By Kaiming Tan, Neurobiology, Physiology, and Behavior, ‘16

Author’s Note: As a student who is engaged in biological sciences research, I often read research publications and perform experiments in laboratory classes and research projects. A common theme across these studies is that different labs use various model organisms. For instance, labs that research infectious human diseases tend to use primates because of their similarity to humans, whereas geneticists tend to use fruit flies because of their clearly defined and inherited traits. I often wondered what drives the selection of specific model organisms and whether there is any scientific justification behind it. This manuscript introduces Krogh’s principle, a principle which is commonly applied in in vivo research studies to aid in determining the appropriate model organism. Additionally, a brief research proposal is presented to demonstrate Krogh’s principle on a practical level.

Key Words

Krogh’s principle, animal behavior, sensory ecology, Leach’s storm-petrel, bladder grasshopper, research proposal

Introduction

Krogh’s principle is the gold standard used to choose experimental models in the fields of animal behavior and sensory ecology. The principle states that for every research question, there is a preferred model organism to study, which allows researchers to produce experimental results that help to answer the research question. These preferred organisms often have one or more specialized traits that are particularly well-suited for a researcher’s objectives [1]. August Krogh developed this principle in the mid 19th century, and ever since, scientists worldwide have adapted this principle when selecting model organisms for their research studies.

Krogh’s Principle in Use

Dr. Robyn Hudson is a world-renowned scientist and a leading expert on the effects of chemical olfactory cues on animal behavior. Dr. Hudson applied Krogh’s principle in her research on olfactory learning and development by choosing the European rabbit as a model for her research. Rabbit pups are born blind, but they have a fully-developed sense of smell, also known as olfaction. Additionally, baby pups are fed in the dark conditions of their underground nests. They are also altricial, meaning they are entirely dependent on their mother’s brief nursing. In Hudson’s experiment, the rabbit pups were only visited by their mother once a day to be nursed for about three to four minutes [2]. Thus, the pups require olfaction to look for their mother’s nipples for milk in order to survive. Given the natural history of this species, scientists can conclude that the baby rabbit’s ability to search for their mother’s nipples is primarily due to olfaction. 

Additionally, Krogh’s principle applies to Dr. Hudson’s choice of research subjects because rabbits are easy to rear and observe in a laboratory setting. European rabbit pups have evolved plastic mechanisms calibrated by circumstantial odor experience in preceding and current environments. Olfactory plasticity allows the rabbits to modify their behavior in response to olfactory cues such as different scents. This enables the rabbits to learn behaviors evoked by odors and makes the behavior easy to measure and manipulate by researchers [2,3]. As a result, scientists can measure the rabbits’ behavioral ecology with respect to foraging, which could make the European rabbits a preferred model for olfactory learning. These rabbits are perfectly suited for this type of study because they are born with an innate sense of olfaction, allowing Dr. Hudson and her research team an excellent opportunity for a study aimed at olfactory learning and development.

Another illustrative example of Krogh’s principle in use comes from the work of Dr. Brian Hoover. Dr. Hoover’s research interests included the role of olfaction as it pertained to determining mating preferences in avian species. Dr. Hoover investigated mating patterns in Leach’s storm-petrel (Oceanodroma leucorhoa) to explore the chemical basis of mate choice through avian olfaction. There are several reasons he chose the Leach’s storm-petrel as the model organism for this study. The Leach’s storm-petrel has among the largest olfactory bulbs of any bird, thus they have an excellent sense of smell [4]. Olfaction is critical for the Leach’s storm-petrel to locate prey [5]. In addition, the Leach’s storm-petrels are genetically monogamous and produce only one chick per year. This scenario allows the offspring to have higher genetic quality. An organism’s genotype must be best-fitted for its survival. As the Leach’s storm-petrel only gives one offspring per year, its genetic makeup needs to be suitable to sustain it in its environment so that its survival rate is high. Thus, scientists can collect data about offspring quality and observe the adults’ mating patterns, thereby simplifying the data collection process while maintaining the accuracy of the mating patterns measured. 

The population of Leach’s storm-petrel was abundant in the study, which equated to a large and accessible sample size. Sample size is an important consideration for data analysis as it is the determinant of statistical power, the ability to report findings with statistical confidence. There are other species that could have been used in this study, including the mallards (Anas platyrhynchos) due to their large olfactory bulbs. The mallard’s reproductive behavior is also driven by olfactory cues [6]. However, the mallards are not an ideal model organism compared to the Leach’s storm-petrels in this study because the mallards are polygamous. Therefore, the mallards would not show clear mating preferences compared to the monogamous Leach’s storm-petrel [5,7]. Both Dr. Hudson and Dr. Hoover utilized Krogh’s principle when designing their respective research studies. Applying Krogh’s principle allows for the intersection of practical study methodology, high quality data, and conclusions that are generalizable beyond the species studied. 

Application of Krogh’s Principle: An Experimental Proposal on the Effect of Noise Pollution on Insect Communication    

Now that we have examined historical uses, both recent and distant, of Krogh’s principle, we will now examine an application of Krogh’s principle for future research. In light of how useful Krogh’s principle is, it makes sense to propose an additional study on the auditory interference of grasshoppers. This research proposal will explore whether artificial noise in the environment affects insect hearing or communication. Insect perception of a sound is masked by environmental noise pollution. Based on Krogh’s principle, Bullacris membracioides (the bladder grasshopper) will be the model organism for this study due to their anatomical and behavioral advantages. Male and female bladder grasshoppers (Bullacris membracioides) call each other during mating using the duet behavior. The duet behavior occurs when a male grasshopper produces a song that is then repeated by a receptive female. Therefore, perception of the male call by the female can be measured by the female’s reply [8]. Anatomically, female bladder grasshoppers possess a sensitive auditory system of six pairs of ears (A1-A6). The A1 auditory organ contains 2,000 sensilla, which allows them to hear sounds over great distances of up to two kilometers. This is in contrast to other species of grasshoppers (i.e. Achurum carinatum) where females can only hear male calls only within 1-2 meters [9].

Bladder grasshopper female calls range in frequency from 1.5-3.2 kHz [8]. A common habitat for the bladder grasshopper is on the roadside. Road noises from the motorcycles can be loud (110 dB) and within the frequency range (700 Hz to 1.5 kHz) that could interfere with the auditory system of grasshoppers. Bladder grasshoppers typically mate in daytime, which is the same time as peak traffic noises. As a result, common noise pollution may disrupt perception of grasshopper calls and interfere with mating behavior [10] . In addition, it has been shown that grasshoppers exhibit phonotaxi (an organism’s movement in response to sound) in laboratory conditions [8-11]. To test whether artificial noise can disrupt the duetting behavior of the grasshoppers, a female grasshopper will be placed in a glass aquarium in front of an omnidirectional speaker that plays a recording of a male’s song mounted on a parabolic disk. Testing will be done at distance levels such as 100, 200, 500, 1,000, and 2,000 meters to examine whether distance correlates to the amount of time the female takes to respond. The experiment will be repeated with male calls with traffic noise, traffic noise alone, and no sound at all. Studying the effects of noise pollution on the auditory system using the bladder grasshopper is an example of Krogh’s principle because they are easy to raise and rear in a laboratory setting. Bladder grasshopper’s advantageous hearing made them the model organism of choice in relation to Krogh’s principle, making the hearing behavior more practical to measure and manipulate in response to different noise levels. 

Conclusion

Krogh’s principle is an important concept to keep in mind while designing research studies. Many scientists, including Dr. Hudson and Dr. Hoover, around the world have applied this principle in their research to better answer research questions. To further elucidate the utility of Krogh’s principle, an experimental proposal was made concerning the effects of noise pollution on insect communication. The anatomical and behavioral characteristics considered in selecting the bladder grasshopper for this experiment were illustrated. Krogh’s principle provides useful guidance for scientists to select the most representative and practical model organism to study. 

Acknowledgments

The author would like to thank Dr. Gabrielle Nevitt (Professor of the Department of Neurobiology, Physiology and Behavior at University of California, Davis) for supporting this research project and providing feedback on early versions of this manuscript.

Editor’s Note: A previous version of this article was published on April 18, 2020. The article was updated on June 19, 2020 to correct citation style.

References

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