Why is energy lost as it moves from producers to primary consumers?

Each consumer is consuming for a number of reasons, only
one of them being physical growth. Calories consumed are turned into body heat, healing,
reproduction, and motion, all of which are necessary life processes, but none of which
retain captured calories in the food chain. An animal must expend calories in order to
obtain food. Those expended calories are not available to be eaten by members of the
next trophic level, so they in essence leave the food
chain.

Ecologists estimate that plants, at the base of most
food chains, convert only about 1% of the sunlight they receive into plant material
which is then available as food. For animals, an average of about 10% of the energy a
creature consumes is retained as additional body mass, which is the only thing that can
be passed up the food chain to the next trophic
level.

Foraging efficiency is the term for how animals
maximize calorie intake versus calories burned to attain food; it is a very interesting
ecological topic. Check out the link below for an example of how it works in a grazing
species, the bighorn sheep of western North America.