Sangheili

The Sangheili are a sapient species of sinewy, bipedal sauroids. As a founding species and primary ruling class of the Covenant alongside the San'Shyuum, the Sangheili are one of the oldest active spacefaring species in the Orion Arm.

Evolution and prehistory
The Sangheili are semi-predatory omnivores (or mesocarnivores) that evolved from larger predators. As opposed to humans, who began as nomadic endurance hunters who could outrun faster prey over long periods of time, the Sangheili relied on their speed and agility as hunting tactics; this also means their means of thermal regulation in sustained exertion are not as effective (for example, they do not perspire). Their impressive physical prowess is less of an indication of an apex predator role, but rather the harsh and competitive evolutionary environment of Sanghelios, where they are a lower mid-tier predator at best, having sacrificed strength and size for brain capacity. Their primitive cousins would often be forced to resort to scavenging rather than hunting for live prey, although as they grew more intelligent, their hunting strategies also diversified, and with pack tactics they could best even far physically superior prey. It was also around this point that they transitioned from primarily quadrupedal to bipedal locomotion, first intermittently and later near-exclusively. Pack hunting formed an integral part of the Sangheili's formative millennia, and is suspected to have been a key contributor to their development of language and intelligence. Indeed, in more than one early Sangheili civilization, their word for themselves translated to "hunters" or some close equivalent. The early Sangheili organized themselves into tight-knit tribal units in which eggs were brooded and young raised communally - early predecessors to the species' later standard group families and clans. Depending on the latitude, the proto-Sangheili were crepuscular or metaturnal, hunting for food in twilight or in sporadic intervals throughout Sanghelios' somewhat atypical day-night cycle.

As opportunistic mesocarnivores, the primitive Sangheili favored animal-based nutrient sources ranging from sea life and insectoids to sauroids, but they would often supplement their diets with plant-based foods such as fruits, roots, algae and fungi by necessity. The eventual development of agriculture saw a handful of additions to this. Sanghelios does not exactly have direct equivalents to terrestrial grain, and their staple crops are most akin to legumes and algae, as well as some types of fungi. The Sangheili may have genetically modified some crops early on in their history to be more nutritious or easily digestible to themselves. Even now, grain such as irukan is mostly grown for livestock's consumption, as well as non-Sangheili serfs; Sangheili can eat it, but it does not give them much in the way of nutrition. The development of agriculture eventually led to the formation of small tribal communities, which quickly settled down; the species never experienced a prolonged and nearly-universal nomadic phase, as they never relied as much on routinely crossing long distances as humans did.

Anatomy and physiology
One of the Sangheili's most distinctive morphological characteristics is their quad set of hinged mandibles. Many related species on Sanghelios share this trait, which originally evolved for feeding purposes while the species' ancestors were still quadrupedal and largely unable to use their forelimbs for feeding. The mandibles, much stronger and more prominent at the time, were used to both immobilize and kill prey as well as guide it into the mouth. The modern Sangheili's smaller mandibles are actually a neotenic trait retained from an ancestral mutation at one point in the species' evolution. Normally, as their ancestors would mature into adulthood, the mandibles would grow considerably larger. However, this would also directly limit the brain size, as the muscles responsible for moving them took up considerable space around the cranium. This can be seen with several of the species' surviving relatives, which retain a thinner, elongated skull, with some even having an additional head crest to which the muscles attach. As the species shifted increasingly from quadrupedal to bipedal locomotion and began using the forelimbs to manipulate objects, the need for the mandibles for their original purpose grew less acute, and the retention of the more neotenic, less pronounced and weaker mandible structure freed up space in the cranium for a larger brain to develop. Now, on most Sangheili ethnicities, the mandibles are largely vestigial and have little actual utility aside from conveying expressions. Because of this history, larger and more forward-jutting mandibles exhibited by some phenotypes are generally seen as more atavistic or "primitive" traits in Sangheili.