of hominid evolution by the forces of natural selection. Plenty of animals walk rhythmically and hear the sounds of the heartbeat in the womb, but only humans have the ability to be engaged (entrained) in rhythmically coordinated vocalizations and other activities. According to Jordania, development of the sense of rhythm was central for the achievement of the specific neurological state of the battle trance, crucial for the development of the effective defense system of early hominids. Rhythmic war cry, rhythmic drumming by shamans, rhythmic drilling of the soldiers and contemporary professional combat forces listening to the heavy rhythmic rock music all use the ability of rhythm to unite human individuals into a shared collective identity where group members put the interests of the group above their individual interests and safety. Some types of parrots can know rhythm. Neurologist Oliver Sacks states that chimpanzees and other animals show no similar appreciation of rhythm yet posits that human affinity for rhythm is fundamental, so that a person's sense of rhythm cannot be lost (e.g. by stroke). "There is not a single report of an animal being trained to tap, peck, or move in synchrony with an auditory beat", Sacks write, "No doubt many pet lovers will dispute this notion, and indeed many animals, from the Lipizzaner horses of the Spanish Riding School of Vienna to performing circus animals appear to 'dance' to music. It is not clear whether they are doing so or are responding to subtle visual or tactile cues from the humans around them." Human rhythmic arts are possibly to some extent rooted in courtship ritual. The establishment of a basic beat requires the perception of a regular sequence of distinct short-duration pulses and, as a subjective perception of loudness is relative to background noise levels, a pulse must decay to silence before the next occurs if it is to be really distinct. For this reason, the fast-transient sounds of percussion instruments lend themselves to the definition of rhythm. Musical cultures that rely upon such instruments may develop multi-layered polyrhythm and simultaneous rhythms in more than one time signature, called polymeter. Such are the cross-rhythms of Sub-Saharan Africa and the interlocking kotekan rhythms of the gamelan. For information on rhythm in Indian music see Tala (music). For other Asian approaches to rhythm see Rhythm in Persian music, Rhythm in Arabic music and Usul—Rhythm in Turkish music and Dumbek rhythms. Terminology