This metric ("m") is natural in several ways:

let A and B be individuals. Let everything else be equal between the individuals unless otherwise stated and let us use the (b) metric unless otherwise stated. Then the metric is natural is the sense that:

1) if A lives longer than B, then m(A)>m(B)

2) if A does more work / consumes more energy than B, without stealing it from a more efficient consumer then m(A)>m(B)

3) (if we're counting descendents too) if A has more offsping than B then m(A)>m(B)

4) if A is bigger than B then generally 2) is implied

5) if A does not kill capriciously, then m(A)>m(B)

6) if A recycles waste and uses the extra energy then m(A)>m(B)

7) if A does not "burn down the forests" without what most people consider good cause, then m(A)>m(B)

8) descendents set up solar panels on mars or otherwise tap new

energy sources: then m(A)>m(B)

9) descendents spread out geographically, otherwise act the same: then m(A)>m(B)