Three Renowned Hypotheses and Their Experimental Validation

Arthur Hasler, an ecologist (left), is renowned for unraveling the homing behavior of coho salmon. Center for Limnology, University of Wisconsin-Madison

Key Insights

Coho salmon (Oncorhynchus kisutch) are remarkable creatures. Native to the Pacific Northwest, they start their journey in freshwater streams before migrating to the ocean. Upon reaching maturity, they return to their birthplace, often journeying up to 400 miles (644 kilometers) to reach their destination.

The late Arthur Davis Hasler played a pivotal role in understanding this phenomenon. As an ecologist and biologist at the University of Wisconsin, he was fascinated by how these fish navigate back to their home streams. In 1960, he employed a Hypothesis-Presentation.pdf">fundamental scientific principle — the hypothesis — to uncover the truth.

What exactly is a hypothesis? It is a provisional, testable explanation for a natural occurrence. Hypotheses are specific in scope, unlike theories, which encompass a wide range of phenomena and are supported by multiple lines of evidence. A prediction, on the other hand, is the expected outcome if the hypothesis or theory holds true.

Returning to 1960 and Hasler's work with salmon, one unproven idea suggested that Coho salmon relied on eyesight to find their home streams. Hasler aimed to test this hypothesis. He gathered fish that had already returned to their native streams, blindfolded some, and released them in a distant location. If the eyesight hypothesis were correct, fewer blindfolded fish would return home.

The results were surprising. Both blindfolded and non-blindfolded fish returned at the same rate. (Further experiments revealed that smell, not sight, is crucial for the species' homing ability.)

While Hasler's blindfold hypothesis didn't hold up, other experiments have achieved greater success. Here, we explore three of the most famous experiments in history and the hypotheses they examined.

Ivan Pavlov and His Dogs (1903-1935)

The Hypothesis: If dogs can develop conditioned responses (such as drooling), then repeatedly pairing a

neutral stimulus (like a metronome or bell) with food should lead the dog to associate the stimulus with eating. Over time, the dog should drool predictably upon encountering the stimulus, even without the presence of food.

The Experiment: Ivan Pavlov, a Nobel Prize winner and vocal opponent of Soviet communism, is famously linked to man's best friend. Starting in 1903, the Russian scientist conducted a series of experiments spanning decades, focusing on dogs and conditioned responses.

Present food to a hungry dog, and it will salivate. In this scenario, the stimulus (the food) naturally elicits a specific response (drooling). This reaction is instinctive and not learned.

In contrast, the steady ticking of a metronome or the ringing of a bell serves as a neutral stimulus. For a dog, this sound holds no intrinsic significance, and if the animal has never encountered it before, it won't trigger an automatic response. However, the sight of food certainly will.

By consistently playing the metronome or bell before feeding, Pavlov and his team conditioned the dogs to associate the sound with eating. Over time, the noise alone began to elicit drooling before any food was presented.

As detailed in "Ivan Pavlov: A Russian Life in Science" by Daniel P. Todes, Pavlov's breakthrough was his ability to measure each dog's reaction by quantifying the saliva produced. Each dog reliably drooled at a consistent rate when exposed to its specific artificial food-related cue.

Pavlov and his team also explored other hypotheses about animal physiology using conditioned responses. In one experiment, a dog was tested on its ability to distinguish time. The dog received food only when a metronome clicked at 60 strokes per minute but not at 40 strokes per minute. Remarkably, the dog salivated in response to the faster rhythm but not the slower one, demonstrating its ability to differentiate between the two.

The Verdict: Through proper conditioning and patience, it's possible to train a hungry dog to respond to neutral stimuli by salivating predictably and in a manner that can be scientifically measured.

Ivan Pavlov demonstrated that a hungry dog could be trained to salivate in response to neutral stimuli through conditioning. ©Mytour

Isaac Newton's Radiant Prisms (1665)

The Hypothesis: If white sunlight comprises all colors in the visible spectrum — each traveling at distinct wavelengths — then each color should bend at a unique angle when sunlight passes through a glass prism.

The Experiments: Before Isaac Newton, color remained a scientific enigma. In the summer of 1665, he began experimenting with glass prisms in a darkened room in Cambridge, England.

He created a quarter-inch (0.63-centimeter) circular opening in one of the window shutters, permitting a solitary beam of sunlight to enter the room. When Newton positioned a prism in the path of this ray, a rectangular spectrum of multicolored light appeared on the opposite wall.

This spectrum displayed distinct bands of red, orange, yellow, green, blue, indigo, and violet light. The patch measured 1 inches (33.65 centimeters) in height but only 2.6 inches (6.6 centimeters) in width.

Newton concluded that these vivid colors were inherently present in sunlight, but the prism refracted them at varying angles, causing the colors to separate.

However, he wasn't entirely convinced. To confirm, Newton repeated the experiment with a slight modification. He introduced a second prism to intercept the rainbow-like spectrum. As the refracted colors passed through the new prism, they merged back into a circular white beam. Essentially, Newton took white light, split it into its constituent colors, and then recombined them. A fascinating demonstration!

The Verdict: Sunlight is indeed a combination of all the colors of the rainbow, and these can be individually separated through the process of light refraction.

In 1665, Isaac Newton conducted experiments to validate his hypothesis that sunlight is a combination of all rainbow colors and that these colors can be separated through light refraction. Apic/Getty Images

Robert Paine's Revealing Starfish (1963-1969)

The Hypothesis: If predators control the populations of their prey, then removing a key predator should lead to an increase in the prey species' population.

The Experiment: Introducing Pisaster ochraceus, commonly known as the purple sea star or purple starfish.

With its expandable stomach, this starfish preys on mussels, limpets, barnacles, snails, and other unfortunate creatures. Along the coastal rocks and tidal pools of Washington state, it reigns as the top predator.

This creature turned Robert Paine into a renowned figure in science. As an ecologist, Paine was deeply intrigued by the ecological impact of apex predators. In June 1963, he launched a groundbreaking experiment in Mukkaw Bay, Washington. For several years, he maintained a starfish-free zone on a rocky stretch of the shoreline.

The task was labor-intensive. Paine frequently had to remove stray sea stars from his designated area, often using a crowbar. He would then toss them back into the sea.

Prior to the experiment, Paine noted 15 distinct species of animals and algae in the study area. By June 1964, a year into his starfish removal, the count had fallen to eight.

Without the presence of purple sea stars, barnacle numbers surged dramatically. These were eventually overtaken by California mussels, which became the dominant species. By clustering densely on the rocks, the mussels outcompeted other organisms, rendering the area unsuitable for most original inhabitants. Even sponges, anemones, and algae — which Pisaster ochraceus does not consume — were mostly driven out.

All those species flourished on a separate stretch of shoreline that Paine chose not to disturb. Subsequent experiments led him to conclude that Pisaster ochraceus is a "keystone species," an organism with an outsized impact on its ecosystem. Remove the keystone, and the entire system becomes unbalanced.

The Verdict: Apex predators influence more than just their prey. The removal of a top predator triggers a cascade of effects that can drastically alter an ecosystem.

Ecologist Robert Paine removed all purple sea stars from a rocky area in Mukkaw Bay, anticipating a surge in mussel, barnacle, and snail populations. His prediction was incorrect. Jerry Kirkhart/Flickr (CC By 2.0)