ARIZONA ANTELOPE FOUNDATION

             PRONGHORN HORN SHEATH GROWTH, AGE, AND PRECIPITATION ON
A RANCH IN SOUTHERN NEW MEXICO

DAVID E. BROWN, Life Sciences Department, Arizona State University, P. O. Box 87501, Tempe,AZ 85287

WILLIAM F. FAGAN, Department of Biology, University of Maryland, College Park, MD 20742

BEAU TURNER, Turner Foundation, Inc., One CNN Center, Suite 1090, South Tower, Atlanta, GA 30303

   Abstract: Pronghorn horn sheath length and mass decreased with age for a population in a semiarid area of southern New Mexico. Horns of bucks 7 >years were significantly smaller than younger animals (P<0.03). Considering the effects of pronghorn age alone, sheaths of hunted animals decreased by an average of 0.28 cm (0.11 in) of length and 0.53 Boone and Crockett (B&C) points for every year of age.  Winter rain (October through March) during the year prior to a hunt exerted a significant negative effect on B&C scores but not on sheath length.  For B&C scores, age remained a significant predictor after the negative effects of increased winter rainfall had been accounted for statistically.

Key Words: Pronghorn, horn size, pronghorn age, precipitation and horn development.

    The Southwest, especially Arizona, is well known for the trophy quality of its pronghorn. Conventional wisdom has been that horn growth and development is a function of age, nutrition, genetics, and possibly the length of the growing season. Biologists have generally assumed that maximum horn growth occurs during the first 4 or 5 years, after which the size of the deciduous sheaths increases at a slower rate (see e.g., Kitchen and O’Gara 1982). This view was questioned, however, in 1985 when a new record book pronghorn was taken in Arizona. The cementum annuli of this buck’s incisors indicated him to be only 3-4 years of age, even though the buck measured 94.6 B&C points (Lewis 2000).

    Only three studies of pronghorn horn growth have been conducted, the most recent being on the Fort Belknap Indian Reservation in Montana in which 85 animals were aged over an 8 year period (Mitchell and Maher 2001). Those authors found that pronghorn attained adult-sized horns when 2 or 3 years old, and that these two age classes contained the largest horn measurements. To gain a better understanding of the relationship between pronghorn age and horn growth in the Southwest, we measured and aged a series of bucks taken in southern New Mexico between 1994 and 2002.

 STUDY AREA

    Our study area was the privately owned Armendaris Ranch, a former land grant, now managed by the Turner Corporation. Most of the ranch resides at elevations between 1375 and 1525 m, and the mean annual precipitation is < 25 cm.  Approximately 74,600 ha (ca. 52%) of the ranch are classified as pronghorn habitat, in which the primary vegetation is semidesert grassland populated by such grasses and shrubs as black grama (Bouteloua eriopoda) and palmilla (Yucca elata) (Brown 1994). Most of the remaining vegetation is Chihuahuan desertscrub and the climate is warm-temperate with an average of 213 frost-free days per year (Truth or Consequences, NM). Bison (Bison bison) are the only permitted grazing animals, other large herbivores being restricted to free-roaming gemsbok (Oryx gazella), mule deer (Odocoileus hemionus), and desert bighorn sheep (Ovis canadensis). Most wildfires are allowed to burn.

   The pronghorn population is subject to climate-induced variation but was estimated to number more than 1,000 animals in 2000. Pronghorn hunting on the Armendaris Ranch since 1994 has been extremely conservative with the combined harvest of archers in late August and rifle hunters in September never exceeding 20 bucks in any given year (Table 1). Hunters are guided to the best areas and hunt success is usually 100% with hunters allowed to take as large an animal as desired.

 

METHODS

   Hunter-killed pronghorn were measured in inches by two outfitter-guides according to Boone and Crockett record book scoring procedures (Lewis 2000). Of the measurements taken, we only used the length of the longest horn sheath and the total Boone and Crockett score, the latter to provide a measurement of horn sheath mass. Incisors were taken from each pronghorn harvested and sent in a labeled manila envelope to Matson’s Laboratory in Milltown,Montana, where the teeth were sectioned and aged according to the number of cementum annuli (McCutchen 1969). Although the animals selected and the horn measurements were biased in that hunters usually took the largest animal available to them, this bias tended to be the same each year. Also, because the numbers of animals harvested was highly conservative, we assumed that the harvest of individual bucks in any given year had little impact on either the selection or size of animals taken in subsequent years.

   We quantified horn sheath growth in two ways, “Horn Length” and “Boone and Crockett Score” (BCS), and conducted all of the analyses described below independently for each measure of horn size. We considered two periods of rainfall that might conceivably influence horn sheath growth: 1) “Summer Rain” (April--September) immediately prior to the hunting season in which the animals were killed, and 2) “Winter Rain” (October--March prior to the hunting season (Brown et al. 2002).  We also built a composite variable by summing the two rain variables (yielding total rain in the full year prior to hunting season) for use in other analyses.

   Initially, we treated pronghorn age as a continuous variable and used an analysis of variance or ANOVA to determine mean horn length and BCS differences between years and age classes at a significance level of P <0.05.  However, a parallel effort in which we treated pronghorn age as a series of binary categorical variables, aggregating individuals at or below a particular age threshold, and contrasting their horn growth patterns with those of all older individuals, yielded entirely comparable results.

   Relationships between rainfall and pronghorn horn growth were assessed using multiple regression (using GLM, Systat).  Our starting point in all analyses was a fully saturated model with both seasonal rain variables and pronghorn age as predictor variables.  If any predictor variables were not significant, we then used backward stepwise elimination to reduce the GLM model  (one variable at a time) to one in which all of the remaining predictor variables were significant.  We also tested for the effects of pronghorn age ignoring any contributions from rainfall and for any systematic change over time (i.e., effects of year sequence independent of age or rainfall)

RESULTS

   Data were obtained on 100 pronghorn over a 9-year period (Table 1). Annual sample sizes varied from one in 1996 to 20 in 2000. Ages ranged from 2 to 14 with all year-classes being represented except 13 year-olds. Horn lengths varied from 33.02 to 42.81 cm (13.00 to 17.13) in (horn length data were not kept in 1994) and BCS ranged from 63.75 to 89.63 points with a mean of 78.12 (Table 1). Twenty-three of the animals had a BCS of 80 points or more; only 4 of these bucks were >6-years old.

   As was expected, pronghorn horn sizes and BCS varied by year and by age. Both measurements tracked closely together and showed that bucks 7 years and older were significantly smaller than animals between the ages of 2 and 6 (P<0.001 for sheath length, P=0.023 for BCS; Figure 1).   

Table 1. Pronghorn age and horn measurements in inches for Armendaris Ranch, 1994-2002

 

 

 

No. of

 

 

 

 

 

 

 

 

 

 

 

 

 

Mean Horn

Mean B&C1

Year

Bucks

2 yrs

3 yrs

4 yrs

5 yrs

6 yrs

7 yrs

8 yrs

9yrs

10 yrs

11 yrs

12 yrs

13 yrs

14 yrs

Length

Score

1994

6

 

2

4

 

 

 

 

 

 

 

 

 

 

 

74.75

1995

8

 

 

6

2

 

 

 

 

 

 

 

 

 

15.11

76.15

1996

1

 

 

 

 

1

 

 

 

 

 

 

 

 

15.13

77.77

1997

10

 

 

1

3

2

1

 

1

1

 

 

 

1

15.75

81.93

1998

12

 

2

2

 

3

1

 

2

1

 

1

 

 

15.34

78.2

1999

9

1

 

1

3

2

 

1

 

1

 

 

 

 

15.56

80.42

2000

20

1

3

5

6

 

2

1

 

 

1

1

 

 

15.39

81.56

2001

17

 

1

5

1

3

 

2

1

2

2

 

 

 

15.02

74.17

2002

17

2

 

4

6

2

1

1

 

1

 

 

 

 

14.93

76.7

Total

100

4

8

28

21

13

5

5

4

6

3

2

 

1

 

 

Mean B&C

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Score

 

76.7

79.9

77.3

79.7

78.5

77.6

78.1

74.2

74.3

72.3

70.9

 

76.7

 

 

Mean Horn

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Length2

15.9

16.2

15.3

15.6

15.7

14.6

15.0

14.7

14.6

14.4

14.0

 

16.4

 

 

1B & C: Boone and Crockett Score
2Excludes individuals from 1994

   Ignoring any effects of rainfall, pronghorn age was a significant predictor of both sheath length and BCS in regression analyses (P = 0.005 and P = 0.009, respectively, Fig. 1).  On average, pronghorn sheaths from harvested bucks decreased by 0.11 in of length and 0.53 B&C points for every year of age (Fig. 2). 

 


Figure 1. Mean (+/- SE) maximum horn sheath length and BCS by age class for pronghorns taken on the Armendaris Ranch in southern New Mexico 1994-2002.


Figure 2.  Pronghorn BCS as a function of age for bucks taken on the Armendaris Ranch in southern New Mexico 1994-2002.

When the effects of rainfall (see Brown et al. 2002) were examined, multiple regression analyses using backward stepwise elimination resulted in a model for BCS that included both winter rainfall and age as significant predictors (P < 0.02 and P <0.03, respectively). On average, every inch of winter rain decreased pronghorn BCS by 0.63 points whereas every year of age >2 corresponded to a 0.44 decrease in BCS.   For horn sheath lengths, we detected no significant effect of any rainfall pattern and only age was a significant predictor (P < 0.006).  On average, every year of age corresponded to a 0.28 cm (0.11 inches) decrease in horn sheath length.  We found no significant effects of year sequence in any analysis.

 DISCUSSION

   Based upon BCS from hunter samples, the mean size of pronghorn horn sheaths decreased with age.  Horn sheaths of animals at least 7 years old were significantly smaller than those of animals aged 2 to 6. At least some animals attained large horn sheaths at an early age (2 to 3 yrs). This decline in horn sheath length and mass in older animals is in sharp contrast to such bovids as bison and bighorn sheep that do not shed their horn sheaths (Kitchen and O”Gara 1982).

  

 A possible concern with the methodology of this study is that the animals used to determine the dependence of horn size on age were not a random sample.  Instead they were harvested, at least in part, because they possessed large horns. Theoretically, this kind of repeated harvesting bias in favor of pronghorn with large horns could generate a relationship where horn size decreased with age.  In other words, if animals within a population kept the same relative horn sizes from one year to the next, and if the largest animals were killed each year, then the animals that reached old age would tend to be those with smaller horns.  Such selectivity may contribute to the patterns we document, but, for several reasons, we do not believe they account for the entire relationship.  First, the Armendaris Ranch population is hunted conservatively, with only a small fraction of the animals killed in a given year. Second, due to herd dispersion and variable encounters with hunters, size selectivity will be imperfect, with many large-horned individuals of a given age surviving to the next season. Third, we believe that horn size is likely to vary within individuals across years due to the vagaries of resource acquisition; and Fourth, there is no evidence of declining horn length or BC scores over time (Lewis 2000, this study).

   Because maximum horn size can be attained at an early age, and if bucks > 6 years of age exhibit declining horn sheath measurements, there would be little incentive to manage for older age animals if trophy bucks are a primary management goal. Nonetheless, it remains to be seen if a population consisting primarily of young pronghorn bucks is a desirable management goal from the standpoint of population growth or behavior in that the influence of pronghorn age on social status and reproductive function is not yet fully known (Lee et al. 1998).

    Horn sizes also varied as a function of rainfall as they did in Montana (Mitchell and Maher 2001). In particular, there was also a significant negative dependence on precipitation over the preceding October through March, but only for horn sheath length, not the BCS, which is an indicator of mass. The reasons for these relationships are unclear, and appear contradictory. Horn growth presumably takes place from before the time that the old horn sheaths are shed in November until some time the following summer, perhaps even up until the time that the buck is harvested. Hence, the preceding winter rainfall and the preceding summer precipitation should both have a positive effect on the production of nutritious forage with a corresponding increase in horn growth and size. The negative effect of rainfall on horn growth is therefore something of an enigma, because winter rainfall has been shown to have a positive effect on fawn survival on the Armendaris Ranch and elsewhere (Brown et al. 2002). We will continue this study in future years to determine if this negative relationship is true or merely an artifact.

ACKNOWLEDGEMENTS

Tom Waddell of the Armendaris Ranch was instrumental in the inception, design and conduct of the study. Tom also maintained files on all of the precipitation and horn measurement data. Neil Lawson and Shawn Russell, of the Armendaris Ranch measured most of the animals and Jennifer Elaine Brown, Phoenix, AZ, Alicia McKee, ArizonaStateUniversity, Tempe, AZ,  and David Verhelst, Tempe, AZ, assisted with the data analysis.

LITERATURE CITED

BROWN, D. E., ed. 1994. Biotic communities: southwestern United States and northwestern Mexico. University Utah Press, Salt Lake City.

BROWN, D. E., W. F. FAGAN, R. M. LEE, H. G. SHAW, and B. TURNER. 2002.Winter precipitation and pronghorn fawn survival in the Southwest. Proc. 20th Pronghorn Pronghorn Workshop, Kearney, NB.

KITCHEN, D.W. and B.W. O’GARA. 1982. Pronghorn (Antilocapra americana) Pp. 960-971 in J. A. Chapman and G. A. Feldharner, editors. Wild mammals of North America: biology, management, and economics. JohnsHopkinsUniversity, BaltimoreMD.

LEE, R. M., J. D. YOAKUM, B.W. O’GARA, T. M. POJAR and R. M. OCKENFELS, eds. 1998.  Pronghorn management guide. Pronghorn Pronghorn Workshop, Arizona Game and Fish Department and Arizona Pronghorn Foundation.

LEWIS, N. L., ed. 2000.  Arizona wildlife trophies: millennium edition. Arizona Wildlife Federation, Mesa,AZ.

McCUTCHEN, H. E. 1969. Age determination of pronghorns by the incisor cementum. J. Wildl. Manage. 33:172-175.

MITCHELL, C. D. and C.R. MAHER. 2001. Are horn characteristics related to age in male pronghorns? Wildl. Soc. Bull. 29:908-916