The discussion is not a restatement of the results. For example, it is not enough to say that "metabolic rate went up." You must explain why it increased. Relate your results to specific principles (for example, biochemical) underlying your results. Compare your data to similar studies in the literature: i.e. "These data agree with the findings of Spray (1972) who found..." or "Other investigators have reported similar findings (Spray, 1972; Johnson et al. 1971)" to establish whether your results are typical.
You can discuss the significance of your results at different levels. At the biochemical level, what caused the results you observed? At the other end of the scale, an important role of the discussion is to determine how the results reflect the actual operation of the phenomenon in nature. Of what value is it to the organism? Interpret your data rigorously. Do not speculate beyond what the data will support.
If necessary, briefly mention sources of error and problems which may limit the utility of your data if you know they had a great impact on your results, but do not dwell upon them. A discussion section consisting entirely of error analysis is boring and pointless to read.