New acoustic data explains a crucial whale behavior
These underwater sing-a-longs may have hidden meanings.
Blue whales are the largest animals currently roaming the Earth, so you'd think scientists would have an easy time tracking and learning about their behavior. Not quite.
Ecology research is often constrained to observation, which can be challenging when your subject is 200 tons and travels miles across the ocean on an annual basis. Previous studies have used either tracking devices or passive acoustic monitoring to learn about the daily life of these incredible animals, but these studies have not been able to draw many holistic conclusions about behavior.
New research published Thursday in the journal Current Biology combines both individual and population wide acoustic data to identify an acoustic signature demarcating the whales' annual migration. Essentially, the whales' songs contain patterns that reveal behavior in unprecedented detail.
Understanding how these animals communicate with each other not only informs our study of their behavior, but can also help future researchers better anticipate how they might react to climate-driven changes in their environments.
Good vibrations — When it comes to whale communications, acoustic vibrations through the ocean are key, explains the study's first author and graduate researcher at Stanford University's Hopkins Marine Station William Oestreich in a statement.
"Sound is a vital mode of communication in the ocean environment, especially over long distances," said Oestreich. "Light, or any sort of visual cue, is often not as effective in the ocean as it is on land. So many marine organisms use sound for a variety of purposes, including communicating and targeting food through echolocation."
Researchers previously believed these songs were primarily to do with breeding, but the new research reveals these animals have more uses for their sing-a-longs than originally thought. When it comes to uncovering the hidden meaning of these whale songs, the researchers say they stumbled upon them by accident when analyzing existing acoustic data.
Listen to their incredible vocalizations here:
An excerpt of the blue whale song captured in this study played back at 10 x the original speed.
How to eavesdrop on a whale — Because chasing a blue whale through the ocean with a boom mic outstretched to capture their calls is hardly a practical way to go about things, the researchers instead used two different approaches: a hydrophone and individual acoustic trackers.
Like a microphone would capture acoustic vibrations as they move through air, a hydrophone is designed to capture acoustic vibrations as they move through water. The size of a human hand, hydrophones can capture two terabytes of data per month of acoustic behavior from different whale populations.
Analyzing five years' worth of data collected from a hydrophone placed 18-miles off of Monterey Bay, California, the researchers noticed some distinct patterns emerging. The data show that, year after year, the blue whale calls seemed to be loudest during October and November and tended to happen mostly at night.
While this information was a useful first clue, it still didn't paint the whole picture of why this behavior might be happening. To fill in those gaps, the team also turned to acoustic data collected from individually tagged whales as well.
These 15 different biotags work similarly to a lav mic on a TV weatherman. They allowed researchers to analyze song patterns from individuals animals in a group to determine whether a behavior was actually occurring through the entire population. The researchers were excited to see that the same behavior they'd measured from miles away using their deep ocean hydrophone also appeared in their close-range data of these individual whales.
"[Oestreich] and I were both like, 'Hello, behavior'," said the study's senior author and biological oceanographer, John Ryan, in a statement.
An image from the study of what a tagged blue what looks like.
Using this data, the researchers were able to identify that blue whales sing more at night during feeding periods (presumably because they're too busy munching during the day), but that this pattern inverts during their migration season and they instead sing more during the day.
Scientists can't yet say what these calls mean to the whales making and receiving them, but they speculate that they might be used a signal to different whales in the group that it's time to stop feasting on krill and make their 4000-mile journey south for the year.
More than a travel schedule — While learning more about how these animals might communicate their travel schedule to each other is important from a behavioral standpoint, the researchers point out this technique could also be used in the future to identify other types of behavior patterns as well. For example, they're hopeful it could help researchers better understand how blue whales react to climate-driven changes in their environment, such as food scarcity.
Understanding this behavior could hopefully lead to more targeted conservation efforts for these already endangered animals.
Abstract: Linking individual and population scales is fundamental to many concepts in ecology, including migration. This behavior is a critical yet increasingly threatened part of the life history of diverse organisms. Research on migratory behavior is constrained by observational scale, limiting ecological understanding and precise management of migratory populations in expansive, inaccessible marine ecosystems. This knowledge gap is magnified for dispersed oceanic predators such as endangered blue whales (Balaenoptera musculus). As capital breeders, blue whales migrate vast distances annually between foraging and breeding grounds, and their population fitness depends on synchrony of migration with phenology of prey populations. Despite previous studies of individual-level blue whale vocal behavior via bio-logging and population-level acoustic presence via passive acoustic monitoring, detection of the life history transition from foraging to migration remains challenging. Here, we integrate direct high-resolution measures of individual behavior and continuous broad-scale acoustic monitoring of regional song production (Figure 1A) to identify an acoustic signature of the transition from foraging to migration in the Northeast Pacific population. We find that foraging blue whales sing primarily at night, whereas migratory whales sing primarily during the day. The ability to acoustically detect population-level transitions in behavior provides a tool to more comprehensively study the life history, fitness, and plasticity of population behavior in a dispersed, capital breeding population. Real-time detection of this behavioral signal can also inform dynamic management efforts to mitigate anthropogenic threats to this endangered population.
