Research
Current and past research involvement.
Original Research
Can an animal’s habitat affect its voice?
The Acoustic Adaptation Hypothesis
Whether it is singing, screaming, growling, chirping, or speaking, sounds are the primary way many animals communicate with each other. But these animals often face an issue– how do they make themselves heard when blocked by dense vegetation, winds that carry away their voice, or the background chatter of their competitors?
The Acoustic Adaptation Hypothesis, first thought up in 1945, asked a simple question– do animals adapt the physical structure of their sounds so that they can be easily received, even despite a cluttered ‘acoustic space’? This might be done by adjusting the pitch, volume, frequency, or speed of their calls to combat environmental degradation of sound.
Even nearly 50 years later, the answer remains unclear. Many things in animal’s habitat certainly distort sound. Noise from wind, rain, or the ocean can cover a call. Dense vegetation and bare ground can scatter a sound, while large boulders and cliffsides echo and amplify it. Wet, humid air can dampen sound until it is unintelligible even at short distances!
My recent research focuses on habitat-driven song adaptation in wren species. So far, it seems that acoustic adaptation is likely species and habitat dependent. For example, Cactus Wrens (pictured) have a chattering song that is particularly good at fighting large fluctuations in air temperature, wind, and humidity, while Canyon Wrens have a pure-tone, descending song which is well-suited for traveling across large spans of shrub habitat. Meanwhile, House Wrens, who occupy a wide variety of habitats, don’t seem to have a song adapted to any one in particular. Such lack of adaptation of voice to a specific environment might actually help them in occupying these very different habitats!
How is this important in the context of conservation? Unfortunately, noise pollution from urban centers, agricultural machinery, and low-flying aircrafts can make it difficult for animals to contact potential mates, causing population decline. For example, large amounts of concrete and the presence of skyscrapers are capable of distorting songs, and traffic noise can drown out a bird completely. Check out this article to learn more about how birds are reacting to urbanization of habitats. Free downloads of my complete review of the acoustic adaptation hypothesis can also be found here!
How is female song different than male song?
Evolution of Female Song
Most people are aware that male birds sing to attract mates and defend their territories– but did you know some females sing, too?
Female song was thought to be far more common in tropical climates, but recent work by my colleague Dr. Nadje Najar estimates that about 25% of North American warbler species have been documented singing– and that’s only one group of songbirds! Given a decades long hyper-focus on male song in birds, many in the field believe that this may be an underestimation. It is possible that female song has been ignored, or more likely, that females are sometimes wrongly categorized as males because they sing.
In species where females sing, they usually sing the same songs as their male counterparts. In these cases, both males and females might be filling the same role with their song– likely territory defense or pair bonding. But one of my focal species, Canyon Wrens, have what we call ‘sex-specific signals,’ meaning they are physically structured differently than male songs. You can listen to the differences between a male and female Canyon Wren song here.
In a recent study, we found that, because they are structured differently than male songs and therefore propagate over habitats differently, female song likely plays a different role than male song in this species. Specifically, male song retains volume and clarity over large distances, and is used for defending very large territories from other, distant males. Meanwhile, female song fades relatively quickly and is used almost exclusively when competing females get to close to the nest. Both songs are for defense– but at different distances, and for different sexes!
To learn more about the importance of studying female song in birds, check out this podcast featuring my graduate adviser and renowned ornithologist Dr. Lauryn Benedict.
How stressful is it to grow up with siblings?
The Stress of Siblings
We all know that stress is commonly a bad thing– beyond being uncomfortable, stress causes an elevated heart rate, diminished organ functioning, and in some cases, deteriorated health and growth. But can stress be a good thing?
What we know as ‘stress’ is regulated primarily by one hormone, which we call CORT. Humans, along with primates, large carnivores, and hooved mammals, use a version of CORT called cortisol. Small mammals, reptiles, and birds use a similar version of this hormone called corticosterone.
Biologically, stress can be categorized into two categories. Baseline CORT is a background level of stress hormone always present in the body, and ‘stress-induced’ CORT, which is produced during actively stressful moments. In humans, stress-induced CORT might come from immediate incidents causing stress, like taking a test or engaging in a job interview. In animals, it is usually produced when an animal is suddenly exposed to danger, like when a predator tries to eat them.
During my undergraduate career, I studied how these two types of stress are affected by the presence of siblings. Theoretically, having several siblings should mean increased stress derived from increased competition for food, which in turn might lead to reduced body condition and growth rates. Therefore, growing up with a lot of siblings should over-activate the CORT system and lead to detrimental effects on growth and body function in young animals.
However, after studying two species of North American Swallow, the Tree Swallow and Violet-green Swallow (young pictured), I found that although both types of CORT are elevated when baby birds grow up in large groups of siblings, growth and body condition are not affected. Surprisingly, this likely means that elevated levels of CORT may not actually be a negative consequence of large families, but rather a regulation process which allows birds to maintain proper condition if they are exposed to sibling competition. Elevated levels of baseline CORT stimulates a baby bird with a lot of siblings to beg more frequently than one with less competition. As a result, they are fed the same amount of food, and with the same frequency, as those with smaller families, and therefore grow up at the same rate.
In the end, it turns out that stress hormones are not at negative result of sibling competition, but a physiological process by which animals might ensure they maintain proper condition despite competition! For more details on this study, my article on stress in swallows can be downloaded for free here.
Other Involvements
Click the photos below to access project websites.
Oregon Sensitive Species
With the help of the Oregon Biodiversity Information Center at Portland State University, the Oregon Department of Fish & Wildlife uses decades of critical research to designate species protection status. This includes unlisted, but sensitive species such as the American Pika, which is– without protection– expected to go extinct within the century.
Piping Plover Conservation
The Virginia Technical Institute’s Shorebird program works across North America to monitor declining shorebirds. On Fire Island, New York, near threatened Piping Plovers fight against both climate-caused habitat destruction and the island’s beach visitors for space to breed.
Publications
Benedict, L., Hardt, B., and Dargis, L. (2021) Form and function predict acoustic transmission properties of the songs of male and female canyon wrens. Frontiers in Ecology and Evolution.
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Hardt, B., Benedict, L. (2020) Can You Hear Me Now? A Review of Signal Propagation and the Acoustic Adaptation Hypothesis. Bioacousics. https://doi.org/10.1080/09524622.2020.1858448
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Hardt, B., Ardia, D., Bashaw, M. & Rivers, J. (2018) Experimental brood enlargement differentially influences the magnitude of the corticosterone stress response in closely related, co-occurring songbirds. Functional Ecology, 32, 2008–2018.
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