Very little is known about a Gonarch's life cycle. What is known is that a Gonarch is the last stage of a Headcrab's life and that very few Headcrabs reach this stage. While appearing very similar to a Headcrab's anatomy there are a few differences. Gonarchs are supported by four razor sharp limbs. Standing at several meters tall, they dominate Gordon in size and power. Gonarchs are easily one of the largest and toughest known creatures from Xen. Heavily armored with an exoskeleton, they can take massive amounts of projectile and explosive damage, taking even more explosive damage to kill than a Gargantua.

The large bag of flesh which hangs from the creature, commonly referred to as its "sac", seems to be a reproductive housing. During the battle with Gordon, it releases underdeveloped baby Headcrabs at a rapid rate. These baby Headcrabs, while mostly harmless, can be greatly troublesome en masse. Nothing is currently known about how Gonarchs reproduce. While Gonarchs appear to be mindless breeding machines, there is some evidence that they can feel emotion of some kind. When one of its offspring is killed, it will make an unusual and sad-sounding cry, suggesting it mourns for its dead young. Gonarchs are known to make other noises as well--most notably, when they are angry or injured, they will emit a loud cry, very similar to that of an elephant.

Just as with Headcrabs, Gonarchs seem to have an apparent lack of sensory organs, which suggests that their movement is based on touch and sensing vibrations. During the attack on Gordon by the Gonarch known as "Big Mama", it never comes close to running off the edge of its platform. This suggests that Gonarchs may retain memories of their surroundings, or perhaps lay scent markers to warn themselves of obstacles or edges. Even without any apparent sensory organs, Big Mama manages to home in on Gordon within seconds, further suggesting that perhaps vibrations and scents play a key role in a Gonarch's life (much like a spider which lays webbing and feels vibrations from the web to catch prey). Gonarchs also have an apparent lack of a "head" structure, suggesting that any brain may be burrowed deep within the armored exoskeleton for further protection. While no Gonarch has been observed ingesting any material, it does not appear to have a mouth of any sort, which brings into question its diet and method of eating. But it might be that the headcrab eats a lot before reaching the Gonarch stage, so that the Gonarch will not eat and use the nutrients from the food to feed the underdeveloped headcrabs and it will die of malnutrition.

Entering a Gonarch's lair results in immediate attack, suggesting that they are highly territorial creatures, that they are highly protective of their young, or both. They attack by charging at their victim, then attempting to puncture them with their massive legs. They also have somewhat of a ranged attack, by forcing yellowish balls of acidic material from the top of their shells, which they hurl through the air like mortars, raining down upon the victim. With all their traits combined, Gonarchs prove to be skilled at defending themselves and their young, and are one of the toughest enemies Gordon faces in Xen. Upon death, a Gonarch explodes violently.


Within Valve Software, the creature is known as "Big Momma" (cf. its model name, big_mom.mdl), a name that was given to it early on by staff developing Half-Life. Regarding the species name, one theory is that it is a portmanteau of the words "gonad" and "monarch". "Gonad" stems from the resemblance of the large sac that hangs beneath the Gonarch to a scrotum, while "monarch" references the Gonarch's role as a "king" (= boss) enemy. This theory is supported by the book Half-Life 2: Raising the Bar, in which the following appears :

"Sometimes we're just sitting around and we're tired and somebody comes up with a goofy idea like, "Why don't we put a giant testicle on a 20-foot-tall armored spider?"
―Gabe Newell

It is unknown if any Gonarchs were present on Earth after it was conquered by the Combine, due to the fact that it is unknown how long it takes a Headcrab to mature into a Gonarch or if any were teleported during the portal storms. It is also unknown what happens to whatever host it uses if it has one.


Main article: Combine Big Momma Pod

In the early stages of the game development, the "Combine Big Momma Pod", also known as the "Birther", was a device used to create Headcrabs.

Although the Gonarch is the final phase of the standard Headcrab evolution, none are seen in Half-Life 2.