How Does Prey Detect Predators
Prey animals can detect predators using many of the same senses. Vison, smell (except in birds apparently), and sound all are used to detect predators.
Animals also have evolved feature detecting cells for predators that are in their brains at birth. Frogs that have never encountered a snake in their life still carry out proper defensive behavior. Some species lower their head and stick their arms out. Other species straighen their legs to stand taller (below). Both strategies make the frog harder for a snake to swallow.
Frogs also have feature detecting cells in the thalamus of their brains that visually detect predators. Remember how the visual part of a frog's brain is sensitive to particular features of its prey. This same part of the brain triggers feature detecting cells in the thalamus if the stimulus is oriented vertically to resemble a snake. Recent work confirms that humans and non-human primates also have so called 'snake detecting' cells in their visual thalamus.