Animal Behavior: _{Causes and approaches to studying behavior}

History of Animal Behavior Research

The formal study of animal behavior began with the publications of 19th-century naturalists, such as Charles Darwin’s book The Expression of the Emotions in Man and Animals. Darwin was ahead of his time in proposing that emotions are found in species other than humans. Darwin’s theories were based on his observations of animals ranging from roosters to primates. He also linked body gestures with emotions, such as the raised lip of a coyote exposing its teeth, signaling it’s ready to fight but also sending a warning (Darwin 1872).

During the 19th century, scientists focused on studying animal behavior as explainable through stimuli and responses in laboratory settings, also known as “conditioning.” For example, Russian physiologist Ivan Pavlov’s famous experiments demonstrated that one could condition a dog to associate sounds with food, evidenced by the dog salivating. The emerging field of experimental psychology worked to understand the behavior of animals as sets of reflexes originating in the cerebral cortex and conditioned by their environments (Clark 2004).

While experimental psychology brought rigor to the study of animal behavior, it lacked the context of an animal’s natural surroundings. Controlled laboratory conditions minimize and simplify the environmental inputs into behavior, making some experimental questions easier to ask. But by stripping away most of the “environmental inputs” dictating how an animal behaves, the behaviors observed in the laboratory may be oversimplified or abnormal.

However, the scientific community did not formalize the study of behavior in natural settings until the 1930s. Dutchman Nikolaas Tinbergen observed that some behaviors are present at birth, such as Herring Gull chicks (Larus argentatus) pecking a red spot on a parent’s bill to beg for regurgitated food. Austrian Konrad Lorenz observed how Greylag Geese (Anser anser) and other birds follow the first moving object they see during a critical period of learning, a behavior known as “imprinting.” (Burkhardt 2005). Sometimes, a chicken hatched in captivity (such as on a farm) will imprint on a person and become inseparable.

The first practitioners of a new discipline called “ethology” were thinking about animal behavior in the broader context of evolution and wrestling with a key question: What are the underlying causes of animal behaviors?

Causes of Animal Behavior

Why are these goslings following their mother (Figure 2)?

What approaches could we take in answering why goslings follow the mother (Figure 2)? Tinbergen outlined four significant perspectives to explaining animal behavior: causation, development, function, and evolutionary history.

“Tinbergen’s Four” (applied to the goslings’ behavior):

• Causation (mechanism) – The goslings’ mother began walking and calling to them, activating hormones in their brains that turn on the following behavior, which is executed through the skeletal-muscular system in the form of walking.
• Development – When the goslings first opened their eyes, they imprinted on the first living organism they saw (typically the mother bird); the goslings’ brains and bodies matured to a developmental stage that, coupled with practice, imbued the goslings with the ability to follow her.
• Function (adaptation) – The goslings that closely follow the mother are less vulnerable to predators, thus have better survival outcomes. They are more likely to survive to reproductive age and pass on genes responsible for the attachment behavior. Conversely, those who don’t follow, or follow poorly, are more at risk and less likely to pass on genes.
• Evolutionary History – These geese are from a long line of ancestry in which parents watched closely over their hatchlings. At some point in the past, the parent-following behavior emerged. Because of the survival benefits it brought, the behavior was favored by repeated rounds of natural selection until it became a universal aspect of goose behavior.

When scientists study animal behavior, they look to distinguish the “proximate” (or immediate) triggers from the “ultimate” (longer term) evolutionary underpinnings of the behavior. Proximate triggers include the sound of the mother goose that stimulates her goslings to follow. Ultimate evolutionary roots of the behavior include genetics that confer survival advantages. There are also “intermediate” factors to consider, such as the developmental stage of the goslings, that are relevant to understanding the behavior.

Approaches to Studying Animal Behavior

Because behavior can be examined from various perspectives, the methods for studying behavior are also varied. Many disciplines including ethology, experimental psychology, animal physiology, and natural history, to name a few, attempt to understand animal behavior. Let’s look at how those disciplines, collectively, help us understand the causes of animal behavior, beginning with an analogy: How would you describe what makes the car in Figure 3 drive its route?

The most proximate explanation would be the car’s engine. You could describe how it works with batteries, pistons, belts, and other parts to move the car’s wheels using gasoline and/or electricity. Or how the driver turns the steering wheel to direct the car’s path. But that is not a complete answer to the question. There are also bigger picture (ultimate) explanations:

• The person driving the car, which reflects the hiring process of the food delivery service;
• Who ordered the food at what time, which shapes the route the car takes;
• How the road system was designed to allow cars to move along certain paths.

Similarly, animal behavior can be understood from various perspectives ranging from proximate mechanisms to the ultimate drivers. For example, consider how birds communicate through song, which has striking parallels to human communication.

Causation

Behavior results from stimulus-response mechanisms controlled by the endocrine system (hormones), the nervous system (nerves), and their interactions. When an animal experiences a stimulus, such as a sound, it’s nerve receptors (in this case in the ears) send messages from nerve to nerve like a telephone network, while its hormones send chemical messages through the bloodstream. The nerves may stimulate a hormonal response or vice versa, with the brain mediating. This “conversation” causes behavior.

Bengalese finches (Lonchura striata domestica) are a domesticated species that has proved useful for research on bird communication. Japanese biologist Tomoko G. Fujii studied the neural basis for these finches responding to sounds from other Bengalese finches. By sedating the birds, Fujii and colleagues could watch which parts of the finches’ brains responded to recorded birdsongs. The researchers found that nerve cells were especially activated in part of the amygdala, a structure known to play a role in social-reproductive behavior in many animals, including humans (Fujii et al. 2016).

But birdsong also depends on how these nerve signals get translated into changes in the vocal organs. Brazilian biologist Amanda Monte used fine dissection and three-dimensional imaging to examine how the vocal organs of a South American species, the Black Jacobin hummingbird (Florisuga fusca). Credited with the highest pitched song of any bird, Black Jacobins sing a series of notes outside of human hearing range. By mapping cartilage, bones, and muscles, Monte identified a suite of muscles controlling their complex songs (Monte et al. 2020).

As young animals grow and develop, their behaviors develop as well. Just as young children begin to walk and talk, young birds begin to fly and sing songs specific to their species. A bird species can be identified by its song, such as through the Merlin app (Cornell Lab of Ornithology). But how do birds develop their species-specific songs?

"Two great truths of population zoology: (1) no two individuals are alike, and (2) both environment and genetic endowment make a contribution to nearly every trait.”

- German evolutionary biologist Ernst W. Mayr

In the 1950s, British ethologist William Homan Thorpe found that Common Chaffinches (Fringilla coelebs) raised with recorded adult songs sing normal songs as adults. Chaffinches raised together, but without hearing the songs of adult males, sing complex, but abnormal adult songs. And chaffinches raised alone in a lab setting sing much simpler songs as adults (Thorpe 1958). Thorpe’s discoveries suggested that birdsong resulted from both innate (inherent or inborn) and environmental influences.

Since Thorpe’s discoveries, decades of research since have confirmed that birds have an innate ability to learn the songs of their species; however, they will only learn the proper songs by being exposed to them. Further, that exposure must happen during a critical period of their development. Most bird species learn their songs during the first year of life and practice the sounds much as human babies babble before they learn to speak. So, birds learn to sing the songs they hear while growing up.

But that’s not the whole explanation. Research by Canadian biologist Ian Thomas showed that young male Savannah Sparrows (Passerculus sandwichensis) learn more songs than they need. As adults, the sparrows reduce their song repertoire to whichever is most like their neighbors’ songs. Having a similar song may help reduce aggression among neighbors defending their territories (Thomas et al. 2021). It’s like human neighbors putting the same yard sign up to demonstrate solidarity.

Still, understanding how internal structures (like vocal cords) and external influences (like neighbors’ songs) shape birdsong does not fully explain it. The immediate function of an animal’s behavior may require a broader perspective.

For example, listen to this recording: Northern Mockingbird Songs. Do you hear familiar sounds? Northern Mockingbirds (Mimus polyglottos) mimic sounds–other birds, dog barks, car alarms, and more. Australian behavioral ecologist Anastasia H. Dalziell, sought explanations for how their mimicry affects recipients; she and colleagues proposed that vocal mimicry may serve multiple functions, including deterring predators (Dalziell et al. 2014).

Whether it’s birdsong or the myriad of other behaviors exhibit by animals, we can understand the function of a particular behavior through the longer lens of adaptation in their environmental contexts.

Function (Adaptation)

Adaptations are adjustments over time in the fit between organisms and their environments as a result of natural selection (see our Adaptation module). Early theories based on economic approaches assumed that animals were adapted to always optimize their survival and reproductive success (fitness). Scientists developed models to characterize the behaviors they observed and come up with laws to apply across species (see our Modeling in Scientific Research module).

“Inasmuch as food preferences may be at least partially controlled by genetic factors, one would assume that natural selection has favored those genotypes which predispose their owners to favor the ‘right’ foods, that is, those that yield the most in net energy and nutrients per time.”

- American zoologist J. Merritt Emlen, 1968

In the 1970s, British zoologists John R. Krebs and Nicholas B. Davies developed the “Optimal Foraging Model.” The model described how animals foraged or searched for food, something all animals must do to survive (Krebs and Davies 1978). Based on economic principles, the model assumes that animal foraging behavior is “optimized.” In other words, an animal searches for food in a way that procures the most nutrition with the least amount of energy expended. For humans, that would be like buying an inexpensive meal at the restaurant next door.

Clearly, we do not always choose our food in a way that could be called “optimal foraging,” nor do other animals. Canadian behavioral ecologists helped refine Optimal Foraging Theory by pointing out how the risk of predation plays a role in animal foraging (Lima and Dill 1990). Looking for food must also involve tradeoffs between getting the food and avoiding getting killed, fighting off competitors, and other environmental risks.

Nature provides numerous examples of animals behaving in ways to balance their food needs against risk. In studying how Amazon orb-weaving spiders (Hingstepeira folisecens) hunt for prey, Brazilian community ecologist Kátia F. Rito found that the spiders strategically use shelters made of rolled leaves. If prey get trapped in their web near the shelter, the spiders pull on the threads without leaving the shelter, an apparent adaptation to stay concealed from predators. If prey are trapped further away, the spiders change their strategy to run out and attack (Rito at al. 2016).

As environmental conditions change, so do risks, which is reflected in animal behavior. Svalbard reindeer (Rangifer tarandus platyrhynchus) on islands of Norway typically graze on the lush Arctic tundra summer cover of grasses, herbes, sedges, and shrubs. However, changing climate has resulted in thick layers of ice overing the tundra. Norwegian biologist Brage Bremset Hansen and colleagues have studied how the reindeer respond to the shifted availability of forage. By analyzing their feces, the researchers found that they resorted to eating seaweed they could find on the shorelines (Hansen et al. 2019).

Animal feeding behavior is the outcome of a tradeoff between obtaining nutrients as predicted by Optimal Foraging Theory, and various other needs related to survival and reproduction. These tradeoffs and outcomes continue to shape research today.

Evolutionary History

Understanding animal behavior also requires looking beyond the present to the ultimate causes, or evolutionary underpinnings. Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce (see our Adaptation module). Many behaviors have genetic base, from aggression in fruit flies to political behaviors in humans. As a result, they are subject to inheritance and natural selection like physical traits.

Let’s consider bird migration.

Imagine taking a ten-hour walk without food and water, stopping to get a meal,and then doing it again, day after day. That’s what Snow Geese (Anser caerulescens) do. Every year, they make the journey from the eastern United States to as far as Greenland with occasional stops to rest and forage for food. Then they fly back again. And they repeat it the next year. It is a massive expenditure of energy and time. So, why do birds migrate?

A proximate explanation is that birds migrate to spend the winter in warmer conditions and the summer at breeding grounds with suitable temperatures and food. Migration allows animals to take advantage of seasonally available resources. For example, Snow Geese (Figure 4) spend the winter feeding intensively in warmer places with lush vegetation like coastal Pennsylvania. For breeding, Snow Geese traves to the Arctic where they nest in the tundra on ridges with a view of approaching predators.

However, looking at the ultimate causes of bird migration from an evolutionary perspective provides additional insights.

During the Pleistocene (from about 2.6 millions years ago to 11,700 years ago), glaciers came and went in response to climate change. The alternating glacial and interglacial periods caused contractions and shifts in bird ranges. Fossil records of bird bones can help scientists reconstruct bird migrations in the past. Models which assume that a bird species’ needs (or niche, see our Animal Ecology module) remain constant are also used to estimate how birds were distributed in the past.

For example, Italian biologist Lisa Carrera and colleagues modeled how past climate changes affected the distribution of six bird species (Carrera et al. 2022). Carrera and her colleagues chose birds ranging from species that never migrate to species that migrate long distances, wintering in sub-Saharan Africa, such as the Common Quail (Corturnix corturnix). Below (Figure 5) is their comparison of model-based estimations of the Common Quail’s current winter range with its winter range during the Last Glacial Maximum (LGM, 9–26,500 years ago).

Consider Figure 5. What differences do you notice?

From the models (Figure 5), the researchers noticed a reduction in the Common Quail’s breeding range during the LGM, especially along the map’s northern edge. The distribution of Common Quali fossils from that period and region supported the researchers’ model predictions. The study also found that cold-adapted species expanded their ranges during the LGM, while warm-adapted species found climate refuge in Mediterranean Europe.

So, individual birds and bird populations can shift their migratory behavior over time. As climate changes, the question is whether birds can keep up with the rate of change (see our Factors that Control Earth’s Temperature module). Some birds are already shifting their migration timing (showing behavioral flexibility).

But migratory behavior likely results from the interaction of an individual's genetic makeup with its physical and social environment. The Eurasian Blackcap (Sylcia atricapilla) typically migrates from Europe to Africa and the Mediterranean for the winter. However, some individuals stay behind to winter in Western Europe. In experiments, biologists artificially selected for the individuals that did not migrate. Within six generations, this produced a new, non-migratory population (Berthold et al. 1992). The shifts in migration today may be an example of such microevolution (evolution in a small group over a short period).

Even in bird populations that do not typically migrate, the genetic basis for migration remains (at a low level). It’s the raw material that natural selection might one day use so that the birds begin migrating again when conditions demand it (see our Adaptation module). As the earth’s climate changes, a key question is whether migratory birds can rapidly evolve changes in their innate migratory programs. The temperature changes weathered by bird populations during the glaciations of the Pleistocene took thousands of years. Also, as the climate shifts, habitat must be available at new locations for their breeding and winter migration needs.

Recall Tinbergen’s levels of explanation for animal behavior (causation, development, function, and evolutionary history). We can see the complexity they encapsulate‒from behavioral coding in DNA‒to the role of an animal’s interactions with its environment. The modern study of ethology recognizes that the diverse aspects of behavior evolve together in complex ecological contexts. There is also a growing emphasis on understanding individual variations against the backdrop of species-specific behavior.

Individual Behavior

“When I began in 1960, individuality wasn’t an accepted thing to look for; it was about species-specific behaviour. But animal behaviour is not hard science. There’s room for intuition.”

- British Primatologist Jane Goodall

In today’s study of animal behavior, there’s growing emphasis on individual animal personalities. HIstorically, scientists assumed that non-human animals show no individual differences in behavior. But, the emerging field of “Cognitive Ethology” studies the emotional lives of animals, reviving Darwin’s early work on animal emotions (Darwin 1872). American professor of Native American Studies Steve Pavlik pointed out that Native Americans have always been cognitive ethologists, attributing thoughts, emotions, and consciousness to other species, such as mountain lions and black bears (Pavlik 2014). Indeed, Canadian ecopsychologists seek to strengthen animal-human relationships towards the healing and wholeness of Indigenous communities (McGinnis et al. 2019).

Cognitive ethology recognizes that individuals may behave in specific, predictable ways differing from others of the same species, which is not explained by models that lump individuals. Yet, these individualities may affect fitness and even be subject to natural selection. Spanish evolutionary ecologist Pilar López and colleagues examined individual variation in the behavior of male Iberian rock lizards (Lacerta monticola) in response to simulated predator attacks. They found “shy” individuals that fled to safe refuges and remained there longer, in contrast to “bold” individuals that spent more time out of the refuges (López et al. 2005).

A review of research taking individual animal personality into account found a general trend that bolder individuals have greater reproductive success than shyer ones but bolder individuals suffer shorter lifespans. So, both personality types might occur because their individual fitness will depend on the situational context. As environmental conditions change, distinct personalities may prove more successful. (Smith and Blumstein 2008). By having many personality types, a population of animals is better prepared for an uncertain future (see our Population Genetics module).

To add even more complexity to the study of cognitive ethology, consider that personality is shaped not only by genes but by the individual’s experiences. Indeed, experiences leave their mark on the brains of animals by stimulating changes in neural wiring and hormones (the mechanistic elements that are the proximate causes of behavior). Even fruit fly brains respond to experiences with changes in their nervous systems. The field of “neuroethology,” which studies how animals extract and analyze biologically relevant information from the world around them, has matured to include a focus on this responsiveness (or plasticity) of brains.

Therefore, ethology encompasses a suite of many disciplines and perspectives, from mechanistic to evolutionary to learning-focused. The wide variety of perspectives and disciplines became necessary as the original study of animal behavior revealed staggering the complexity of animal behavior.