This section focuses mainly on the slans' biological traits, and also gives some basic general information and some notes on their psychological makeup. For more information about their telepathic abilities, see Telepathy. The relations between humans, slans, and tendrilless slans are covered in the section Slans in Society.
Natural mutations, this offshoot breed of humanity is superior in almost every respect — their internal organs differ from those of normal humans (6), they possess two hearts that function as one (7, 84), and are endowed with tendrils, golden threads mixed in with their hair, which can detect thought waves and give them the ability to read minds (18). Tendrils are outgrowths from the more mysterious regions of the brain — these areas are underdeveloped in humans, though they nonetheless seem to grant vague and murky semi-telepathic powers that sporadically grant intuition and suchlike (84). Tendrils often wave slightly when they detect thought waves (18).
Slans are 2-3 times more intelligent than humans (24) and mature at a faster rate — a 9 year old slan has the mind of a 15 year old human (5), and a 17 year old possess the mind of a 30 year old (68). Slans have superb photographic memories, and they are able to memorize large amounts of data in a short period of time, with perfect recall (131).
They possess phenomenal stamina and strength, which is attributed to the neural connections to the muscles rather than the muscles themselves which are identical to humans' (84). Even a slan child can outrun an adult human male (10). Their strong hands and fingers allow them to climb walls (50, 92). At age 15, Jommy was able to climb 30 stories with Granny on his back. Doing this, however, pushed him to his limits (89).
Slans can go for long periods without sleep (154). When ill or injured they go into a deep sleep to allow the body to repair itself. During this time their subconscious can detect nearby danger and awaken to deal with the emergency (39). When slans are sick they become highly emotional and often think and act without due caution (40, 46).
A slan's average lifespan is 150 years (79). Owing to their longevity and intelligence, they take the long-term view with their plans, some of which can take over a century to run their course (118). Generally, slans are not considered fully mature at 23, but hardship can accelerate the maturation process (164). Slan adults are more alert and possess greater telepathic skill than their children (6, 147).
Attempts to crossbreed humans and slans result in failure. Such attempts have been made more than 100 known times over the centuries (78-79, 84-85).
Genetically engineered offshoots of true slans, initially created as a means of hiding their true nature from hostile humans (186). They are identical to true slans, only without the tendrils (47). They therefore lack the slans' telepathic abilities (46, 96) while still being stronger and more intelligent than humans (96-7). The tendril gene was designed to lay dormant for centuries, and is scheduled to show up in tendrilless offspring within 50 years of the story's end (186-7).
They are very advanced technologically, possessing weapons and ships far more sophisticated than humans (100, 148-150). As well as a complete dominance over most of space within the solar system, they have explored the surfaces of the Moon, Mars, and Venus and gone as far out as Jupiter's moon, all without encountering any indication of true slans or extraterrestrial life (100). They possess heavy weapons so powerful they could reduce even their own mighty spaceships to dust (117). They maintain heavy patrols within the solar system at all times, vigilant for signs of the slans. Such is their fear that they have extensively mined areas of space near Earth (126).
Although they do not possess telepathic abilities, they are capable of using effective shields around their minds which can be raised and lowered at will to prevent their minds being read (93, 98). The cunning tendrilless slans can use this to their advantage, however, by leaking specific thoughts to deceive true slans (97). They have compensated for their lack of telepathic abilities by honing the science of psychology to an extreme degree (99). They used this extensive knowledge in their many newspapers; the subtle difference between tendrilless and human styles was detectable (124).
The tendrilless also have a very efficient system for physical descriptions; within a matter of seconds they can catalog a person's precise features and relay that information on (145).
Both the true slans and tendrilless slans are ruled by their own council (101, 116).
Although their great city of Cimmerium is on Mars, they secretly control much of the human world (116, 123), including the Air Center (49), all channels of communication (146, 183), numerous popular newspapers (124), and important positions in the government (145). The tendrilless go to great lengths to conceal their activities from humanity, though they do take the occasional risk when forced to (112, 120, 149). So powerful and vast were the tendrilless' capabilities that they estimated all of Earth's defenses could be wiped away in a mere two months of warfare (116). Once openly in control, they would enslave mankind (117).
While true slans are generous and altruistic even in the face of relentless hatred (140), the tendrilless are conniving, ruthless, and xenophobic (101). Jommy idly speculated that their lack of telepathic abilities caused their minds to be closed and secret to others, just as human minds are (111). Deceit and selfishness therefore have free reign; free contact between minds may by its very nature ensure honesty and consideration for others.