Studying large creatures, like whales can be difficult. Especially when they spend most of their time deep underwater. But scientists have become pretty creative in their approach to studying them, especially when it comes to quantitative attributes. In this post, we will discuss how scientists can measure the size of sperm whales by using their clicks!
UnCoda-ing Sperm Whale Clicks
Sperm whales, like other toothed whales, or odontocetes, use various types of clicks. to forage, feed, socialize and reproduce. They can produce a sequence of 3 to 20 low-frequency clicks called “codas”. Sperm whales use these codas as a form of social vocalization. These codas appear to vary geographically (like geographical dialects) and can show different patterns that reflect different behaviors. Scientists can characterize these regular interval clicks produced as long series. They are used during deep dives to find prey. Repetitive in nature, they range from about 0.5 to 2 seconds at about 15kHz frequency, reaching a decibel level of 236dB re 1 µPa. As a reference, a jet engine can reach decibels of 140dBs, which we consider loud! Faster repetitions are referred to as ‘creaks’ or ‘buzzes.’
It all clicks now!
Sperm whales’ heads are asymmetrical. Through the right nasal passage, they force air to the monkey lips (also known as phonic lips). You can read more about it in one of our past posts: Echolocation 101: How dolphins see with sound. This force claps them shut, producing a click. The click then bounces from an air-filled sack back to the spermaceti organ, hitting another air-filled sack against the skull. The sound reverberates off of it, through the junk, and is amplified into the watery environment. It will come back through the lower jaw and travel to its ear bone. Now, as stated above, sperm whales produce a repetitive amount of clicks. The time between the clicks is known as the interval between pulses [the inter-pulse interval (IPI)]. And now scientists can use this IPI to determine the size of a sperm whale!
Let’s talk about the IPI
You might be wondering, “how does this IPI give you the length of the sperm whale?” The IPI gives an idea of the length of the head because the clicks hit the back of the skull before traveling out, and coming back when it hits an object. Nishiwaki et al. 1963 found that there is, in fact, an allometric relationship between sperm whale head length and body length. This means a body part size scales as it grows and develops with the whole body. Scientists know the sound speed in the spermaceti organ, and they created an equation to estimate the size of a sperm whale’s head, and by extension, the size of the whale’s body! There are some PAMguard programs or Cacholot Automatic Body Length Estimator (CABLE) that have made this body estimation even more streamlined.
This technique sounds like magic but there are some drawbacks. One caveat is that the sperm whale needs to face the hydrophone or acoustical recorder head-on to get an accurate estimation. Another problem is that scientists need to know the life history of sperm whales recorded to better understand the sizes or if they will be accurate or not. Overall, these programs are definitely advancing science and will continue to improve with continued use.
Did you like this post? Check out our post on sperm whales here.
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