Variation in call pitch among killer whale ecotypes.

Vocal structure can vary between populations due to variation in ecology-dependent selection pressures, such as masking by background noise and the presence of eavesdroppers. Signalers can overcome these obstacles to effective communication by avoiding frequencies that overlap with background noise or the audible range of eavesdroppers. In the Northeastern Pacific three "ecotypes" of killer whale coexist in sympatry, but differ from one another in their diet and habitat use. The minimum frequency (F(min)) and the frequency containing the peak energy between 0 and 10 kHz (F(peak)) of a random sample of calls produced by a population of each ecotype was measured. The offshore ecotype produced calls with a significantly higher F(min) than the other ecotypes, which could be a strategy to avoid masking by low frequency chronic bandlimited wind noise found in the offshore environment. The resident ecotype produced calls with a significantly higher F(min) and F(peak) than the transient ecotype. This could be to reduce detection by their salmonid prey, which has a narrow band, low frequency auditory range.