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below which no behavioral response is assumed and above which responses would occur for all individuals. More recent efforts have considered exposure-response proba- bilistic functions. Neither a step-function threshold nor a single probabilistic function was provided by Southall et al. (2007) or appears in Southall et al. (2021) because of the well-documented variance in responses across spe- cies, sound types, and exposure contexts (see Ellison et al., 2012; Southall, 2017). There is simply no single overarching applicable level or function spanning all species and expo- sure types. The situation is not, however, intractable; it just requires more nuanced, evidence-based, and context- and group-specific means of predicting response probability.
Southall et al. (2021) proposes novel, distinct approaches for deriving probabilistic response functions within the very different contexts for free-ranging marine mammals and those in captivity. For field conditions, sound sources were distinguished not by impulsive or nonimpulsive features but rather by practical, real-world categories (active sonar, seis- mic surveys, continuous/industrial noise, and pile driving). Southall et al. (2021) is largely methodological, proposing
and applying novel analytical concepts on a strategic subset of a vast and growing literature. The need for more system- atic, standardized data in behavioral response studies is highlighted, along with key variables to describe subject- or group-specific metadata (e.g., behavioral state, group size), exposure contextual variables (e.g., spatiotemporal features, exposure novelty), and acoustic exposure metrics (e.g., dura- tion, rise time, sound pressure and sound exposure levels).
A major conclusion is that meta-analyses integrating results across studies will be strengthened where such data are sys- tematically collected and reported.
For captive animals, an assessment of response severity based on Southall et al. (2007) is proposed with more explicit consideration of conditioning, habituation, and potential physiological effects. For free-ranging animals, however, Southall et al. (2021) propose a fundamen- tally different approach. Although similarly identifying progressively severe response categories, Southall et al. propose three explicit parallel tracks (see Figure 3) for behaviors associated with the vital functions of sur- vival (e.g., resting, navigation, defense), feeding (e.g.,
Figure 3. Behavioral response severity spectrum. Examples of observable responses are shown for behaviors relating to survival, feeding, and reproduction in categories of increasing presumed severity (0-9). Severity assessments such as these have been used in systematic expert elicitation of behavioral response severity in noise-exposure studies. From Southall et al., 2021.
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