Whale songs are getting lower in pitch every decade

Since the 1960s, researchers have recorded blue whale songs gradually shifting to lower and lower frequencies — a mystery that took decades to explain. The leading theory is that it reflects population recovery: as whale numbers increase after near-extinction from hunting, males sing at lower frequencies to communicate over longer distances in increasingly crowded seas. Larger, healthier males also naturally produce lower calls, suggesting improved health across the population.

Source: Mcdonald, M.A., Hildebrand, J.A. & Mesnick, S., 'Worldwide decline in tonal frequencies of blue whale songs', Endangered Species Research (2009)