Comparative Biochemistry and Physiology Part A 127 (2000) 225 – 236 Milk intake in blue fox (Alopex lagopus ) and silver fox (Vulpes ulpes ) cubs in the early suckling period Øystein Ahlstrøm a, *, Søren Wamberg b a Department of Animal Science, Agricultural Uniersity of Norway, P.O. Box 5025, N-1432 A  s, Norway b Department of Physiology and Pharmacology, Institute of Medical Biology, Uniersity of Southern Denmark, DK-5000 Odense C, Denmark Received 21 March 2000; received in revised form 29 July 2000; accepted 17 August 2000 Abstract Milk intake of fox cubs (2–16 days of age; body weight, 96–649 g) in ten blue fox litters and ten silver fox litters were measured by the water isotope dilution (WID) technique following a single intraperitoneal injection of tritiated water ( 3 HHO). Litter size varied from four to 14 in blue foxes and from three to eight in silver foxes. Silver fox cubs had higher birth weights than blue foxes. Inter-species body weights and growth rates were apparently dependent on litter size and the dam’s constitution. In both species growth rate increased with age and body weight (7–35 g per day). In the cubs, the biological half-life of body water turnover (BWT) rose from 1.5 days at 2–3 days of age to 2.5 days at 13–16 days of age, although a considerable scatter was seen. The mean daily milk intake of the cubs varied with body weight, from 31 to 193 g per day, whereas daily milk intake per unit of body mass remained stable at 30–35 g per 100 g body weight. The ratio of milk intake to body weight gain varied considerably among cubs, averaging 4.5 g/g during the 3-week experimental period. In suckling fox cubs, the calculated daily intake of metabolically energy (ME) corresponded fairly with the estimated energy requirements for growth and maintenance of the young. Finally, the applicability and the accuracy of the WID technique was evaluated in ten 3-week-old fox cubs, by tube-feeding with a milk replacer for 48 h, which documented that the daily rates of milk intake and water turnover can be accurately measured in suckling fox cubs by the WID technique following a single injection of 3 HHO. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Blue fox; Silver fox; Body growth; Carnivores; Milk intake; Tritium; Water turnover; Energy requirement www.elsevier.com/locate/cbpa 1. Introduction Blue (Alopex lagopus ) and silver fox (Vulpes ulpes ) are seasonal breeders with only one an- nual breeding cycle. In the Northern hemisphere, mating takes place in February/March in silver foxes and in March/April in blue foxes. The gesta- tion period ranges from 49 to 55 days. The fox cubs of the two species are born in April/May and May/June, respectively. In silver foxes, the litter size is four to six and the cub’s weight at birth is approximately 90 g. In blue foxes, the litter size is eight–12 and the birth weight is about 65 g (Nes et al., 1987). The fox cubs grow rapidly and at the end of postnatal week-3, their birth weight has usually increased 4 – 6-fold. During this period, the cubs rely on mother’s milk as the sole source of nourishment. Therefore, the health status of the dam and her capability of supplying adequate amounts of milk are of crucial importance for the survival of the young. The high energy costs of * Corresponding author. Tel.: +47-64-947970; fax: +47- 64-947960. E-mail address: oystein.ahlstrom@ihf.nlh.no (Ø. Ahlstrøm). 1095-6433/00/$ - see front matter © 2000 Elsevier Science Inc. All rights reserved. PII:S1095-6433(00)00269-5