Robert G. White^1*, Karen L. Gerhart¹, Don E. Russell² , Debbie Van De Wetering² , William E. Hauer¹ and Janice E. Rowell^1
1Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775; ²Canadian Wildlife Service, Environment Canada, 91782 Alaska Hwy, Whitehorse, Yukon, Y1A 5B7

In arctic ruminants plane of nutrition is thought to influence fecundity and the duration of lactation (Cameron et al., 1993; Gerhart et al., 1997; White et al., 1997). To determine the extent that nutrition influences population responses it is necessary to determine which females are weaning normally or are undergoing extended lactation. For caribou (Rangifer tarandus granti) we have developed a set of criteria to determine whether a caribou is in full lactation, in the process of weaning or has just weaned her calf, based on milk composition. In this paper we examine the lactation curve for muskoxen (Ovibos moschatus) and apply the criteria developed in caribou to muskoxen to test applicability between species.

The chemical composition of milk was determined by standard analysis on samples taken from the mammary gland by hand stripping. Criteria to determine the reproductive condition of caribou were derived from the analysis of 276 milk samples obtained from females of the porcupine caribou herd (PCH) and the central arctic herd (CAH) in the months of June, September, October and November. Reproductive performance of the PCH was determined over a 3-year period (Gerhart et al., 1997). Muskox milk was obtained opportunistically throughout entire lactation from animals at the Large Animal Research Station (LARS). Through dietary manipulations lactation lasted 9-22 months (White et al., 1997). A test of applicability of the caribou criteria was made on muskoxen of known breeding history.

Milk composition changed markedly with date with a general increase in dry matter (DM), protein (P) and fat (F) and a decline in lactose (L) in November compared with June in both caribou and muskoxen (Luick et al., 1974; Parker et al., 1990). These major changes include a linear increase in P with F and non-linear changes of P with L and L with DM.

Independent of these relations we noted four different populations of milk chemistries in caribou (Types I...IV) following the production of colostrom (Table 1). Through field observations, and those made at LARS, we conclude that for caribou Type I milk typifies that for females in peak milk production, Type II for early weaning, Type III for weaning and Type IV a clear liquid produced immediately after weaning (see Table 1).

For muskoxen, nutritional treatments resulted in extended lactation in 9 of 53 lactation events over a 13-year period (1983-96). Mean duration of normal lactation was 37+ 0.4 (SEM) weeks while extended lactation lasted 78+ 2.4 weeks.

Application of the caribou diagnostic values to muskoxen was not as successful as with caribou, despite of the close interrelations of milk constituents in both species.

Table 1. Caribou milk chemistry in relation to stage of lactation.

The trend in milk composition in terminal lactation in both caribou and muskoxen is typically an increase in P and F associated with a decline in L.. Finally, fat is removed and the remaining clear liquid is rich in N; presumably in the form of amino acids given its sweet taste.

Three to 4 weeks postpartum, 90% of 82 caribou with calves were producing Type I milk and 10% produced Type II. Females that had lost their calves shortly after birth produced Type IV milk. By late September, 98% of 41 females with calves were producing Type II milk.

For caribou, a milk sample taken in October through November can be used to diagnose a female’s stage of the weaning process. Caribou with a calf at foot in November (n=99) were classified as recently weaned (Type IV; 15.1%), weaning (Type III; 48.5%) and extending lactation (Type II; 36.4%).

Work is continuing on developing a set of criteria for muskoxen that could be used to determine whether a given female will extend lactation.

Cameron, R.D., W.T. Smith, S.G. Fancy, K.L. Gerhart and R.G. White. 1993. Calving success of female caribou in relation to body weight. Can. J. Zool. 71:480-486.

Gerhart, K.L., D.E. Russell, D. van de Wetering, R.G. White, and R.D. Cameron. 1997. Pregnancy of adult caribou (Rangifer tarandus): evidence for lactational infertility. J. Zool., Lond. 242:17-30.

Luick, J.R., R.G. White, A.M. Gau, and R. Jenness. 1974. Compositional changes in the milk secreted by reindeer. I. Gross composition and ash. J. Dairy Sci. 57:1325-1333.

Parker, K.L., R.G. White, M.P. Gillingham and D.F. Holleman. 1990. Comparison of energy metabolism in relation to daily activity and milk consumption by caribou and muskox neonates. Can. J. Zool. 68:106-114.

White, R.G., J.E. Rowell, and W.E. Hauer. 1997. The role of nutrition, body condition and lactation on calving success in muskoxen. J. Zool., Lond. 243:13-20.