Things I did not know

Animals raised in captivity under optimum conditions -- Caribou (McEwan and wood, 1966), black-tailed deer (Woods et al. 1962), and whitetailed deer (French et al. 1955; Magruder et al. 1957) -- still underwent a reduction in food-consumption during the winter months, with the retardation of growth in young deer and the loss of weight in adults.

That's from Ronald Skog's 1968 Thesis, which I'm re-reading portions of. My immediate question is... why? Why should animals under nice, warm conditions being fed ad-lib lose weight? Is it because they turn off their digestive track? If so, why eat? Very curious