Hey Brandon, this is the other Brandon. Head shape and size is one of those things that "sometimes they have them, sometimes not". In all honestly a lot of it depends the actual ssp of boa. I see this more in Boa constrictor constrictor than any other ssp. Especially Iquitos Peruvians and Surinames. Although, occidentalis get huge noggins as well. Another variable is individuality (i.e. genetics). Litter mates have some variance among them. Like big heads, longer bodies or just bigger in girth all together right at birth. Not to ramble on, it's just genetics. Think of it this way, some people are taller than others. No different with boas. They all grow and look differently even if very similar.
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Obviously, females will have a larger head and grow to be a larger snake in general. As far as which sex grows the fastest, hard to say. There's no "timeline" or whatever you want to call it for boa growth. So many variables come into play with growth (again, genetics, feeding frequency, sex, type of prey, ssp., cage temps etc etc). So it's hard to determine which sex would actually grow faster. One would think female, but are they actually growing faster, or just longer? Even though males don't grow as big, does that mean they don't grow as fast? And since males don't get as large, do they reach their peak" faster than a female does, or vice versa? Hard to say.
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Growth rates are pretty vague questions because their is no answer. Every boa grows differently. As a generalization, I honestly think they grow at fairly steady rates with one another (ruling out all that junk that aids in growth).
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I don't know if I answered all of your questions. It's late and I didn't put that much thought into this. But here's a picture to help you out. This is a 3 year old Iquitos Peruvian. As you can see, she's got a good size head in comparison to her body. And one other observation that I forgot to state above is foreshadowing. Boas that display large heads when you, usually means you will get a big adult.
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Hopefully Jimi sees this thread and can add anything I left out.