Andrea B. S. Swingley and Robert G. White
Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775-7000, U.S.A.

This study provides evidence that calves may exert a behavioral feedback that influences the duration of lactation in muskoxen. This behavioral feedback may represent a mechanism by which muskox females can detect that their calf is undernourished. Muskox females generally produce one calf annually and nurse their calves for approximately 8-9 months. Between 1988 and 1994, six muskox calves at the Large Animal Research Station, Fairbanks, AK were nursed for approximately 18 months; these calves exhibited prolonged nursing. While females nursed their calves for longer than normal, they did not produce another calf. Preliminary examination of the nursing behavior of calves exhibiting prolonged nursing showed differences from the behavior of calves weaned normally (Schulman and White, 1997). While preliminary analysis suggests that behavioral factors may be involved in the determination of the duration of lactation, other factors need to be investigated. The frequency with which a calf returns to the cow in an attempt to receive more milk has not yet been examined. Calves in need of increased nutritional support would be expected to return to the females more frequently in an attempt to nurse. These undernourished calves would also be expected to have lighter body mass than normal calves during periods in which a female might be physiologically responsive to feedback suggesting the calf's deficiency. We also predict that a benefit from this prolonged nursing would be a heavier calf at weaning provided the cow produces sufficient milk.

In order to test these predictions we examined behavioral and body mass data for six pairs of calves. Each calf pair consisted of a calf exhibiting prolonged nursing and a calf weaned at the normal time from the same cow. Behavioral data were collected as described in Schulman and White (1997). We used three measures of nursing behavior: total time spent nursing per day, mean number of nursing attempts per day, and mean daily nursing bout duration. We divided the season into four age categories to partially standardize for calf age, development, variations in food supply, and the presence or absence of a bull. The rutting period was separated into a separate age category. We employed a randomization test to determine differences in behaviors between prolonged and normal calves, and between the group of prolonged calves and the group of normal calves. We combined results from multiple comparisons using a Fisher test for combining tests of significance (Sokal and Rohlf, 1995). Body mass was recorded for each calf at regular intervals beginning at birth and continuing throughout the first three to four years of life. We used the same prolonged-normal pairs as the behavioral analysis to compare calf body mass at birth, rut, the first possible weaning period, and final weaning for the prolonged calves. Milk intake was measured using the doubly labeled water technique (Holleman et al., 1988) three times in summer, once in early winter, and opportunistically for those cows nursing past one year.

Calves exhibiting prolonged nursing had significantly more nursing attempts than normal calves in the two age categories before rut (category 1: p<0.005; category 2: p<0.005). This supports the prediction that calves in need of increased nutritional support return to females more frequently attempting to nurse. These calves also spent more time per day nursing than normal calves from 6 weeks of age until the end of the post-rut period (category 2: p<0.05; category 3: p<0.05; category 4: p<0.001). This suggests that calves in need of increased nutritional support successfully communicated this deficit to the female, who was able to respond with continued milk production. Results from the comparisons of calf body mass and milk intakes are forthcoming. However, in July of their second year of lactation, calves receive on average 340± 70 ml/d of milk, which could explain their greater body mass at that time than weaned yearlings. Regardless of the outcome of the body mass and milk analyses, the behavioral data clearly describe evidence supporting the hypothesis that the calf provides feedback to the dam that may influence the duration of lactation in muskoxen.

Sokal, R.R. and Rohlf, F.J. 1995. Biometry, 3^rd Edition. W.H. Freeman and Company, New York, 87 Pp.

 
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