Antler Size Relative to Body Mass in Moose: Tradeoffs Associated with Reproduction

ANTLER SIZE RELATIVE TO BODY MASS IN MOOSE: TRADEOFFS ASSOCIATED WITH REPRODUCTION

1 2 3 4 Kelley M. Stewart\ R. Terry Bowyer\ John G. Kie · , and William C. Gasaway ·

1lnstitute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA; 2United States Forest Service, Pacific Northwest Research Station, 1401 Gekeler Lane, La Grande, OR 97850, USA; 3Aiaska Department of Fish and Game, Fairbanks, AK 99701, USA

ABSTRACT: Body size and age are highly correlated with antler size, fighting ability, and reproductive success in male cervids. Production of antlers requires energy above that for maintenance ofbasal functions, and is especially demanding of minerals. In addition to producing antlers, young cervids also incur the cost of completing body growth. Large-bodied males with large antlers invest more in antler development and reproduction at the expense of body condition than do young males. Young males are constrained by the need to complete body growth to attain the body size necessary to compete effectively for females when mature and, hence, invest less in antlers. We tested the hypothesis that adult male moose (A/ces alces) produced larger antlers relative to their body mass than did younger males. We used regression to compare the ratio of antler length per unit body mass (antler length: body mass) with age. Regression analysis indicated a strong curvilinear relationship (R.2= 0.961) between antler length per unit body mass and age. Young males invested less in antlers than older males that had reached a sufficient size to compete effectively for mates; consequently, there was a tradeoff between body growth and antler size. Young males must produce antlers to gain experience in aggressive encounters and establish dominance relationships among their cohort, although investment in antlers is less than that of mature adults. Delaying investment in mating until physically mature and able to compete for females with other large-antlered males is the most successful strategy for maximizing mating success and achieving the greatest fitness in male moose, as well as among other cervids.

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Keywords: age, A lees a/ces, antlers, body mass, cervids, mating success, moose, reproduction

Although several hypotheses have been widely among males from polygynous spe forwarded to explain the function of antlers, cies compared with monogamous ones, lead most researchers agree that horn-like struc ing to more intense competition for mates tures evolved as weapons for intraspecific and stronger sexual selection among males combat (Geist 1966, Clutton-Brock 1982, (Darwin 1871, Alexander et at. 1979, Lincoln 1992). Among polygynous cervids, Clutton-Brock 1987, Weckerly 1998). males compete vigorously for access to Body size, antler size, and social domi females during rut, which favors the evolu nance are positively correlated across an tion of sexual dimorphism and large fighting array ofcervids (Anderson and Medin 1969, weapons, including antlers (Geist 1966, Lin McCullough 1969, Bowyer 1986, Van coln 1994, Lundgren 1996, Weckerly 1998). Ballenberghe and Miquelle 1993 ). Clutton The reason often proposed for this associa Brock et at. ( 1982) used regression to com tion between polygyny and sexual dimor pare mean antler length against mean shoul phism is that mating success varies more der height as an index of body size, and

4William C. Gasaway passed away on 15 July 1998 in Stockton, CA, USA. 77 ANTLER SIZE AND BODY MASS - STEWART ET AL. ALCES VOL. 36, 2000 reported a positive relation between in number of points (tines), and total spread creasing antler and body size. Body size from Gasaway et a!. (1987). We used and age are strongly correlated with antler principal component analyses (PCA; SAS size and mating success (McCullough 1979, Institute Inc. 1987) based on correlations Clutton-Brock et a!. 1982). Fighting suc among variables to reduce the dimensionality cess in red deer ( Cervus elaphus) peaked of those data, and obtained a single variable, between 7 and 10 years old, and declined principal component 1 (PC1) that indexed rapidly in older animals (Clutton-Brock et size. Following PCA, we compared antler a!. 1979, 1982). Male reproductive success size, as indexed by PC 1, with age using in white-tailed deer ( Odocoileus regression analysis (Zar 1996). virginianus) and elk (Cervus elaphus) We obtained data on Eurasian moose was greatest after 5 years of age from Prieditis ( 1979) to evaluate the effects (McCullough 1969, 1979; Noyes eta!. 1996). of body mass (kg) and antler size with age Production of antlers, and supporting (years). Because those data did not include their large mass, requires energy above that measurements of all antler characters and for maintenance of basal functions in ma because we did not have access to original ture males, and may be especially demand data, we used mean antler length (em) as ing of minerals (Ullrey 1983 ). Both young our index of antler size. We confirmed that and old-aged animals have less energy to length was a reliable index to size using data invest in antlers because of the necessity of from Gasaway eta!. ( 1987). We employed completing body growth in young animals, multiple regression with an adjusted coeffi 2 and declining body condition with senes cient of determination (Ra ), weighted by cence in old-aged individuals (McCullough sample size (Neter eta!. 1985), to compare 1979, Clutton-Brock et al. 1982, Ullrey the ratio of antler length per unit body mass 1983, Brown 1990). Such relationships, (antler length : body mass) against age. however, have not been demonstrated for moose (Alces alces). We hypothesized RESULTS that antler growth was a lower energetic Principal component analyses (PCA) priority than body growth and maintenance on antler characteristics of Alaskan moose in younger moose than in older ones; thus, (Gasaway et a!. 1987) reduced the extensive investment in the production of dimensionality of those data (Table 1). The antlers was unlikely until adult body mass first principal component (PC 1) explained was achieved. We examined the relation 73% of the variation in antler measure between patterns of antler size with age, ments. Because the eigenvector associ and tested for a correlation between antler ated with the first principal component(PCl) length and overall antler size using data loaded similarly (range 0.30- 0.35) across from Gasaway eta/. ( 1987). We then used all antler characteristics, PC 1 was used in data from Prieditis ( 1979) to test the hy further analyses as an index of overall size pothesis that adult male moose produced of antlers. Regression analyses indicated a larger antlers relative to their body mass curvilinear relationship between antler size than did younger males. (PC1) and age (R}= 0.60, P,s;. 0.00 1, Fig. 1). That analysis indicated large variation in METHODS antler sizes in each age class, although We obtained data on age of adult moose antlers generally increased in size until about in addition to their antler characters includ 10 years old and then declined (Fig. 1). ing right and left palm length, beam length, We used data from Prieditis (1979) to

78 ALCES VOL. 36, 2000 STEW ART ET AL. - ANTLER SIZE AND BODY MASS

Table 1. Summary statistics for age and antler characteristics of 1,501 Alaskan moose from 1968 to 1983; data from Gasaway eta!. ( 1987).

Variable Mean SE

Age (yrs) 5.4 0.08 Total spread (em) 131.3 0.66 Right palm width (em) 26.8 0.21 Left palm width (em) 26.9 0.21 Right palm length (em) 81.2 0.61 Left palm length (em) 81.1 0.61 Right beam length (em) 17.9 0.06 Left beam length (em) 17.9 0.07 Right number of points 8.9 0.07 Left number of points 9.0 0.07

examine the relation between antler size body mass and age (Ra 2 = 0.961, P< 0.0001, per unit body mass with age. Because Fig. 2). We also conducted that analysis 0 75 fewer antler characteristics were available with metabolic body mass (e.g., BM · ), in that data set, we used antler length as an but the relationship did not improve mark index of size; that variable was highly cor edly (Y= 0.09 + O.l6X -0.01X2, R/= 0.969, related (r = 0.91) with our index of antler P.:::; 0.001, n =11). size (PC!) for Alaskan moose. Weighted regression analyses indicated a curvilinear relationship between antler length per unit

40 .------~ 0 0 8 0 Q l5 -..,... 20 (.) e:.. w 0 N Ci5 a: w -20 ...J 1-

~ -40 0 0 0 8 -60 0.0 5.0 10.0 15.0 20.0 AGE (YEARS) Fig. l. Relationship between antler size (Y), indexed by principal component 1 (PC 1), and age of Alaskan moose. Regression model; Y=-49.94+ 14.30X-0.76X;R/=0.60,P<0.001,n= 1,501. Data for age and antler characteristics were obtained from Gasaway eta!. ( 1987). We expected a parabolic fit because senescence of antlers in older males is common.

79 ANTLER SIZE AND BODY MASS - STEW ART ET AL. ALCES VOL. 36, 2000

0.22

~ 0.20 ~ >-c 0.18 0 Ol .. 0.16 ~ CJz 0.14 w ...J a: 0.12

~z 0.10 < 0.08 0.0 2.0 4.0 6.0 8.0 10.0 12.0 AGE (YEARS) Fig. 2. Relationship between the ratio (Y) of antler length (em) to body mass (kg) as compared with age(X) in years for Eurasian moose. Regression model; Y=0.04 + 0.38X-0.002X2; R/= 0.961, P < 0.0001, n = 11, weighted by sample sizes (n ). Means and sample sizes for age, antler length, and body mass were obtained from Prieditis ( 1979).

DISCUSSION of moose antlers was greatest at 9.5 years. Male moose < 4 and 2: 10 years old Fighting success of male red deer peaked invested less in antlers than breeding-age between 7 and l 0 years old and declined males (Figs. 1 and 2). Moose < 4 years old rapidly in males > 11 years (Clutton-Brock had proportionally smaller antlers for their et a!. 1982). Those observations are sup body size than individuals between 5 and l 0 ported by the declining trend in antler sizes years of age (Fig. 2). McCullough ( 1979) in animals > l 0 years of age among male and Clutton-Brock et a!. ( 1982) reported moose in our sample (Fig. 1). that the greatest reproductive success was Genetics, nutrition, and physical condi among adult males that had completed body tion have substantial effects on antler char growth, and our data supported their obser acteristics (French 1956, Gross 1983, Brown vations that mature males invested more in 1990, Hundertmark eta!. 1998). Moreover, antler development than did younger age antler morphometries have been used to classes. Clutton-Brock eta!. (1982) and evaluate the performance of populations of Lincoln ( 1992) reported that prior to 5 years white-tailed deer (McCullough 1982). Ant old, substantial increments in body growth ler development is costly and requires sub were observed in male red deer, whereas stantial investment of resources; absorption rates of growth were slower in older age ofminerals, particularly calcium, from bones classes. Male moose, in Alaska, achieved during antler growth has been observed in maximum body mass at about 8 years of age several species of cervids (Banks et a!. (Schwartz 1998); our data also indicated 1968, Hillmaneta/.1973,Muireta/. l987). that investment in antlers was greatest after Van Ballenberghe (1983) reported that that age (Figs. l and 2). Indeed, Bubenik et moose in Alaska completed antler growth in a!. (1978) noted that shaft circumference a shorter period than most other cervids,

80 ALCES VOL. 36, 2000 STEW ART ET AL. - ANTLER SIZE AND BODY MASS which required high investment in antlers whereas males in the largest antler classes over a relatively short time. This rapid participated in relatively few sparring bouts. growth and potential for absorption of min Investment in antlers at the expense of erals might be especially costly for moose. body development in younger age classes Ullrey ( 1983) reported histological changes may result in decreased reproductive suc in the bones of white-tailed deer and mule cess overall, because those individuals ulti deer (Odocoileus hemionus), and indicated mately may not be able to obtain the body or that minerals were mobilized from bone antler size needed to be competitive for during antler growth in adults. Indeed, the females. Delaying investment in mating chemical composition of antlers varies with until physically mature and able to compete their size (McCullough 1969, Bowyer 1983) effectively for access to estrous females as well as the particular part of the antler with other large-bodied and large-antlered (Miller et a!. 1985). Existing models of males is likely the most successful strategy calcium metabolism and antler development for maximizing reproductive success, (Moen and Pastor 1998) should consider thereby achieving the greatest fitness m differential investment in antlers with re moose and other cervids. spect to age. In addition, differences in allocation of minerals may have implica ACKNOWLEDGEMENTS tions for osteoporosis in moose (Hindelang We are indebted to J. G. Cook, and B. and Peterson 1996, Hindelang eta!. 1998). L. Dick, for their helpful reviews of our Fighting in cervids is both dangerous manuscript, and to E. Rexstad for statistical and costly; numerous studies have reported advice. This manuscript benefited from relatively high rates of activity and of inju helpful discussions of moose antlers with V. ries resulting from male combat during the Van Ballenberghe, C. C. Schwartz, and K. mating season (Schaller 1967, Bowyer 1981, J. Hundertmark. Funding for this study was Clutton-Brock et a!. 1982, Lincoln 1994). provided by the Institute of Arctic Biology Mature adults invest heavily in antler devel at the University of Alaska Fairbanks, and opment and reproduction at the expense of the U.S. Forest Service. body condition (Miquelle 1990), whereas body growth appears to take precedence REFERENCES over antler development in growing cervids ALEXANDER, R. D., J. L. HOOGLAND, (Chapman 1975). Indeed, juvenile white R. D. HOWARD, M. NOONAN, and tailed deer had lower proportion of total P. W. SHERMAN. 1979. Sexual di mass in antlers than did deer> 4.5 years of morphism, and breeding systems in age (McCullough 1982). Juvenile cervids pinnipeds, ungulates, primates and hu are less likely to invest in antlers at the mans. Pages 402-604 inN. A. Chagnon expense of body growth, particularly be and W. Irons (eds.) Evolutionary Biol cause body size is highly correlated with ogy and Human Social Behavior, an fighting success; and interactions between Anthropological Perspective. Duxbury juveniles and mature adults were typically Press, Boston, MA. won by large males (Clutton-Brock 1982, ANDERSON, A. E. and D. E. MEDIN. Bowyer 1986). Hirth ( 1977) reported that 1969. Antler morphometry in a Colo sparring matches were performed primarily rado mule deer population. J. Wild!. by the youngest males that were growing Manage. 33:510-533. the fastest and were the least certain about BANKS, W. J., G. P. EPLING, R. A. their position in the dominance hierarchy, KAINER, and R. W. DAVIS. 1968.

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