Comparative Psychology Research Paper

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Regardless of their area of expertise, scientists who approach the study of behavior in terms of adaptive function, evolutionary history, and developmental mechanisms can be considered comparative psychologists. Comparative psychology tries to bridge the gap between psychology’s focus on behavior and biology’s focus on evolution. The idea that evolution often starts with a functional change, such as a novel foraging behavior or mating strategy, and morphological changes then ensue has appealed to biologists since the 19th century. The preeminence of function over form is found, for example, in Lamarck’s famous example of giraffes stretching their neck to reach higher foliage, thus resulting in the transmission of longer necks to their offspring. Although modern evolutionary theorists have largely abandoned this Lamarckian mechanism, known as the inheritance of acquired traits, the initiating role of function in evolution has remained central. This principle can be illustrated by a similarly famous example first used by Darwin: the beak of the Galapagos finches. The relation between population changes in beak shape and size and the availability of seeds has become a primary example of the action of Darwin’s natural selection mechanism in wild populations. Thus, the foraging behavior of finches plays a determinant role in the direction of evolutionary change in beak morphology.

Early Greek and Roman philosophers advocated evolutionary-like views, including some resembling the modern notion of natural selection. For the most part, however, ancient views of nature were not clearly evolutionary, as far as is known today. Aristotle, for example, proposed that humans are the acme of the hierarchy of life because of their possession of rationality. The notion that humans stand apart from the rest of the natural world has persisted for cultural and religious reasons throughout the centuries.

After the European Renaissance, naturalistic views of behavior regained strength. Among these views were the influential mechanistic theories of animal behavior proposed by Gomez Pereira and René Descartes. These theories focused on the notion of reflex, a simple unit of behavior that requires an external stimulus, some processing at the level of the central nervous system, and a motor response output. This simple idea would eventually become the basis for early developments in the neurophysiology of behavior and learning during the second half of the 19th century. Simultaneously, Darwin’s theory of evolution by natural selection provided a sound basis for the notion of continuity across species, both physical and behavioral. Darwin himself emphasized that rudimentary mental processes should be found in nonhuman animals, leading to the notion of mental continuity and opening the door to an exploration of the evolutionary roots of psychological processes. These are the main elements of the historical context in which comparative psychology emerged as a discipline.

Clifford L. Morgan is credited with writing the first general Introduction to Comparative Psychology in 1894, in which he proposed his influential canon based on the law of parsimony. In criticism of questionable anecdotal evidence for mental continuity, Morgan wrote, “In no case may we interpret an action as the outcome of the exercise of a higher psychical faculty, if it can be interpreted as the outcome of the exercise of one which stands lower in the psychological scale.” This statement suggests that of several equally effective accounts of some behavioral effects, the one based on the simplest psychological process should be selected. For example, a behavioral phenomenon should not be attributed to an emotional response when it can be completely explained in terms of simpler reflexes. Morgan’s canon is aimed at preventing experimenter bias and anthropomorphic interpretations of behavioral data.

During the 20th century, a movement within the biological sciences gave rise to ethology, a discipline closely related to comparative psychology. Traditionally, ethologists focused on “innate” rather than acquired behaviors and examined behavior in naturalistic or seminaturalistic settings, rather than under the controlled conditions of the laboratory. These approaches tended to converge during the second half of the 20th century. Multidisciplinary integration is the main contemporary feature of comparative psychology. The goal of this research-paper is to provide an overview of the methods and ideas that are likely to drive comparative psychology in the 21st century.

Theory

Evolutionary biologists traditionally focus on cladistic relations between species to determine the phylogenetic history of specific traits. For psychologists, it is difficult to determine the origin of behavior because common ancestors are extinct and the fossil record does not preserve behavioral features, only some morphological characters. This means that comparative psychologists must examine evolutionary trends in terms of the behavior of living species. Evolutionary theory provides two major concepts to organize such research: clade and grade.

A clade is a set of species sharing a common ancestor. For example, chimps, gorillas, orangutans, and other apes are classified in the family Pongidae. The unique characters of this particular clade reflect evolutionary novelties and provide an opportunity to study how such novelties originate and spread across species. Alternatively, species from different clades may evolve common traits independently; theorists refer to these traits as grades, and they transcend phylo-genetic relations. Encephalization in birds and mammals provides an example of the concept of grade. Both birds and mammals exhibit brain sizes that are typically about 10 or so times larger than the brain size known for reptiles. Although birds and mammals evolved from different reptilian groups, they both achieved a significant degree of encephalization independently. Therefore, birds and mammals share a grade in terms of relative brain size, even though they are classified as distantly related in taxonomic terms.

The complementary nature of these concepts can be illustrated with the example of aquatic mammals, such as cetaceans (whales, dolphins) and sirenians (manatees). Genetic evidence has revealed that the closest living relatives of the cetaceans are the artiodactyls (even-toed ungulates), so much so that some taxonomists have combined the two into a single clade: order Cetartiodactyla. Included in this clade are some purely land-dwelling species (cows, deer), some that are entirely aquatic (dolphins), and some land dwellers that spend a good portion of their lives in and around water (hippopotami). Presumably, cetacean ancestors resembled the partially aquatic hippopotamus as part of the transition to purely aquatic life from their land-dwelling ancestors. The same is probably true of the sirenians, whose closest extant relatives are the proboscideans (elephants). Any land-dwelling lineage adapting to an aquatic environment is likely to experience similar transitional stages. In this regard, cetaceans and sirenians share a grade of adaptation to aquatic life. Even though the species are distantly related and evolved under different ecological conditions, within-grade comparisons may be useful to understand the evolutionary trends required for such transitions independently of the details of phylogenetic history.

Evolutionary trends common between species often involve adaptations that allow the animal to specialize less (i.e., greater behavioral plasticity) and therefore exploit more resources in its environment. The fact that these traits make a species less specialized is likely to be the reason for their generality across species. Learning ability, for example, is an adaptation that dramatically increases plasticity, allowing the organism to adjust to environmental changes and predict events based on prior experience. Learning mechanisms tend to be highly conserved across many species. In contrast, adaptive changes in behavior that increase specialization are commonly achieved through adaptations in contextual variables (e.g., motor function, motivation, sensory processing). One hypothesis to explain these observations is that changes in a contextual variable can be more selective for the specific adaptation because they require the modification of only one or a few systems. In contrast, changes in learning mechanisms have the potential to change multiple behavioral systems, with some of those changes being maladaptive. In either case, by looking at grades of learning in (cladistically) primitive species, comparative psychologists are likely to discover how learning evolved in derived species. Comparisons across both grades and clades are vital to the study of comparative psychology.

Evolutionary Principles

Evolutionary principles guide comparative psychologists in the study of behavior. Among these principles, evolution by natural selection has provided an important theoretical guidance for behavioral studies. Three conditions must be met for evolution by natural selection to occur:

Phenotypic variability within a population
Heritability (phenotypic variability must be, at least in part, heritable)
Differential reproductive success of alternative characters.

Natural selection can occur in several ways. Selective pressures can favor individuals with a given phenotype (e.g., long neck) over individuals with the alternative phenotype (e.g., short neck). Across generations, the population becomes dominated by individuals with the favored phenotype because the underlying genes spread more successfully in the population, while the alternative phenotype becomes less common. Cases in which selective pressures favor an extreme version of a trait are known as directional selection. Alternatively, individuals with longer necks and shorter necks may be able to exploit different resources, whereas those with intermediate features may suffer from greater competition. Cases in which both extreme versions of a trait hold a selective advantage are known as disruptive selection. Traits at the extremes of the distribution may also be selected against, favoring moderate expressions, a case known as stabilizing selection.

Selection can favor behavioral phenotypes in the same way it favors morphological characters. For example, artificial selection experiments with the fruit fly Drosophila melanogaster have shown that animals can be selected based upon their tendency to move to the top (negative geotaxis) or to the bottom (positive geotaxis) of a 16-unit T-maze. Such disruptive selective breeding produces strains of animals that express these extreme phenotypes. In naturalistic settings, researchers have observed selection in different species to work in similar ways to produce different behaviors, such as variations in courtship and mating behaviors, nest building, aggression, and other traits.

Presumably, there can be situations in which behavioral or morphological plasticity is favored by selective pressures. Selection for plasticity and learning ability is called the Baldwin effect. In the early years of the 20th century, James M. Baldwin proposed that an initial period of increased plasticity is required for abilities to arise. Once these abilities are established, they may become “instinctive” in subsequent generations because plasticity is no longer an important selected outcome. This loss of plasticity is called genetic assimilation. Genetic assimilation involves the evolution (via natural selection) of characters that were triggered by environmental pressures acting on a plastic phenotype in the ancestors, but that are organized internally in the descendants, independently of the presence of an environmental trigger. In this way, phenotypic plasticity can be an important factor driving evolutionary change.

For individual organisms, the ability to pass on their traits through reproduction is what defines their fitness. One way of determining an individual’s fitness is to observe how many of its total offspring, across its lifetime, survive to adulthood. This is known as lifetime reproductive success (LRS). Because these data are usually difficult to collect, especially in naturalistic settings, researchers sometimes use less comprehensive methods to estimate LRS. For example, looking at reproductive success over one or several breeding seasons, the number of successful copulations, defense of territory, or other similar indicators can provide estimates of LRS.

Comparative Approaches

Comparisons across extant species, though necessary, are not without difficulties. Comparative research may involve direct assessments of behavior in different species, or alternatively, extensive analysis of behavioral processes in a single species at a time. Although relatively few researchers dare to tackle the problems of developing long-term research programs involving two or more species, many model their research with a single species in such terms that comparisons are encouraged, even if only across laboratories. Thus, comparative research is actually more common than it may appear at first sight because most (but not all) research with nonhuman animals in psychology is ultimately concerned with human applications.

Comparative psychologists have followed a common trend shared with those who contribute to evolutionary biology in a more general sense. Evolutionary theories of behavior can adopt two complementary views, one based on adaptive function and the other based on phylogenetic history. The adaptive functional approach has several characteristics:

It stresses the relation between behavior and ecology.
It studies closely related species with contrasting ecology.
It aims at determining the contribution of behavior to reproductive success.
It deals with micro-evolutionary divergence in behavior.

Researchers have implemented the adaptive functional approach in a variety of areas such as in the study of flavor aversion learning in rodents (see Chapter 35). One study tested the hypothesis that rapid flavor-aversion learning is an adaptive trait for species that exhibit a generalized diet, but is less advantageous for species that specialize in a narrow set of foods. This specialist-generalist hypothesis was tested by inducing flavor-aversion learning in several species of rodents from the genus Dipodomys (kangaroo rats). Although D. merriami consumes grains, seeds, and other plant materials, D. microps specializes almost exclusively in the leaves of chenopod plants. As predicted, D. merriami (the generalist) acquired flavor aversions faster than did D. microps (the specialist). Researchers attributed the difference to a greater aversion to novel foods in the generalist than in the specialist, a phenomenon known as neophobia.

The adaptive significance approach is appropriate for characters that exhibit rapid evolutionary change. The morphological analogy would be the variations in beak size and shape exhibited by Galapagos finches. Other biological characters are more stable in evolutionary terms, thus this approach would not help determine how they evolved. For example, if a researcher is interested in studying the evolution of the avian feather, then looking at the Galapagos finches may not be particularly illuminating because this character is likely to be stable in all these species. A comparison of peripheral structures in birds (feathers), dinosaurs (some of which exhibit feather-like structures), and extant reptiles (scales) may be more illuminating. In such a comparison, the emphasis is on the evolutionary (or phylogenetic) history of this particular character. When applied to behavior, this approach has the following characteristics:

It stresses the relation between behavior and phylogenetic history.
It studies distantly related species.
It aims at determining homology, homoplasy, and divergence in mechanisms.
It deals with macroevolutionary trends in behavior.

Most comparative research in psychology involves a phylogenetic approach. For example, during the past few decades, there have been numerous attempts at demonstrating that nonhuman animals ranging from apes and rats to parrots and pigeons have the potential to exhibit higher cognitive functions, including abstract concept formation, self-awareness, and language. Traditional comparative studies have also emphasized the generality of basic learning processes by studying Pavlovian and instrumental conditioning in simple organisms, such as the marine slug Aplysia californica, the nematode Caenorhabditis elegans, or the honeybee Apis mellifera. Systematic research with a variety of vertebrate species has also shown evidence of divergence in basic reinforcement processes, as shown by the study of a phenomenon known as successive negative contrast (SNC). In SNC, the experimenter trains a group of animals with a large incentive that is eventually downshifted to a small incentive. The behavior of these animals deteriorates beyond the level of a control group always given access to the small incentive. SNC and related effects occur in several mammalian species, including primitive marsupials like the opossum, to rats, pigs, monkeys, apes, and humans. However, analogous experiments with bony fish, amphibians, reptiles, and birds have failed to yield evidence of SNC. What these experiments indicate is that nonmammalian vertebrates perceive the downshift in incentive magnitude, but show no sharp deterioration in behavior. Researchers have speculated that SNC depends on mechanisms of emotional arousal (e.g., frustration) that are present only in adult mammals.

Methodological Issues

Comparative studies face complex methodological problems that can be illustrated with the following experiment. Imagine that a researcher wants to establish a direct comparison between the rates of learning in rats and pigeons. The researcher matches training parameters as much as possible by administering practice in the same apparatus, under the same trial-distribution conditions, with the same type and amount of food, using adult animals, and so on. If the species differ in acquisition rates, is it fair to conclude that this demonstrates species differences in learning processes?

The short answer is “no.” The reason for this negative answer lies in the impossibility to demonstrate that objectively equal conditions have the same impact on the behavior of both species. For example, consider the amount of food given as the incentive during training trials, although the same logic applies to all other aspects of the training situation. The same amount may be more rewarding to the rat than to the pigeon (or vice versa). Thus, differences in performance across species may reflect the differential effects of motivational, perceptual, or motor processes, rather than learning mechanisms per se. These factors are called contextual variables. Because of species differences in contextual variables and the impossibility of equating them, researchers have turned away from direct comparisons across species.

The alternative methodology involves an emphasis on the comparative study of functional relations between variables that can affect learning and behavioral outcomes. Thus, a fruitful approach would be to ask whether relatively larger reinforcers yield faster learning in both rats and pigeons, because absolute comparisons can be greatly affected by contextual variables rather than by learning mechanisms. When different species show different behavioral outcomes under analogous variations in conditions of training, the question arises as to whether the mechanisms of learning have diverged. As in other types of research, comparative psychologists studying behavioral plasticity, whether in the context of learning, cognition, or specific behavioral phenomena (e.g., play behavior, courtship, parental care, etc.), frame their findings in terms of the theoretical concepts provided by evolutionary principles, including homology, homoplasy, divergence, adaptation, and so on.

Research in comparative psychology often aims at identifying the mechanisms underlying specific psychological effects. This approach implies the need for a clear definition of “mechanism,” a concept that has acquired different meanings for different researchers. The need to characterize this concept is also highlighted by comparative research aimed at determining whether analogous behavioral outcomes are homologous or homoplasic across species. Homology implies inheritance from a common ancestor, whereas homoplasy refers to the convergence of behavioral outcomes, despite independent evolution, because of common ecological pressures.

A useful method for determining whether species share homology of behavioral mechanisms is to study the phenomenon at different levels of analysis. Behavior can be understood at at least four levels:

Psychological level (e.g., associations, fear, timing clock)
Neurobiological level (e.g., neural networks in specific brain sites)
Neurochemical level (e.g., neurotransmitter systems)
Cell-molecular level (e.g., second-messenger systems, gene expression).

Lower levels are more basic, and therefore tend to be more general across species and behaviors. As the hierarchy of levels progresses, the focus becomes more and more specific to a particular behavioral phenomenon. Across species, the same behavioral output may be achieved from the same mechanisms (homology) or different mechanisms (homoplasy) at any of these levels. For example, a comparison of a common behavioral outcome in two species that yields evidence of similar mechanisms at all four levels supports the hypothesis that the underlying mechanisms are homologous. If, however, species differ in terms of mechanisms at some or all levels, but the behavioral outcomes are similar, this result is consistent with the homoplasy of underlying mechanisms.

Recent research in several areas shows a third alternative. For example, mollusks, insects, and rodents have a very remote common ancestor, an animal that lived more than 600 million years ago and was no more complex in neural terms than a planarian. Despite the independent evolution of the organization of the central nervous system, these animals share some basic cell-molecular mechanisms of learning. In general, when cellular and genetic processes are homologous, but the higher-level character has evolved independently, biologists invoke the concept of parallel evolution. In parallel evolution, the sharing of common building blocks increases the likelihood that independently evolved traits will have similar morphology or function.

Applications

The main contribution of comparative psychology relates to a better understanding of the processes that lead to the evolution and development of animal behavior. Traditionally, comparative studies have been treated as basic research carried out mainly for the purpose of increasing human understanding of nature. Although researchers have focused on behavior, these studies have had implications for an understanding of other problems, including brain evolution, human evolution, economic theory, neural networks, and others.

Inevitably, however, basic research leads to applications aimed at solving specific problems of human relevance. Some of the areas to which comparative psychologists have contributed include the following, in no special order:

Development of psychoactive drugs
Animal models for neurological disorders
Improving animal production
Environmental enrichment for zoo animals
Development of psychotherapeutic techniques applied to mental disorders
Animal training for law enforcement and the TV and film industries
Improving survival and reproductive success of endangered species
Determining the behavioral consequences of pollution in natural populations
Restricting the interaction between wild predators and cattle

Summary

Comparative psychology is among the oldest branches of scientific psychology. Well over one century of behavioral research has led to an impressive improvement in an understanding of the evolutionary and developmental basis of behavior. A century ago educated people had little or no understanding of the perceptual capacity of animals, their learning and memory skills, their ability to make and use tools, their higher cognitive powers, and the extent to which their behavior was an expression of neural activity, to name but a few categories in which significant progress has been made. An intended consequence of these studies has been the assumption that they will provide a better framework to understand human nature. By appreciating the psychological skills of nonhuman species, comparative psychologists have come a long way in their contribution to identifying the characteristics that humans share with other species, as well as those that may be uniquely human—the natural product of a long evolutionary history.

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