Growling kitten

[julietbat]

This is possibly a really stupid question but I'm going to ask it anyway...
DP and I went to view a couple of kittens this afternoon (we saw them last week too). They're nearly 8 weeks old and in a home-based rescue centre (so lots of handling and no cages). They're totally adorable and we're 99% sure we want them.
DP is just a little concerned because while they were playing with their toys the little boy kitten growled twice protectively over his 'trapped' toy when his siblings got too close. Now I thought it was really cute and feisty! But DP is just slightly worried that it might mean he will be aggressive as he grows up. In all other respects he's gorgeous - the owners of the rescue centre say he's really cuddly and we've seen him playing really nicely with his brothers and sisters.
Is the growling an indication of an aggressive personality or just a kitten (one of the smallest in the litter) trying to make his mark??