Well, black rats are larger in general than corns, so it makes sense that its bigger than corns of the same general age. As far as metabolism, I had a pair of baby Kisatchi corns/rats that seemed to grow with every feeding and converted food into growth at a much higher rate then same age corns. So, I think its totally possible it just assimilates faster. I have a WC yearling texas rat and WC yearling black rat that are taking hoppers and seem to be "slim" again soon after feeding. However, the yearling corns I have from last years hatch are pretty much the same.

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