The Kharaa will work together to defend their hives, or attack a large force or well defended base. Otherwise, it's each Kharaa for itself, gestating, evolving, and behaving according to their own objectives. Hit-and-run, ambush, and blitz tactics are common - but just as often, a Kharaa will bulk up and go toe-to-toe with any foe it can find. But for the unifying presence of the bacterium and hivesight, they would be little more than aggressive (and cunning) animals.
-Trans System Authority "Frontiersman" Manual
The Kharaa are the alien race in the Half-life mod Natural Selection. Since Natural Selection is a multiplayer-only mod, both human and alien teams are played by the players. Playing as the very alien Kharaa is a very interesting an unique experience, both because of the way teamwork is forced, and from the way organization is intuitive. Furthermore, the Natural Selection development team has come up with a large amount of very atmospheric background documents that have been placed in the manual that really give life to the world, and these aliens. Everything just comes together in a way that... fits. Kharaa play is based around five key points: Hives, Resources, Evolutions, Territory, and Upgrades.

...the presence of larger life forms (humans) seem to trigger the next stage of alien growth: the Hive. There does not seem to be any malice involved - the bacteria just reacts naturally and aggressively to any competition.
Every game begins with the Kharaa possessing one of three possible hives, at one of three possible hive positions pre-placed in each map. Hives spawn skulks, the basic Kharaa attack form. Every time a player dies, as long as a hive exists, he or she will respawn as a skulk. As a hive can only spawn one player at a time, it is best to collect more hives as quickly as possible. Being able to spawn in players two or three at a time is a great and powerful asset. Hives also strengthen Kharaa, providing better armor absorption, and new abilities as well. Each Kharaa has four possible weapons, one that can always be used, one that requires at least one active hive, one that requires at least two active hives, and one that requires all three hives to be active.

Beyond that, in early versions of the game, hives allowed the Kharaa to advance into higher, deadlier life forms. With two hives came the onset of Fades, and three allowed the mighty and terrible Onos to bear his might upon the marine invaders. Later versions removed this requirement.

An additional hive based restriction is that only one upgrade chamber is allowed per hive. It has been speculated that this restriction will also soon be removed.

The bacterium seems capable of tapping a wide variety of energy sources. Heat and water are diverse enough, but it also finds the rich sludge used for nano-construction highly desirable...

As a result, marines and Kharaa end up fighting for the same resources. Defending these resources is vital to the Kharaa, second only to protecting their hives.

Resources, collected from resource nodes using resource towers, are the currency used in Natural Selection, by both sides. Without resources, all Kharaa are stuck as skulks. Without resources, the Khaara are limited to one hive. Without resources, territory cannot be claimed. Without resources, upgrades cannot be obtained. Nearly every useful tool in the game costs resources, and these resources must be guarded viciously from the marines, who will attempt to steal them from you. Map control can often be thought of in terms of how many resource nodes a side controls at any given moment, as when either team assaults an area, the first thing to be taken down is always the opposing resource tower. If a resource tower is up, that means that section of map is obviously under control... at least for now.

The Kharaa can travel up their own family tree, changing their "race", and evolving extraordinary abilities.
Skulks are pretty weak on the battle field. They're fast, can climb walls, but really they can't take a lot of bullets and they can't attack at range. That's where gestating comes in. Once enough resources have been gathered, any Kharaa (but almost always the skulk) can "evolve" into a Gorge, Lerk, Fade, or Onos. Each species has its ups and down, ins and outs. Any of them, once dead, return to being a skulk, the resources spent on evolving gone.
          Trivia: The skulk was originally named "Bob", more or less as a joke.

Gorges are the builder and support class. They spend the most resources of all the Kharaa, building chambers that allow upgrades, putting up new hives, and constructing defenses or bulkheads. They can also heal other Kharaa, including other gorges, making a large mass of gorges all healing each other a very strong movement indeed. Through in a mix of other races, and the walls will run red.
          Trivia: The gorge was originally named "Pudgy", due to its girth. That name has since morphed into "Fatty", a nickname that is still used by players today.

Lerks are the flying aliens. Early versions had flight so difficult to control that it was used only for finding a perch from which spikes could be rained from a distance-- lerks being the only class with a long range hitscan weapon. But eventually, the code for flight became so intuitive that even a newbie can zip down hallways, biting off the heads of their foes as they attack. Lerks are the most fragile of the aliens, but that's okay. They're the hardest to hit.

Fades are frightening nightmare beasts that look almost human, if it weren't for the huge spikes for hands, the wicked looking fangs, and the symbiote on its shoulder that, given sufficient numbers of hives, can launch acid across the room. In early versions of the game, when Fades showed up, that was it for the marines. In later versions, their acid rockets were nerfed and their teleportation abilities improved, making them into hit and run guerilla troopers. They're expensive, but reasonably beefy, incredibly fast, and almost impossible to kill when in the right hands. Marines these days don't even try killing them, they just want to back the horrifying buggers off for a little while.

Onos are massive tanks. In the original betas of the game, they didn't even have a health bar... they were literally made invulnerable to everything. When an Onos showed up, the game was supposed to be over. They were given health later, to make it possible if unlikely to kill one. And bit by bit, the Onos was nerfed, such that they're hardly the feared beasts they once were. Don't think them useless though. They have massive amounts of health and have the "Stomp" ability, which freezes marines in their tracks, ready to be devoured or killed in other ways.
          Trivia: The Onos was originally named "Tiny", also more or less as a joke. The name Onos is derived from the phrase "Oh No!" which is supposedly the first thing heard over the radio from frontiersmen who witnessed it. Which leads to the question, why isn't the Onos' name actually "Oshi" instead?

As near as we can tell, trace amounts of the bacteria begin to collect in areas containing water or heat sources. The bacteria reproduce, slowly forming a mat, visible to the naked eye. It also seems to direct airborne streams down the hallways and passageways, and anywhere it finds another source of energy (water or heat) it forms another mat, from which it sends out more streams, etcetera. It is safe to assume that every square inch of a facility is rapidly infiltrated with at least trace amounts of bacteria, anchored at source points that become potential hive locations (so far, we have never seen more than three). All of these bacteria are in contact with one another, sharing data on their environment, and triggering responses to stimuli (like energy sources, or danger). They form a network, called the Bacterium.
Map control is crucial in Natural Selection. Being able to freely move from location to location, while denying that same ability to your opponents, is the key to victory. With territory comes resources, and with resources comes both more territory, stronger territory, or bigger ugly sticks with which to smack the enemy in the face.

For Khaara, map control first means hives, second means movement chambers, thirdly means constant reconnaissance, and lastly means fortification. Speed of movement across the map is vital, as, except in a few very buggy versions of the game, structures cannot hold a location for long. Only player controlled aliens can do that. Skulks need to sweep across the map, using their speed to get places, and find out where the marines are going, and then using voice chat or, for those without a microphone (get one! this game really needs all players talking to each other!) text chat, summon the rest of the team to lay the royal smackdown and open the jumbo sized can of whoop-ass.

But sometimes you just need to lay out the resources and put down a few chambers. Sensory networks can provide great support as long as they are well hidden, and offense chambers backed up with defense chambers can provide a much needed speedbump that will slow down marine progress long enough to get your team mobilized and to the area in need of defense. Marines have been known to take advantage of this swarming of Kharaa to set off false alarms on the opposite side of the map from the real assault, but that's why the players really need to communicate with each other. It's that important.

When it was first noticed that the aliens seemed to be modifying themselves over the course of a prolonged engagement - growing more resistant to gunfire, or becoming utterly silent - it was assumed that they were reacting to our actions, and developing immunities much like a virus does. The truth is more complex...

Each of these chambers serve a purpose in the field, but defensive, movement, and sensory chambers play another important role. They act as supplements to the knowledge contained in hives, allowing the Kharaa to specialize themselves even further with entirely new abilities.

Certain life forms simply need a few extra enhancements in order to operate at peak performance. Skulks really benefit from the invisibility that cloaking provides, while gorges love the resource saving ability of redemption. Fades practically need regeneration just to survive in the field and remain effective, and the Onos is able to draw even more fire away from the others as long as he has carapace.
  • Defensive upgrades are most useful for the higher level evolutions Fade and Onos, although they have their uses for all Kharaa.
  • Movement upgrades are useful for all the Kharaa, but the true importance of the chamber is connecting two or more hives together.
  • Sensory upgrades are most useful for skulks. They're only icing on the cake for most everyone else... nice, but hardly required. Furthermore, two out of three sensory upgrades have a direct counter that the marines can use... cloaking is countered by motion tracking, sensor sweeps, and observatories. Focus no longer gives one-hit kills once the marines upgrade their armor a single level. This further limits the sensory chamber's use as the game progresses.
From this, can you figure out the optimal chamber order?

The obvious 2nd chamber to be built is the movement chamber. The upgrades are moderately useful to all evolutions, but most importantly, it enables the Kharaa to rapidly respond to attacks anywhere on the map. A vital utility that can only come into play at the 2nd hive. Building it first would be a waste, and building it last would be too late.

Which leaves the question as to what chamber should be built first. Sensory or Defense? At a glance, you might think that sensory should be built first, as its upgrades are useful to the units available at the beginning of the game, and its usefulness expires shortly afterwards. But that would be assuming that the Kharaa is capable, when facing a marine team of equal skill, of claiming all three hives. In practice, this is next to impossible.

Early in any game, the marines are capable of picking up and fortifying a hive. This is generally done almost immediately. In order to dislodge marines from their fortifications, higher level evolutions are needed... Fades and Onos. And Fades and Onos need defense chambers to operate at peak capacity.

Right from the beginning, version 1.0 all the way through to 3.0b4a, the current version at this date, the order of chambers has always been "DMS"... Defense, Movement, Sensory. And because sensory requires the third hive, and the third hive is almost never available in real games, sensory is almost never seen. Furthermore, when three hives are miraculously gained, it is beyond the point of their usefulness, leading many people to believe that the chamber is worthless altogether. And for the most part, they're right.

If speculation is correct, future versions of the game will unlink the "one chamber type per hive" restriction, making it possible to have sensory chambers at the beginning of the game without ruining your Fades' and Onos' chances of breaking a marine installation... because when it's necessary to do so, the defense chambers can be dropped on time.

To end this writeup, I'd like to quote from the official Natural Selection backstory, "Six Days in Sanji" by Jeff Paris.

The crackle of roughly filtered audio came across the comm line, then the sound of people running, one person's breath in particular loud and heavy. A cut, and we heard boots on ladders - from the clangs it sounded like many boots.

Daring typed onto our HUDs: [Daring: Someone named "Magnai" is the recorder in most of these. Can't find more than a few seconds at a time. I'm scanning for speech spikes, something more informative.]

Next, the unmistakable sound of weapons being handed out, ammo inserted, safeties clicked off.

[Daring: They would have had a modest armory, nothing fancy or heavy. Sanjii was not a real target, even for Chinese sabotage.]

More men running, a few terse, frightened words - a rebuke that could only mean "be quiet!" A splice to a new clip, a door whirring open, a few steps, then "Kharaa! Kharaa!" gunfire, inhuman screeches, the disturbingly clear sound of bodies hitting the floor.

[Daring: estimate - four men, non-military, with machine pistols and low-grade rifles. Elapsed time of combat: 4 seconds.]

A new clip: a room, many men and women, rapid-fire discussion. Again, even in Mongol, it was clear: a war room. Desperate plans. Something shatters, "Kharaa!", gunfire, furniture knocked over, screams, cloth and flesh ripping, clips reloading, weapons falling from hands to the floor.

[Daring: it goes on for another fifteen seconds. They are not victorious. Anyone speak Mongol? This "Kharaa" appears again and again..]