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Nipple dimensions and reproductive status of northeastern Minnesota female black bears (Ursus americanus).


Knowledge about the reproductive status of individual wild animals is essential for faunal research on life history or population dynamics and for wildlife management. Nipple dimensions have been used with varying degrees of success to estimate reproductive status for wolves (Canis lupus) (Mech et al., 1993), fisher (Martes pennanti) (Frost, 1994), white-tailed deer (Odocoileus virginianus) (Sauer and Severinghaus, 1977) and small mammals (McCravy and Rose, 1992).

Beck (1991) discriminated between parous (adult) and nulliparous (cubs, yearlings, and subadult) black bears using the sum of diameters of the four anteriormost nipples. He did not report any relationship between nipple width and lactation, relying instead on nipple pigmentation as evidence of active or past suckling, though he described the use of nipple pigmentation as accurate only for experienced observers. Additionally, nipple pigmentation only separates nulliparous from parous bears, but does not indicate whether the bear is actively lactating (i.e., with cubs) at the time of measurement. In this paper, we examine the relationship between nipple dimensions and reproductive status of Minnesota black bears.


Nipple dimensions were taken on female black bears captured on the Superior National Forest in northeastern Minnesota between 1969 and 1982 as part of a long-term, comprehensive study of black bear ecology. The study area and bear capture techniques were described in Rogers (1987).

The width and length of one of the anterior four nipples were measured to the nearest 0.1 mm using calipers. A nipple volume index (NVI) was calculated as: {[[(width/2).sup.2]] X length}. NVI was transformed using the natural log for analyses.

Reproductive status was assigned into one of seven categories at the time of capture, based on knowledge of reproductive condition of the individual bear gained from repeated observations over the course of the study (Rogers, 1987). Reproductive status categories from field captures were collapsed into three reproductive classes following Mech et al. (1993). Nulliparous bears had never produced cubs and were principally yearlings, but included some subadults (2- or 3-yr old) and adults. Parous, nonlactating bears had produced cubs in past years but none in the current year and were unaccompanied by cubs. Parous, lactating bears had produced their first or subsequent litter in the current year and were accompanied by nursing cubs.

Differences in nipple dimensions and volume were tested using analysis of variance. Pairwise comparisons of treatment means were made using a Tukey post hoc test, with significance corrected for multiple comparisons. These procedures were done using SYSTAT (Wilkinson, 1990). Sokal and Rohlf's (1981) single-sample comparison and appropriate sample statistics were used to calculate classification cut-points for reproductive classes from nipple dimensions or volume (Mech et al., 1993).


Seventy-two female black bears were captured [greater than or equal to] 1 time, for a total of 265 usable measurements of nipple dimensions. All measurements of yearlings (n = 9) and subadults (n = 67) were classed as nulliparous, as were an additional 45 measurements of adult bears. All parous bears were classed as adults. Nipple measurements of parous bears were taken 78 times on nonlactating and 66 times on lactating individuals.

Nipple dimensions and NVI for nulliparous black bears remained relatively constant over the calendar year, with volume averaging [less than]0.1 cc in all months [ILLUSTRATION FOR FIGURE 1 OMITTED]. Nipple dimensions and volume for parous lactating bears decreased throughout the spring months; increased through the summer months and remained large, except for a dip in September, through October; and then decreased. Nipple dimensions and volume of parous nonlactating bears peaked in April, and then generally declined until December when they started to increase. Nipple dimensions and volume of parous, nonlactating bears were generally intermediate between nulliparous and parous lactating bears over the calendar year.

Nipples differed significantly in length, width and volume by reproductive class (Table 1). Nipples were progressively longer and wider, and consequently had increased volume, between nulliparous and [TABULAR DATA FOR TABLE 1 OMITTED] parous, nonlactating bears and between nonlactating and lactating parous bears. Nipple length differed significantly in pairwise comparisons between nulliparous and both classes of parous bears, but not between nonlactating and lactating, parous bears. Nipple width and volume differed significantly in all pairwise comparisons by reproductive class.

The differences in nipple dimensions between nulliparous and parous bears were used to construct a classification table to identify a bear of unknown reproductive status (Table 2). A bear with a nipple length of [less than]8.1 mm was classified as nulliparous or [greater than]8.1 mm as parous with 90% confidence. Using nipple width, the full separation of nulliparous and parous with 85% confidence occurs at 7.2 mm. We were unable to develop a classification criterion for nonlactating and lactating parous bears with [greater than or equal to]75% confidence.


Consistent with most other studies of the relationship between nipple dimensions and reproductive status (Mech et al., 1993; Frost, 1994), nipple length, width and volume of northeastern Minnesota black bears differed significantly between nulliparous bears and parous bears (Table 1). Nipple width and calculated volume, but not length, differed between nonlactating and lactating parous bears.

Nipple dimensions of nulliparous black bears from northeastern Minnesota were consistently smaller than parous bears, with NVI averaging less than 0.1 cc throughout the calendar year [ILLUSTRATION FOR FIGURE 1 OMITTED]. Nipple dimensions for parous nonlactating bears were generally similar to those of parous lactating bears during the 1st half of the calendar year. Nipple dimensions of parous lactating bears were then larger than those of nonlactating bears during the 2nd half of the year. By the end of the year and the return to hibernation, nipple dimensions of parous bears that had been lactating decreased as cubs were weaned and nipple dimensions of nonlactating bears were increasing as they entered the later months of pregnancy.

Nipple dimensions can be used with considerable confidence to separate nulliparous from parous black bears and classify a bear of unknown reproductive status (Table 2). Nipple pigmentation has also been used for discriminating nulliparous and parous bears (Beck, 1991). Pigmentation was not recorded in this study, so its relationship to reproductive status and nipple dimensions could not be assessed.

While differences in nipple dimensions between nonlactating and lactating, parous bears were often significant, they could not be used with much assurance to classify bears of unknown reproductive status (Table 2). There was considerable overlap in classification cut-off points between nonlactating and lactating, parous bears, especially. at the 95% level of confidence, and even at the 75% level of confidence.

Acknowledgments. - Our thanks to L. L. Rogers and the many field assistants who collected these nipple dimension data over the years of this study. R. M. Degraaf, E. C. Hellgren, R. M. Pace, M. R. Vaughan and three anonymous reviewers provided constructive reviews of the draft manuscript.



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ROBERT T. BROOKS, U.S. Forest Service, Northeastern Forest Experiment Station, Amherst, Massachussetts 01003 and RONALD McROBERTS, U.S. Forest Service, North Central Forest Experiment Station, St. Paul, Minnesota 55108.
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Author:Brooks, Robert T.
Publication:The American Midland Naturalist
Date:Jan 1, 1997
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