Perhaps the animals they hunted, or feared, produced sounds in the range above 10kHz. 
 
 Kevin 
 
 
 
 
 
 
 On 2011, Feb 4, at 6:18 PM, Piotr Majdak wrote: 
Nice shots, Kevin. What's the upper frequency limit of those apes? For
chimpanzees it is approx. 30 kHz... 
Piotr
 
Kevin Austin wrote:
 
  I may be prejudiced [specious bias], but I think these ears have greater prominence than mine. Kevin
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