Northern Prairie Wildlife Research Center
At mean MEIs (65-442 kJ/kg0.75 per day) that remained below winter maintenance for captive deer (561 kJ/kg0.75 per day; Ullrey et al. 1970; Robbins 1983) throughout the study, the absence of a relation between recent daily MEIs (2 days prior to urine sampling) and urinary A:C ratios was in contrast to the findings of studies of captive elk experiencing positive and negative energy balance (Vagnoni et al. 1996; Garrott et al. 1997). Additional evidence for the absence of such a relation in our deer was obtained when towards the end of the study, MEI was reduced further (65.3-86.6%) in all subjects and effects on A:C ratios were inconsistent (Table 1). Most noteworthy was that high A:C ratios in treatment deer that had already lost the most body mass (22.5-29.0%) either remained high (Nos. 79 and 92) or increased markedly (No. 65; Table 1).
|Table 1. Effect of short-term (15-19 April) severe nutritional restrictions on recent (2 days prior to handling) daily mass-specific metabolizable energy intake (MEI) and urinary allantoin:creatinine (A:C) ratios of slightly and highly food-restricted white-tailed deer (Odocoileus virginianus) sampled on 11 and 19 April 1988 in Grand Rapids, Minnesota.|
|Deer No.||Sex||Before severe restriction||After severe restriction|
mass loss (%)
|Urinary A:C ratio (Ámol:Ámol)||Cumulative
mass loss (%)
|Urinary A:C ratio (Ámol:Ámol)|
|LPLE restricted dieta|
|LPLE control diet|
|Note: Cumulative mass
loss (since 4 February) data are from DelGiudice et al. (1994b).
Deer 79 and 92 died on 18 and 19 April, respectively. Recent mean daily
MEI could not be calculated for these deer, but it was limited to 65-70
kJ/kg0.75 body mass or less.
a Until this period of extreme nutritional restriction (15-19 April), the slightly undernourished group was fed a low-protein, low-energy (LPLE) diet (see the text) ad libitum and the highly undernourished groups was fed restricted amounts of the LPLE feed.
The winter maintenance requirement of captive elk, 552 kJ MEI/kg0.75 per day, is similar to that of captive white-tailed deer (Robbins 1983; Jiang and Hudson 1992). The primary nutritional difference between this deer study and the two elk studies was that in the latter the mean MEIs ranged from submaintenance to well above maintenance level (e.g., 200-1200 kJ/kg0.75; Garrott et al. 1997). Most of the mean A:C ratios (33) related to recent mean MEI were in the maintenance range or above (Garrott et al. 1997). Thirteen of the mean MEIs related to mean A:C ratios in the elk studies were below maintenance level, and only 5 of those were below 400 kJ/kg0.75 per day (Fig. 2 in Garrott et al. 1997). Further, changes in body mass from the two elk studies ranged from a maximum mean loss of 11% to a gain of 9%, whereas in this study, mean body-mass losses ranged from 4.2 to 24.2% and individual mass losses from 0 to 32.2%.
The curvilinear relation between percent mass loss and urinary A:C ratios for the treatment and control deer indicates the importance of considering more than the effect of recent MEI when interpreting A:C ratios. This was unexpected, based on findings from captive elk and domestic ruminants experiencing smaller body-mass losses (Chen et al. 1990a; Verbic et al. 1990; Balcells et al. 1991; Vagnoni et al. 1996; Garrott et al. 1997). The increasing trend of A:C ratios in the nutritionally restricted deer began at about 24% body-mass loss (Fig. 4), and values were comparable to those from captive elk with mean MEIs at and above winter maintenance and gaining body mass (Garrott et al. 1997). A:C ratios of severely restricted deer comparable to and higher than those of control deer were occurring when body-mass losses were as moderate as 8%.
Our data from deer in negative energy balance and losing body mass indicate that factors other than MEI affect A:C ratios when deer undergo acute severe energy restriction or long-term nutritional restriction and body-mass loss. The elevation of the urinary A:C ratio with severe restriction could be attributable to either an increase in 24-h excretion of urinary allantoin, a reduction in 24-h excretion of creatinine, or both. There are insufficient published data to determine the relative importance of these two mechanisms to the changes in A:C ratio; however, by integrating chemistry data from collections of snow-urines of free-ranging elk and simulation modeling of their physiology, DelGiudice et al. (2001) presented evidence that supports the importance of diminishing urinary creatinine output to the elevation of UN:C ratios with prolonged nutritional restriction. Further, short-term severe energy restriction can reduce urinary creatinine output as well (Van Niekerk 1962).
The literature on domestic ruminants and additional data from our deer provide some useful insights. Endogenous urinary allantoin excretion differs among species. Using intragastric infusion to preclude the influence of diet on urinary allantoin production, Chen et al. (1990a) reported that endogenous urinary allantoin excretion was 421 µmol/kg0.75 per day in cattle and 92 µmol/kg0.75 per day in sheep. We estimated A:C ratios associated with these endogenous allantoin outputs to be about 0.7 and 0.1-0.2 µmol:µmol, respectively. Using 24-h urinary allantoin and creatinine output from cattle and sheep at maintenance (Fujihara et al. 1987; Chen et al. 1990a; Puchala et al. 1993; Giesecke et al. 1994), we calculated mean urinary A:C ratios of 3.8 and 0.6 µmol:µmol, respectively. Consequently, we determined that in cattle and sheep at maintenance, allantoin derived from tissue turnover (i.e., endogenous) contributes roughly 17-19% of the allantoin to the A:C ratio. This is similar to findings of Fujihara et al. (1991) for domestic sheep and goats.
Although our deer were experiencing various degrees of negative energy balance throughout the study, their A:C ratios were more comparable to those of sheep than those of cattle. As in sheep, this may be attributable to low levels of xanthine oxidase activity in the blood of deer (Chen et al. 1990a). Similarly, the predicted endogenous A:C ratio of captive elk (0.1-0.2 µmol:µmol by extrapolation; Fig. 2 in Garrott et al. 1997) and A:C ratios of elk at maintenance (approximately 0.3-0.4 µmol:µmol) were much closer to those of sheep. Using data from Maloiy et al. (1970), we determined that A:C ratios of red deer were even lower than those of sheep fed similar diets. These findings suggest that the 24-h urinary excretion of allantoin by wild ungulates is relatively low, but the endogenous contribution to maintenance A:C ratios may also be about 17-19%.
The catabolism of endogenous protein accelerates with severe or prolonged dietary energy restriction of deer (Torbit et al. 1985; Moen and DelGiudice 1997), which presumably would increase endogenous allantoin excretion. Urinary UN:C ratios of our study deer were directly related (r2 = 0.52, P = 0.04) to their cumulative body-mass losses (DelGiudice et al. 1994b); urea is the end-product of both dietary and endogenous protein metabolism. Further, the urinary 3-MeH:C ratio of these deer was even more strongly related (r2 = 0.82, P = 0.0001) to their body-mass loss (DelGiudice et al. 1998), and 3-methylhistidine is derived solely from the catabolism of muscle tissue. Like the A:C ratios, the increasing trend of UN:C and 3-MeH:C ratios began at about 20-24% body-mass loss, which may indicate that fat reserves had reached a critical low level and the catabolism of endogenous protein was accelerating. Chen (1989) noted that 24-h urinary allantoin excretion in steers increased by 33% during the first 3 days of fasting; however, the results of studies of the effects of short-term fasting in sheep have been inconsistent (Rys et al. 1975; Fujihara et al. 1991). Five- to 8-day fasts in sheep have resulted in 80-87% reductions in 24-h urinary allantoin excretion (Fujihara et al. 1991).
As with domestic ruminants, 24-h urinary creatinine excretion is relatively stable in captive deer at or above a maintenance level of nutrition (Antoniewicz and Pisulewski 1982; Fujihara et al. 1987; Puchala et al. 1993; DelGiudice et al. 1995, 1997), and creatinine coefficients (millimoles excreted per kilogram of body mass in 24 h) are reasonably similar among mammalian species (Greenblatt et al. 1976; Wallin 1979; Cool 1992; DelGiudice et al. 1995). While we found little in the literature concerning the effects of nutritional restriction on the 24-h excretion of creatinine in deer or other wild ungulates, reductions greater than 33% have been noted with short-term dietary undernutrition of various domestic ruminant and monogastric species (Blaxter and Wood 1951; Van Niekerk 1962; Chetal et al. 1975; Long et al. 1981; Hovell et al. 1983, 1987; Fujihara et al. 1991). Whether ruminants adapt physiologically with more prolonged undernutrition, and how that might affect creatinine excretion, are not known, but we observed four- to five-fold increases in mean A:C ratios of the treated deer, with even larger increases for individual subjects. Assuming that excretion of allantoin remained stable, creatinine output would have to decrease by 50% just to double the A:C ratio. Consequently, it is very likely that on a mass-specific basis, increases in endogenous urinary allantoin and diminished creatinine excretion were both contributing to the elevated A:C ratios of nutritionally restricted deer.
The logical progression of research dictated that a study such as ours extend earlier efforts by determining whether moderate to severe winter nutritional restriction and body-mass losses similar to those expected in northern free-ranging ungulates affected urinary A:C ratios in a way not predicted by the boundaries and results of previous efforts. For relative and quantitative nutritional assessments, establishing reference values of urinary chemistries relative to known levels of energy intake and body condition is essential to accurate interpretation of their physiological meaning and significance. This has been widely acknowledged in the medical and veterinary sciences and clinics (Davidson and Henry 1969; Coles 1980; Benjamin 1981; Shils and Young 1988). Reference data, whether for their relative value or actual value, are at least as important for the interpretation of nutritional assessments of free-ranging wild ungulates and other animals (Seal et al. 1981; Harder and Kirkpatrick 1994; DelGiudice 1995; Vagnoni et al. 1996).
Our data suggest that higher or increasing energy intake is not the only cause of higher or increasing A:C ratios. Pils et al. (1999) reported A:C values from snow-urines of free-ranging elk calves that began increasing by mid-January to early February, earlier than for the cows sampled during each of five winters. By late March to early April of four winters, mean ratios increased to higher than 0.50 µmol:µmol. This was up to two times higher than those of the adult cows. With no previous reports addressing the effects of prolonged nutritional restriction and the full range of body-mass losses on A:C ratios in cervids, Pils et al. (1999) interpreted the calves' ratios as being directly related to MEI. According to Garrott et al. (1997), this could indicate an MEI (950 kJ/kg0.75) far greater than the mass-specific winter maintenance requirement of elk, suggesting that calves should be gaining mass. Based on our study and an increased understanding of undernutrition and urinary A:C ratios, a more plausible explanation is that the early increasing trend of A:C ratios of the calves was indicative of greater nutritional restriction than in the cows. This is strongly supported by urinary UN:C ratios in snow-urines collected from known elk cows and calves and trends in calves per 100 cows in the same study area during two of the five winters (White et al. 1995).
Our study indicates that caution must be exercised when interpreting urinary A:C ratios of free-ranging ungulates. More rigorous quantification of the physiological relations influencing the urinary excretion of allantoin is required. Additional research should include simultaneous examination of the effects of short-term and prolonged nutritional restriction and recovery on body composition, and 24-h urinary output of allantoin and creatinine. Study focused on 24-h urinary excretion of endogenous allantoin, applying methods used with domestic ruminants (e.g., intragastric infusion), would be useful for improving our understanding of the influence of tissue catabolism on A:C ratios at various planes of nutrition and physical condition. Further, field studies sequentially estimating digestible energy intake of free-ranging ungulates while monitoring them using snow-urine A:C ratios as winter progresses may enhance the value of this characteristic as a nutritional index.