I'm making stuff up :P but in a fandom this old there’s gotta be convergent headcannoning going on somewhere.
Headcrabs in HL are a wild-type spawned from the headcrab brood mother on Xen. All headcrabs are reproductive and reproduce mostly via parthenogenesis, with variety arising from random expression of mutations or deliberate genetic tampering. Brood mothers can devour smaller crabs and 'import' their genetic material for additional diversity, but there's no mechanism akin to sexual reproduction in their life history. They never stop growing if left to their own devices in a place with plenty of food, but they reproduce in proportion to their size and age. So a brood mother can spawn vast quantities of more crabs, while the little ones in the vents might only lay one or two eggs per host body.
The lifecycle goes Crab hatching feeding on insects -> mature crab searching for host body -> finds live host body and latches onto cranium -> co-opts nervous system and uses body for defense and to acquire additional nutrients -> when a threshold of stored nutrition is reached, pilot the body to a sheltered location, lay eggs in body cavity and detach -> begin search for fresh host body. Once a certain physical size is attained, a brood mother no longer requires a host body and can both protect itself and devour nearby life forms to acquire nutrients.
If no additional nutrition is available or there's a population threshold reached, crabs will go into a dormant phase and wait for either population drop or additional nutrition to become available (i.e. living bodies in proximity). They can access limited, primitive body functions of the host, so the quality, age and skillset of the host body can influence how successful the crab is at acquiring nutrition and reproducing. They demonstrate some selection based on host body fitness and defensive capabilities.
Crabs with specific traits or mutations or crabs genetically engineered by the Combine for military purposes can use host bodies in different and often novel ways. They are highly adaptable but a body must have a base level of cognitive function to be co-opted successfully. A crab can take a human body and most primates, and would probably have success with dolphins and similar species if it could acquire them, but can't take anything smaller or of a lower cognitive level than a mid-size primate. That said, there are legends about head-crab wolves, lions and bears being encountered in particularly remote areas, where they have successfully adapted to a lack of their preferred human hosts.
When Dr. Kleiner makes head-humping comments he’s being a physicist, not a biologist; Lamarr is actually trying to get a grip to pierce the skull and lay eggs. Not that that’s any better.
