One aspect of A:C i thoroughly enjoyed was the introduction of the Neomorph as a perfect foil to the xenomorph species.
The answer to this lies in the Life History traits of each species.
Where the xenomorph's weakness lies, the Neomorph flourishes and visa versa. Together the two species/mutations form the perfect organism.
From an ecologist perspective there are two main types of life history traits in the animal kingdom: r-strategists and K-strategists.
One could argue the Xenomorph is a K-strategist species. They are slow to reproduce (usually.....unless a plot device demands it), reproduce in smaller numbers with a focus on longevity and survival. They are much more developed and formidable than the Neomorph, and represent the apex predator of the 'Morph species.
The xenomorph's main weakness is their altricial offspring. This stage of their development is when they are at their most vulnerable; small, weaker and somewhat defenceless upon birthing. The juvenile chestburster prefers to escape and hide, allowing itself to develop itself into the killing machine it will become....if its not killed during this stage.
The Neomorph on the other hand are classical r-strategists. They develop rapidly with a quick life-cycle. They prefer to overrun and dominate an ecosystem through numbers rather than individual prowess.
HOWEVER, their young are precocial as we see in A:C. They are born capable, independent and ready to defend themselves violently if threatened.
When invading a new ecosystem the Neomorph can rapidly reproduce and thrive with their form of aggressive violence even upon birth. They will quickly out-compete all indigenous species and pave the way for the apex predator Xenomorph to grow; a species whose juveniles might be picked off in a new ecosystem.
So perhaps we are interpreting David's perfect creation wrong. The two species combined form a perfect invasive species, not one on their own, as they both have strength's and weaknesses.
I appreciate A:C a great deal due to the contrasting Xeno species portrayed in the film. For what the film is, there is a great deal of subtle concepts in this film that people need to take the time to understand, you will find many riches.
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