Water intake and its thermal energy cost in reindeer fed lichen or various protein rations during winter.

Captive reindeer were fed four different rations which supplied equal energy but varying amount of crude protein: lichens (3%), mixed hay plus lichens (10%) and two feed concentrates with medium (12%) or high (18%) protein content. Kinetics of total body water were measured with tritiated water from February to March 1985, and thermal energy cost of daily water intake was estimated. Biological half-time of water was shorter, and daily water intake significantly higher on the hay plus lichens diet and on the medium- or high-protein concentrates diet than on the lichens diet during March 1985. Similar differences were found between reindeer on medium-protein concentrate and on lichens at the end of the corresponding feeding period during April 1986. Daily water inflow was positively correlated with a dietary supply of digestible crude protein (r = 0.916). Thermal energy costs of daily water intake were highest 1.9 MJ (3.7 l-1) in reindeer on high-protein concentrate during March 1985, and 2.0 MJ (3.9 l-1) in reindeer on medium-protein concentrate during April 1986. The reindeer fed on lichens had minimal and nearly twice as small thermal energy cost of daily water intake (mean 1.1 MJ 2.1 l-1) as on either of the concentrates. Our results show that even a moderate feed protein ration can significantly increase free water intake and its thermal energy cost in reindeer as compared to dominant natural feed (lichens) during winter. Careful protein supplementation is recommended to support body condition with a concomitant addition of easily soluble carbohydrates to compensate for increased thermal costs of water intake.