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I have a bipedal, primate species that I want to be able to move quickly for short periods of time. In other words, rather than possessing the "perseverance" of humans, I want this species to work in quick bursts - perhaps an ambush-type predator that works in groups.

While still maintaining a roughly humanoid, upright form (and still evolving to sapience), what is the maximum running speed such a species could attain? What biological changes would be necessary to propel them to such bursts of speed?

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Running speed is effectively a function of stride length and frequency, and can be increased by increasing either or both.

In order to increase stride length, we can do two things:

  1. Increase the length of the legs,

and/or:

  1. Increase the unsupported distance in the stride.

To increase the stride frequency, there are again two ways of doing this:

  1. Decrease the mass of the lower limbs, shifting muscle toward the portions of the limb that sweep the lowest total distance,

and/or:

  1. Increase the mass of muscle.

Additionally, by storing more of the energy of ground impact and releasing it at the appropriate time, we can use less energy to achieve the same speed.

Since human physiology is already pretty well adapted to long-distance running, it is likely that any future adaptation would either add to this or leave the metabolic factors alone; it is unlikely that humans - being persistence hunters - would do an evolutionary U-turn and become ambush predators. It is especially unlikely in view of the requirement for maximum bipedal speed, since an ambush predator needs to be strong enough to immediately grapple and incapacitate prey (that would favour a form with considerable upper-body mass, counter-productive to great speeds), while a persistence hunter must merely keep up the chase long enough to exhaust the quarry, and the faster the hunter can go, the sooner the quarry will tire.

Given that we will retain a persistence hunter lifestyle, we can anticipate that the ribcage will become even larger in order to house enlarged lungs and heart. Aside from that, a small, lean frame is desirable in order to reduce weight.

Doing these things to a humanoid frame would lead to a being that looks remarkably like a satyr. The legs are long with the muscle placed high, with the foot elongated and with a digitigrade stance. The hooves will probably be solid as in a horse's, though it is not out of the question to have cloven hooves as in a deer's. The waist would be narrow and the chest broad and deep. The head would be fairly delicate in appearance (so perhaps more like the mythical faun?) so as to reduce weight, and the arms might also be relatively slender.

Horns are pretty unlikely despite being part of the satyr/faun myths, but would still fit if you chose to add them - horns as sexual signals have the point that it is disadvantageous to have them in everyday life, but the bearer has managed to thrive despite their inconvenience.

As to the speeds achievable by a satyr/faun? The answer to HDE 226868's Biology SE question suggests that the maximum speed achievable by anatomically modern humans is on the order of 44 kph. Our satyrs, having legs perhaps 60-90cm longer than those of a modern human and having their mass redistributed more favourably, may be able to achieve speeds comparable to that of an ostrich, perhaps up to 70 kph in an exceptional individual. However, we can expect an ostrich to have somewhat better metabolic performance, as avian lungs are more efficient than mammalian, however these satyr/fauns would have better endurance, as humans are perhaps the best evolved animals on earth for shedding excess body heat.

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