copy from a partner. Several possible reasons have been advanced to explain the presence of sexual reproduction:

• Muller's Ratchet: Mutations accumulate in asexual lineages as there is little chance to get rid of them again.
• Vicar of Bray hypothesis: Ability to combine individually advantageous genes allow for rapid adaptations to change
• Red Queen Hypothesis: By constantly reconfiguring the precise genetic set that a parasite or pathogen will face, animals with longer lifespans can provide a moving target.

Asexual lineages tend to disappear over time while sexual lineages tend to persist much longer.

Evolution of Anisogamy

Disruptive selection in gamete characteristics underlies the evolution of Anisogamy. Anisogamy is the condition in which one type of gamete becomes increasingly large (i.e., the egg) to provide the zygote with storage for a head start in development. As size increases, gametes becomes less mobile. In turn selection produces another type of gamete (i.e., sperm) that is optimized for locomotion to find and fuse with the egg. The larger gamete is termed the female sex, the smaller gamete is that of the male. Compared to isogamy this allows for both increased provisions for zygote and optimized mobility.

Bateman's Principle: increased reproductive investment is accompanied by increased selectiveness for mating partners. While female gametes are energetically expensive, large and limited in number, male gametes are cheap, mobile and available in large numbers. This asymmetry leads to reluctant females as the choosy sex, while ardent males are the competing sex. A.J. Bateman (1948) phrased this rule based on work where he had examined variation in reproductive succcess between male and female fruitflies. He had placed 3 male and 3 female fruitflies into a container and subsequently assessed the reproductive success of all individuals. Most females in contrast had reproduced with little variation, while only a few males had accounted for all reproduction and most males had no success at all. Limits to reproductive success thus differ between sexes where males are generally limited by the number of successful matings, while that of females is primarily determined by the rate of egg production. This favors behavioral traits for choosy females and ardent males. Differential reproductive investment will polarize operational sex ratio (i.e., most females and only a few males get mating opportunities).

Trivers-Willard Hypothesis: Moreover, as only the most competitive males achieve mating success, a bias in sex ratio may emerge where dominant/healthy females produce a relative excess of males which stand a disproportionate chance to develop into the successful male. Female offspring are the safe bet, while males may either be highly suceessful or a spectacular bust.

Evolution of Sex Ratio

Sex ratio describes the relative number of males to females in a population and is most commonly around 50:50. Such a sex ratio might not seem very efficient considering that a male can fertilize several females. Fisher's theoretical work illustrated that such a balance forms a general equilibrium point. If there are fewer males than females, then males face better odds in mating. An advantage for males favors females who produces extra sons. The same argument follows for deviations in the opposite direction. Thus, if a population ever comes to deviate from a ratio of 50:50, natural selection will tend to drive it back to that balance.

• primary sex ratio: ratio of males to females at conception
• operational sex ratio: relative number of receptive males per receptive female in the population

Sexual Selection and Ornamentation in Males

• Species recognition
• Sexual selection: refers to the process by which changes in gene frequencies result from individuals that are better than others at either competing for or at attracting mates -- it is the evolution of traits based on differences in mating success among individuals if (1) some traits increase the ability to compete with individuals of the same sex for access to mates, or (2) some traits increase the ability to attract individuals of the opposite sex. Ornamentation in males, which are commonly the competing sex, may result from different evolutionary forces. Sexual selection is always harsher on the competing sex.

Intrasexual Competition and interference

Dominant individuals often gain preferred access to mates, desirable territory, or other advantages that will enhance the individual's chances for transmitting its genes into future generations.

• Precopulatory: A common form is based on an individual's ability to physically dominate a rival. In situations where groups of mates can be readily monopolized this will likely lead to increasing size in the competing sex. Fur seal females, which rely on a small stratch of beach for giving birth, can be monopolized easily in harems.

Winning in ritualized contests, producing a louder signal, or masking an opponent's call; dominance in social groups; territorial exclusion; alternative mating strategies; sexual interference

• Postcopulatory: mate guarding; anti-aphrodisiacs; mating plugs; partners remain attached e.g. wolves; Bruce effect; Infanticide; sperm competition: e.g. primate or bat mating systems

Intersexual Choice: Why does she consider the trait "attractive"?

• Passive Attraction Theory: Sensory Bias, conspicuous signals make an individual more likely to attract the attention of a mating partner
• Nuptual gifts: spermatophore; territory, protection, resources; courtship indicative of parental investment. e.g., Dung beetle
• Good Genes Hypotheses: general state of health, indicator of viability and quality. Hamilton and Zuk's "Revealing Signal Theory": bright ornaments reveal a genuinely healthy individual in good condition. In conditions where parasites ralter male showiness and parasite resistance is largely inherited, females ought to choose those with bright coloration.
• Zahavi's Strategic Choice Handicap Theory: the presence of a costly trait is indicative of otherwise good underlying genes that allow an individual to prosper despite this handicap
• Fisher's Run-away Selection: This "Sexy Sons Hypothesis" works by alligning the presence of a particular morphological characteristic in males with a preference for it in females
• Genetic Compatibility: MHC locus genes in human mate choice

Every individual alive today, the highest as well as the lowest, is derived in an unbroken line from the first and lowest forms - August Frederick Lopold Weismann, German biologist/geneticist (1834-1914)

Speciation

• Anagenesis
• Cladogenesis

Mating Systems

Several types of Mating systems can be recognized

• Promiscuity
• Monogamy (one male, one female partner)
• Polygyny (one male, multiple female partners)
• Polyandry (one female, multiple male partners)
• Polygynandry (multiple female, multiple male partners; Promiscuity)

Mating systems are highly fluid evolutionry entities. They are dynamic and highly optimized systems where many different factors figure into the equation. Selection will always foster the most successful tradeoffs for these parameters, but many strategies may lead to success. Mating partners are rarely selected at random. In most species it is strongly controlled by diverse factors. Although many individual strategies may prove successful, a variety of considerations provide predictive power for the type of mating system that is present:

Differences in Reproductive Investment

• Parental investment is any behavior toward offspring that increases the chances of the offspring's survival at the cost of the parent's. The presence of a large asymmetry in parental care paired with an ability to monopoluize mates favors conditions for male-male competition and polygynous mating systems where a small number of males account for a disproportionate amount of the breeding. As size matters as a cheat-proof arbitator, selection for increased body size and asymmetries between the sexes become common.
• Resource Distribution: When resources (e.g., nesting sites, food resources) can be dominated by a small subset of males, a polygynous mating system will likely emerge:

Certainty of Paternity

Consider mammals, birds, and fish and phrase your prediction as to the sex that you would be expected to invest more into the offspring.

• external vs. internal fertilization
• control of fertilization site

Social monogamy is relatively rare in mammals but common in birds. Eggs, which develop internally in mammals, restrict the females ability to shift parental duties to the male. Birds, where egg development is external, is more conducive to such efforts. Differences in constraints and costs between sexes in parental care.

Polygyny

• resource-defense polygyny: Polygyny should be more common in patchy environments with variation in territory quality. e.g., honey guides; female fitness depends on quality of resource which is controlled by the male
• female-defence polygyny: e.g., seals; females aggregate at favorable sites and males monopolizes access to female harems
• male-dominance polygyny: e.g., crayfish
• scramble polygyny: e.g., frogs, little overt competition between males

Monogamy

Resource considerations may foster the emergence of monogamous mating systems. When resources are difficult to dominate (e.g., a scattered renewable food source) things get more complicated. Monogamy is a possible outcome which exists either over a lifetime (e.g., greylag geese) or serially (e.g., ducks). Also if females are widely distributed, males may not be able to monopolize them. As females may mate with another male, monogamous males may play a guarding roling towards the female. The male may derive some fitness benefits in situations where his parental contributions increased surviva; of his offspring. Under such conditions, female choice will encourage shifting part of the burden of parental care to the male.

Polyandry

Polyandry is defined as “the mating of one female with more than one male while each male mates with only one female.” Exclusive polyandry (as opposed to polyandry in concert with polygyny) is very rare, occurring in only about 1% of animal populations, most being shorebirds like the sandpiper. The basis of polyandry is a sex role reversal. The females compete for the males and are larger and more colorful, while the males take on the parental role. With the sex role reversal, a natural selection against older males evolves. This is accomplished by the females tending to select the males with the best sperm in order to give the female the most offspring possible. Younger males will more likely have fertile sperm; therefore impregnating the female on more instances than an older male with less fertile sperm. When multiple helpers are needed for successful reproduction, females specialize in egg production while pairing with multiple males in polyandry (e.g., Jacanas).

Types

There are two major types of polyandry: simultaneous and sequential. Simultaneous polyandry is when the female controls a very large territory. Inside this territory, the female has multiple smaller nesting territories with different males. The female mates with all males simultaneously, keeping control of the smaller territories. Another form of simultaneous polyandry, cooperative simultaneous polyandry, is when the female only has one nesting area where she mates with multiple males producing a clutch of eggs of mixed parentage with all males contributing to the eggs. Sequential polyandry, the most common form, is where the female mates and produces a clutch of eggs with one male, then leaves the male to incubate and rear the eggs while moving on to another male in a different nesting territory. Here, the female moves from one male to another, leaving the male in full responsibility of the eggs instead of sharing the responsibility.

Benefits

There are many genetic benefits of polyandry. The first being fertility insurance. This hypothesis suggests that by mating with multiple males, the female is guaranteed to fertilize all of her eggs. The multiple partners potentially make up for one male that may not be able to fertilize the eggs. The good genes hypothesis states that the females have multiple mates because she is in search of the male that will pass along the best genes to her offspring. By finding this male, the female is increasing the survival rate of her offspring. The genetic compatibility hypothesis is one that suggests that the female finds multiple mates in order to find the most compatible genetic match for her eggs. While looking for a good match, she is also eliminating the males that are least compatible with her eggs.

The material benefits of polyandry can be seen through three hypotheses. The more resource hypothesis suggests that the more mates the female has, the more males she has to care for her clutch. The better protection hypothesis states that by having multiple partners, the female is better protected from predators. The infanticide reduction hypothesis is one that claims that since the female has multiple males, she has a lower infanticide rate because the males do not know which progeny belong to them. This prevents the males from killing other male’s young. Although some infanticide occurs between females and other female’s eggs, it is minimal among males.

Resources

• University of Kentucky Research
• Stanford Alumni Research
• University of Michigan Research

Promiscuity

Alternatively, in situations where social groups are able to form, females may mate with any male member of the group leading to Promiscuity (Polygynandry). Sperm competition will in this case most likely determine paternal identity (e.g., elephants, chimpanzees). In situations where food is scattered and limited, both parents need to provision the young.

Examples

Mimicry

A major concern of animals and other critters is to protect themselves from predators in order to survive and reproduce and pass their genes off to a new generation. Many animals have evolved adaptations known as antipredator devices such as camouflage and chemical toxins. Animals use camouflage to blend in with their environments in an attempt to be unrecognizable by predators. Other organisms such as the monarch butterfly contain chemical toxins that are secreted into the predator’s mouth when it attempts to eat the butterfly. The monarch butterfly also has warning coloration that gives a warning sign to predators to remind them that the butterfly is toxic and should not be eaten.

These antipredator devices are so successful that other organisms have been known to mimic them. The organism that is mimicked is known as the model and the third party that is deceived by the model and its mimic is known as the receiver. The mimics have learned to take advantage of the color patterns and markings that predators have learned by experience to avoid. The model is usually a species that has an abundant population and has successfully warded off predators with an antipredator device.

Organisms have learned to mimic their surroundings or environment in an attempt to “hide” from predators. For example, lizards have learned to mimic tree trunk color which proves to be very successful as predators will simply move past them as they believe that they are simply looking at a tree. Another example of this type of mimicry can be seen with the Katydid who will mimic a leaf in both color and shape in an attempt to be hidden.

Some prey animals have evolved certain patterns on their bodies that mimic other animals in an attempt to startle their predators. The most common example of this type of mimicry can be found in some moths and butterflies who flash eye spots on their wings to predators. These eye spots startle the predator who believes that the eyes belong to a much larger animal that may be a threat to them.

In one form of mimicry known as aggressive mimicry, an organism will mimic a signal that is either deceptive or attractive to its prey. One example of this involves the praying mantis who will mimic flowers to attract insects that they can then capture and eat. Organisms can also imitate the behaviors of other organisms. Moth caterpillars, for example, will imitate the motion and body movements of a snake in order to scare off predators that are usually a prey item for snakes.

One of the most popular types of mimicry involves the warning coloration found on inedible or toxic organisms such as the monarch butterfly. Once these toxic organisms have adapted this warning coloration which warns predators to stay away, other organisms may start to mimic this warning coloration in an attempt to stay alive. Batesian mimics are those mimics that imitate unpalatable species even though they are palatable. Therefore, one species is harmful while the other is harmless. The wasp is a great example of Batesian mimicry. The wasp is the model species in this example as it possesses a sting which enables it to escape from predators. The bright warning coloration of the wasp has been mimicked by many other insects. Even though the mimics are harmless, the predator will avoid them due to bad experiences with wasps with the same coloration. With Mullerian mimicry, many unpalatable species share a similar color pattern. Mullerian mimicry proves to be successful as the predator only has to be exposed to one of the species in order to learn to stay away from all the other species with the same warning color patterns. The black and yellow striped bodies of social wasps, solitary digger wasps, and caterpillars of the cinnabar moths warn predators that the organism is inedible. This is a great example of Mullerian mimicry as all of these unpalatable, unrelated species have a shared color pattern that keeps predators away.

Mimicry is a very successful antipredator device that species have evolved over many generations. As one can see, organisms have come to mimic many different characteristics such as color patterns and behaviors. However, selection only favors the mimics when they are less common than the model. Therefore, the fitness of mimics is “negatively frequency-dependent.”

Industrial Melanism of Peppered Moths

Biston betularia f. typica, the white-bodied peppered moth.

Within one hundred years (1850 to 1950); the dotted whitish form of the peppered moth (Biston betularia) was almost entirely replaced by the melanic (black) form. The melanic form appeared to be best suited for survival against the soot that had collected on forest tree trunks from pollution because the moths could camouflage with their resting area on the tree. The dotted whitish form of the peppered moth was no longer predominant in this environment because they were easily detected and predated on. How does this happen? Many animals have anti-predator adaptations. Adaptations are defined as “a heritable trait that either spread because of natural selection and has been maintained by selection to the present or is currently spreading relative to alternative traits because of natural selection (Alcock, 2001).” Anti-predator adaptations suggest that a heritable trait, which enables the organism to hide from predators by seeking cover against a background, has spread by natural selection because of reproductive success. H.B. D. Kettlewell’s experiments on the peppered moths, as well as, those conducted by R.J. Howlett and M.E.N. Majerus have proven that the peppered moth’s preference for their resting places on trees are anti-predator adaptations.

In 1955, H.B. D. Kettlewell published his study on pepper moths: Selection Experiments on Industrial Melanism in the Lepidoptera. Kettlewell hypothesized that the dotted whitish form of the peppered moth were more likely to be eaten than the melanic form because they could be easily detected against the soot covered trees. His studies showed that the moths that were easily identified by humans were at a higher risk of predation from birds. The dotted whitish form was at higher risk of predation than the melanic form in the polluted environment.

Howlett and Majerus further examined this hypothesis in their study: The Understanding of Industrial Melanism in the Peppered Moth, published in 1987. They tested it by pinning 50 of both forms of the peppered moths on pale and dark tree trunks.

Calculations: Moths in Polluted (dark) Woodland

Dotted Whitish Form: (30/50) x 100 = 60% Melanic Form: (20/50) x 100 = 40%

60% - 40% = 20% The Dotted Whitish form is predated on 20% more than the melanic form in the polluted woodland.

Moths in Non-polluted (pale) Woodland

Dotted Whitish Form: (15/50) x 100 = 30% Melanic Form: approx. (30/50) x 100 = 60%

60% - 30% = 30% The Melanic form is predated on 30% more than the dotted whitish form in the nonpolluted woodland.

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Their studies showed that the dotted whittish form was preyed on 20% more than the melanic form in the polluted woodland. The melanic form is predated on 30% more than the dotted whitish form in the nonpolluted woodland.

In addition to the difference in predation, their studies also showed that in both the polluted and nonpolluted environments the moths that were located on the limb joints instead of the trunks were less likely to be preyed on.

Howlett and Majerus showed that the dotted whitish form was at a higher risk of predation when resting against the polluted (dark) trees. The melanic form had a greater chance of survival and reproduction because they were less likely to be detected against the soot-covered trees, the same findings of Kettlewell years before. Their experiment proved that the melanic form was an anti-predator adaptation, which is why the dotted whitish form had become so rare. The melanic form had become the dominant trait for survival and reproduction in the polluted woodland.

Antipredator Adaptations used by the Monarch Butterfly

The relationship between pray and predators continually changes. Prey need to find ways to outsmart predators in order to survive and reproduce. One way monarch butterflies increase their fitness is by forming huge groups of up to ten million. As their group size increases, the probability that any one monarch butterfly will be captured decreases. Moreover, the butterflies located in the center of a large group are more likely to survive than those on the outside (Pike, 1999). This is known as the “dilution effect (Alcock, 2001).”

Monarch butterflies that choose to migrate to closed-area overwintering sites are less likely to be attacked by a predator. Also, by reacting as a group to the movement of a predator, monarch butterflies are better able to scare away predators (Pike, 1999). This “mass startle effect” is thought to stun the predators and provide time for the butterfly to escape (true shock and awe indeed).

The aforementioned are collective ways in which the butterfly behaves in order to elude a predator. There are, however, certain individual inherent features that the monarch butterflies possess that increase their probability of avoiding a predator.

Monarch butterflies contain chemicals that are toxic to many predators. Evidently, this makes other similar but harmless species envious. In Batesian mimicry, a palatable species attempts to mimic an unpalatable species in an attempt to increase its own fitness. The monarch butterfly species is one that some Batesian mimics model themselves after perhaps because the monarch butterflies are so successful at avoiding predators (Mappes, 1997). The Batesian mimics, although they are not harmful to predators, experience increased fitness because they model a potentially harmful species such as the monarch butterfly. The viceroy butterfly (Limenitis archippus) is one such butterfly that models itself after the monarch (Pike, 1999).

Although monarch butterflies do not gain an increase in fitness as models of Batesian mimics, they do benefit from Mullerian mimicry. In Mullerian mimicry, two unrelated, toxic species converge on a similar morphology. If more than one unpalatable species has a similar morphological trait, then the predators may more easily recognize a Mullerian mimic as potentially harmful. This saves both the mimics and the predators time and energy.

Lastly, monarch butterflies display aposematism or warning coloration. This warning coloration is meant to be very conspicuous (Alcock, 2001). Monarch butterflies make themselves conspicuous by having bright orange areas on its wings. Predators quickly learn that prey containing these bright colors are potentially harmful. For example, when a blue jay consumes a monarch butterfly, it vomits shortly after. From that point on, the blue jay associates features of the monarch butterfly, such as its bright colors, as unpalatable (Alcock, 2001). Because of their morphological features, Batesian mimics, Mullerian mimics, and many other aposematic species all gain from the monarch’s unpalatability.

References

• Mappes J. & Alatalo R.V. 1997. Batesian mimicry and signal accuracy. Evolution 51: 2051-2053.
• Pike, K. Antipredator Adaptations by Monarch Butterflies. Entomology at Colorado State University, Posted in 1999. Colorado State University. Accessed December 6, 2004.

References

• Alcock, J. 2001. Animal Behavior: An Evolutionary Approach (7 ed.) Sinauer Associates Inc., Massachusetts. pp. 341
• Andersson, M. 1994. Costs of Mate Choice; Direct Benefits of Mate Choice. In Sexual Selection. (pp. 45) Princeton, New Jersey: Princeton University Press.
• Bateman, A.J. 1948. Intra-sexual selection in Drosophila. Heredity 2: 349-368.
• Bateson, P. 1983. Sexual Selection by Female Choice by Peter O'Donald. in Mate Choice. (pp.53). Cambridge: Cambridge University Press.