article_snippet.sent: The researchers found that predator 'hoos' in particular were produced at frequencies well below the threshold of hearing for eagles and other birds of prey.
article_snippet.sent: The researchers found raptor hoos were less intense, shorter, lower pitch and had a small frequency span.
article_snippet.sent: Raptors hear best in the 1-4kHz range while raptor hoos were consistently below the 1kHz threshold.
article_snippet.sent: While both gibbon sexes displayed similar hoo calls, female calls were lower in frequency than male ones.
article_snippet.sent: The researchers say this is surprising, as among mammals, males tend to have lower frequency voices than females.
article_snippet.sent: Females also typically did not produce hoo vocalisations when encountering neighbours and often remained passive.