Birth mass scaling in elk (Cervus elaphus).
Understanding the factors that lead to variation in mass of an individual at birth is critical to understanding life history strategies of a species. In ungulates, for example, individuals with a larger birth mass have a higher probability of post-parturition and overwinter survival (Verme, 1965; Thorne et al., 1976), are able to reach sexual maturity more quickly (Lomas and Bender, 2007), and can achieve a greater asymptotic body mass (Donadio et al., 2012; but see Wolcott et al., 2015). Therefore, understanding what impacts birth mass might be useful to understanding adult body size variation. To a large degree, maternal body size influences the birth mass of neonates (Skogland, 1984; Eloranta and Neimen, 1986). Across small to large ungulates comprised mostly of monotocous species, birth mass scales to the 0.75 power of body mass of the dam, meaning smaller dams give birth to larger neonates in proportion to the dams body size (Robbins and Robbins, 1979). The 0.75 scalar might be an evolutionary response to reduce vulnerability of small neonates to predation and the thermal environment. The 0.75 scalar also might reflect constraints on energy allocation to reproduction as metabolic rate also scales to the 0.75 power of body mass (Robbins and Robbins, 1979; Loison and Strand, 2005).
Far fewer investigations have examined intraspecific scaling relationships between maternal body mass and birth mass of their neonates (Loison and Strand, 2005). In part, this is related to the paucity of studies that measure the mass of females and their offspring on the day of birth. Also, dams should display a wide range of body masses yet have access to forage consisting of similar nutritional quality to make direct comparisons. Cervus elaphus (hereafter, elk) is an ideal species to estimate intraspecific scaling relationships between maternal body mass and birth mass of their neonates for a few reasons. First, elk can exhibit a wide range of body masses, with adult female elk ranging roughly 100-250 kg. Second, elk are a monotocous species. Therefore, neonate birth mass is not complicated by in utero competition between littermates during fetal development (Guinness et al., 1978; Wolcott et al., 2015). Our objective herein is to estimate the scaling relationship between maternal body mass and neonate birth mass in elk.
Data for this study were extracted from the peer-review literature. Three criteria were used when collecting data from articles. One, measurements of maternal body mass and birth mass of neonates were recorded on the day of birth. Two, the study collected data from a number of dams, not just an individual. Three, animals had ad lib. access to a nutrient rich and readily digestible diet. The studies selected for our analyses comprised studies of tame elk grazing irrigated pastures with supplements or animals fed rations consisting of silage and pellets. Data consisted of eleven groups of dam and offspring from seven different sources. These studies reported means from 6-15 dams and offspring.
For maternal body mass, we assumed measurements were recorded shortly before parturition. In analyses, however, maternal body mass was corrected by subtracting birth mass of the neonate to reduce problems with a predictor being part of the response variable (e.g., Parra et al., 2014). Sex of the offspring can influence birth mass (Wolcott et al, 2015). However, neonate sex ratios were not consistent across groups. Consequently, we assigned four birth sex categories, including more female than male young, more male than female young, equal number of male and female young and no information was presented on neonate sex. The scaling relationship was estimated using linear models where we log transformed maternal body mass and neonate birth mass. To assess the potential confounding influences from neonate sex and sample sizes of means we conducted two extra sums of squares tests (Sokal and Rohlf, 2012). One extra sums of squares test compared a reduced model, only one predictor--maternal body mass, to a complete model that incorporated an additional predictor, birth sex categories. The other extra sums of squares test compared the identical reduced model from before with a model that incorporated neonate birth mass and study sample size as predictors. Although not ideal, we approached our analyses in this manner because of the limited number of data points available from the literature.
The mass of elk dams in our data ranged from 80 to 260 kg (Table 1). Neonate birth mass ranged from 8.1 to 18.6 kg. Birth sex categories did not influence the scaling relationship ([F.sub.3,6] = 1.1, P = 0.417), nor did study sample size ([F.sub.1,8] = 0.8, P = 0.400). Hence, we used the reduced model with only the predictor of maternal body mass. A negative allometric scalar of 0.78 (95% confidence interval: 0.59-0.97) was estimated (Fig. 1; [R.sup.2] = 0.90, [F.sub.1,9] = 84.3, P < 0.001).
A negative allometric relationship with maternal body mass in elk suggests the following: on an absolute basis, neonate birth mass is greater when mothers are larger, yet, in proportion to maternal body mass larger dams give birth to smaller offspring. The estimated scalar between maternal body mass and neonate birth mass is similar to Loison and Strand (2005) for reindeer (Rangifer tarandus) and the interspecific scaling relationship estimated by Robbins and Robbins (1979). Birth mass in elk appears to be constrained by the maternal allocation of energy towards reproduction. The 0.78 scalar of neonate birth mass to maternal body mass estimated herein is not statistically different from the 0.75 scalar of metabolic rate to body mass (Loison and Strand, 2005). Further, milk yield, another reproductive attribute critical to provisioning nutrients to offspring, also has been reported to scale to the 0.75 power with maternal body mass (Landete-Castillejos et al, 2003a). On an intraspecific basis,
reproductive attributes appear to be constrained by the influence of body size on energy allocation to reproduction. It is unclear from our study whether the negative allometric scaling relationship is an evolutionary response to reduce vulnerability of neonates to predation and the thermal environment (Robbins and Robbins, 1979).
[FIGURE 1 OMITTED]
Body size variation across a geographic range is often explained by seasonal nutritional availability and climate (Parker et at, 2009). The allometric relationship between maternal body mass and birth mass also might have a role in explaining body size variation. Large maternal body size might be a means to alleviate the large demands of maternal investment into young when food availability is restricted during long winters or when peaks in food supplies mismatch the latter stage of pregnancy and lactation (Landete-Castillerjos et at, 2003b).
Our analysis included a wide range of body masses for mothers, which increases our confidence in the estimated scalar. Furthermore, we were able to show categories of birth sex and sample size of the means used in our analyses did not influence the relationship. However, this study also had limitations. Although the elk in the studies had ad lib. access to high quality forage, the varying types of diets might have impacted body masses and the estimated scalar. Further, the data points used in the analyses were means of body masses that could have conceivably masked variation in the relationship. Hence, the strength of the relationship between dam mass and birth mass ([R.sup.2] = 0.90) might be due to masked variation. Had we had access to masses of individual elk from each study, the [R.sup.2] value might have been less.
Another limitation was the lack of data to assess potential differences among subspecies in the relationship between body mass and birth mass (Landete-Castillijos et al., 2003a). Unfortunately, we had limited data for C. e. scotius, hipspanicus, nelsoni and hybrids between European and North American subspecies. The limitations of small sample sizes are particularly acute at the subspecies level because subspecies is correlated to body mass. The smaller body masses are for the European subspecies, the hybrids are intermediate in body mass and the North American subspecies are the heaviest in body mass. Data gathered from numerous individuals of each subspecies and not data summarized as means is probably needed to adequately assess the influence of subspecies.
We estimated the scaling relationship between neonate birth mass and maternal body mass. The negative allometric relationship suggests that greater maternal body mass relates to mothers giving birth to proportionately smaller offspring, which is probably related to the increased metabolic and maternal requirements of larger mothers. If studies in the future find that birth mass and other attributes of maternal provisioning scale to the 0.75 power of body mass, then these allometric relationships might have implications for explaining the wide range of body sizes displayed by elk throughout their geographic range.
Acknowledgments.--We would like to thank Daniel M. Wolcott, Adam Duarte, and R. Terry' Bowyer for their constructive input on the manuscript.
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GAYATRI BHASKAR and FLOYD W. WECKERLY, Department of Biology, Texas State University, San Marcos 78666. Submitted 13 July 2015; Accepted 20 December 2015.
TABLE 1.--A summary of body mass of mothers and birth mass of neonates of elk (Ceruus elaphus) collected from literature Sex Body mass Birth mass Sample size categories 93.80 8.13 14 (7[male], 7[female]) Equal 103.70 8.48 6 (6[male]) More male 104.00 8.83 10 (4[male], 6[female]) More female 107.80 9.73 15 (9[male], 6[female]) More male 122.50 8.46 11 (Adjusted) Equal 123.42 10.34 7 (3[male], 4[female]) More female 137.37 12.90 7 (2[male], 5[female]) More female 154.00 14.50 6 (5[male], 1[female]) More male 253.00 17.80 6 (no data) None 260.00 17.85 12 (5[male], 7[female]) More female 262.00 18.60 6 (no data) None Body mass Source 93.80 Landete-Castillejos et al. (2003a) 103.70 Landete-Castillejos et al. (2003b) 104.00 Landete-Castillejos et al. (2003a) 107.80 Asher et al (2011) 122.50 Asher et al (2005a) * 123.42 Archer et al. (2013) 137.37 Archer et al. (2013) 154.00 Asher et al (2005b) 253.00 Thorne et al. (1976) 260.00 Hudson and Adamczewski (1990) 262.00 Thorne et al. (1976) * Placed in the equal category because study reported average birth mass adjusted for sex
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|Title Annotation:||Notes and Discussion Piece|
|Publication:||The American Midland Naturalist|
|Date:||Apr 1, 2016|
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