Jan 25, 1997 - Northern red-backed voles (Clethrionomys rUlilus) undergo a pronounced arumal cycle in .... tive tract, right hind quarter, muscle of left hind.
SEASONAL CHANGES IN BODY MASS, COMPOSITION, AND ORGANS OF NORTHERN RED-BACKED VOLES IN INTERIOR ALASKA GERALD
L.
ZUERCHER, DAN[EL D. ROBY, AND ERIC
A.
RExSTAD
Department of Biology and Wildlife, Institute of Arctic Biology, and Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks, Fairbanks, AK 99775 Present address of GLZ: Kansas Cooperative Fish and Wildlife Research Unit, Division of Biology, 205 Leasure Hall, Kansas State University, Manhattan, KS 66506 Present address of DDR: Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, 104 Nash Hall, Oregon State University, Corvallis, OR 97331
Northern red-backed voles (Clethrionomys rUlilus) undergo a pronounced arumal cycle in body mass and are heaviest in summer and lightest in winter. We trapped voles throughout 1994 to determine how changes in body composition and organ size contributed to this cycle. Body mass peaked in summer for females and spring for males. Seasonal changes in body mass were primarily due to changes in lean mass. Body mass was 30-50% lower in winter than summer, and water content of lean mass was lowest in winter. Total body fat was low throughout the year but peaked (as with body mass) in spring (males) or early summer (females). Energy reserves in the fonn of fat depots are apparently most crucial during the breeding season. A low relative ash content in early summer was possibly due to a cation imbalance in the diet. Absolute and relative sizes of different body components contributed to the annual cycle in total body mass. All body components (except brown adipose tissue) declined in absolute mass, dry mass, and percent water during autumn, with skeletomuscular components contributing most to loss of total body mass. Most body components declined in proportion to declines in total body mass. However, liver, reproductive tract, and muscle mass of males declined proportionally more than total body mass; heart, brain, and bone declined proportionally less. Whole body analyses suggest that the annual cycle of body mass in C. rutilus is driven by seasonal changes in optimal body size. Component analyses are consistent with the hypothesis that the primary selective force driving seasonal changes in body components is the enhanced overwinter survival of C. rutilus with relatively small body size. Key words: Clethrionomys rutilus, red-backed voles, body composition, body mass, components, seasonality, Alaska pared with strict folivores (Hansson, 1990). Declines in body mass of voles from summer to winter also have been attributed to declines in total body water associated with reduced water fraction of lean tissues (Evans, 1973). In C. rutilus. a dissociation between turnover of body water and metabolic rate occurs in winter, suggesting reduced ingestion of free water (Feist, 1984; Holleman et al., 1982). The effect on body mass of declines in body water, coupled
Microtine rodents, and other small Arctic mammals, exhibit seasonal changes in sizes of body and, various organs (Churchfield, 1981; Zegers and Merritt, 1988). Such changes in body composition are adaptive by enhancing overwinter survival through reduction of energy requirements at a time when food energy is limiting (Fuller, 1969). Among microtines, greater declines in body mass have been observed in mixed folivores-granivores and strict granivores COffiJournal of Mammalogy. 80(2):443-459. 1999
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with declines in protein, has been documented in several rodents (Sawicka-Kapusta, 1974; Virgl and Messier, 1993). Despite the presumed adaptive significance of fat depots for overwinter survival, there is disagreement over the importance of fat as an energy reserve for small mammals. Fat stores in mouse-sized mammals are too small to meet energy requirements for maintenance for >2 days (Bronson et aI., 1991). For microtines in captivity, amount of body fat can vary daily depending on the feeding schedule (Bronson, 1987). As a result, it is difficult to detect seasonal differences in fat reserves. Batzli and Esseks (1992) document no significant differences among seasons in the percent body fat of brown lemmings (Lemmus trimucronatus). Further, fat reserves in winter were lower in Microtus agrestis despite their higher energy requirements (Evans, 1973). Fat was the most variable constituent of body composition in CLethrionomys gLareo/us, but there was no clear pattern of seasonal change in fat reserves (SawickaKapusta, 1974). We documented the annual cycle of change in body mass and body composition in C. rutilus. Which body components contribute to recession of body mass in autumn and increase in spring is unknown. Our hypothesis was that all tissues and organs retained their relative mass in the organism as body mass changes through the annual cycle. In other words, the exponent of the allometric relationship between log component mass and log body mass was 1.0. Our alternative hypothesis was that as body mass changes, relative sizes of various body components also change. Allometrically, the exponent of the equation relating log component mass and log body mass is > 1.0 or < 1.0. Scaling of various organs to body mass has been investigated in detail for mammals (Schmidt-Nielsen, 1984), and allometric equations have been derived using empirical data from a variety of mammals of differing body mass. For example, liver mass scales to the 0.87 power and kidney
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mass to the 0.85 power of body mass. Heart mass and blood volume scale to the 1.0 power of body mass in a variety of mammals. Metabolic rate, on the other hand, scales to the 0.75 power of body mass, and masses of some organs or tissues might be expected to follow metabolic rate (SchmidtNielsen, 1990). Consequently, an explanation for changes in relative mass of a body component as total body mass changes could reside in problems of scaling-a fundamental physiological constraint. Alternatively, the change in relative mass of body components with change in body size may reflect adjustment or acclimatization to different sets of environmental conditions or selective forces in summer as opposed to winter. This would predict that changes in relative mass of body components are adaptive by reallocating resources during a particular season. In a highly seasonal environment, the body plan that maximizes fitness is likely to change dramatically with change in season. The population of northern red-backed voles used in our study has been studied previously, and individual weight loss from summer to winter is well documented (Whitney, 1976, 1977). The first goal of this study was to investigate how changes in the composition of carcasses of voles contribute to seasonal changes in total body mass. To accomplish this, we tested the following hypotheses: 1) body composition (percentage of water, fat, ash, and ash-free lean dry mass) does not change with season or in relation to change in total body mass; 2) seasonal patterns of change in body mass and composition do not differ between sexes; and 3) body composition in summer does not differ between subadults and adults. Our second objective was to describe seasonal changes in body components as they relate to the annual cycle in overall body mass and composition of C. rutilus. Seasonal changes in size of various body components were examined in terms of absolute mass, relative mass, and percent wa-
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ter. We sought to identify body components primarily responsible for the annual cycle in total body mass and body composition in C. rutilus. We also sought to detennine if seasonal changes in percent water of total lean mass were a product of changes in water content of a few body components, or a generalized pattern for all tissues. If relative size and composition of some body components were not conserved throughout the annual cycle of body mass, we hoped to gain some insight into the potential adaptive significance of those differences. We were interested particularly in how those changes relate to survival and reproduction of a small endotherm in a highly seasonal environment. MATERIALS AND METHODS
Free-ranging C. rutilus were trapped during each of six predetermined phases of the annual cycle in 1994 (late winter, 21 January-16 February; spring, 21 March-25 April; early sununer, 25 May-25 June; late summer, 15 July-IS August; autumn, 20 September-lO October; early winter, 1 December-30 December) on the campus of the University of Alaska Fairbanks in interior Alaska (64°N, 14rW). Habitat at trap sites was a boreal forest mixture of black spruce (Picea mariana), white spruce (P. glauca), paper birch (Betula papyrijera), and alder (Alnus crispus). During spring, early and late summer, and autumn, 5- by 6.25- by 16.5-cm Shennan live traps and 6- by 8- by 24-cm U gglan multiple capture live traps were baited with a mixture of peanut butter and oatmeal and examined twice daily (0600 and 1800 h). Newly trapped voles were weighed with a Pesola spring scale (±0.5 g). In the laboratory, voles were euthanized with Halothane, weighed on a Mettler analytical balance (±0.1 mg), and placed in a freezer at - 20°C until di$section and body composition analyses could be performed. During early and late winter, snap traps baited with a mixture of peanut butter, oatmeal, and vegetable oil were set and examined three times daily (0600, 1300, and 2000 h).
Reproductive activity in females was determined by the presence of embryos. lactation, or open vagina. Active males were determined by descended testes (McCravy and Rose, 1992) of
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>5.0 rom in length. Although that index to reproductive activity in males was somewhat arbitrary, few reproductive males (6 of 63) had testes 4.5-5.5 tnm. For those individuals, comparison of morphological measures (i.e., body length and mass) to other males caught during the same trapping period and at the same trapping site aided in determination of reproductive status. Also, Whitney's (1976) criterion for distinguishing subadults « 17 g) from adults (2: 17 g) during early and late sununer was used. All protocols involving live animals were approved by the Institutional Animal Care and Use Committee at the University of Alaska Fairbanks. Wet mass before dissection was recorded (::to.1 mg) using a Mettler analytical balance. and each carcass was dissected into 17 components (brain, head, heart, liver, kidneys, stomach, small intestine, cecum, large intestine, reproductive tract, right hind quarter, muscle of left hind quarter, bone of left hind quarter, integument, subcutaneous white adipose tissue, brown adipose tissue, and the remaining carcass). Each component was placed in a pre-weighed aluminum pan and weighed on a Mettler analytiCal balance to detennine wet mass (±0.1 mg). Each component was then placed in a forced-air convection drying oven (ca. 60°C), dried to constant mass, and reweighed to detennine dry mass. Water content of each component was calculated from the difference between wet and dry masses. For each component, absolute wet mass, absolute dry mass, relative wet mass (percent lean body mass), and relative dry mass (percent lean dry body mass), as well as percent water in each component were calculated. Dissected components were combined and ground using a mortar and pestle. Each ground carcass was divided into 2-4 subsamples, each of 2.0-2.5 g, so that the entire carcass was extracted to determine total body fat. Total fat was extracted from samples using a Soxtec HT-12 soxhlet apparatus (Tecator, Inc., Herndon, VA) with petroleum ether as the solvent system (Dobush et al., 1985). Lean body mass (LBM) was determined from the difference between total body mass (measured immediately after death) and total body fat. The remaining lean dry matter was placed in a glass scintillation vial and burned in a muffle furnace (ca. 500°C for 24 h) to determine ash content. Absolute values and percentages of total body mass for water, fat, ash, and ash-free lean dry matter (mostly pro-
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TABLE I.-Comparisons of seasonal body mass of male and female Clethrionomys rutilus trapped on the campus of the University of Alaska Fairbanks during 1994. Due to absence of significant differences, voles caught during early and late summer sessions were combined into "Summer" and those caught during early and late winter sessions were combined into "Winter." Body mass is given as mean ± SD (n). Analysis of Variance
Body mass (g) Season
Males
Spring Summer Autumn Winter
23.00 ± 1.92 (6) 21.09 ::t 2.66 (14) 18.04 ::t 1.92 (5) 15.00 ± 1.94 (14)
F-statistic (dJ.)
Females 20.42 :t:: 1.92 30.58 :!:: 1.20 19.24 ± 2.10 14.71 ± 0.62
tein), and percent water of lean mass were calculated for each carcass. Results from the analysis of whole carcass attributes were used to investigate how changes in body composition contribute to changes in total body mass. Analysis of variance (ANOVA) was used to determine differences in body mass between sexes for each season. Twelve variables describing whole animal composition were analyzed for differences among seasons, sexes, reproductive statuses, and their interactions. Five variables were analyzed using ANOVA: total body mass; total body water; total ash-free lean dry mass; total body ash; and total body fat. Seven percentage and index variables were analyzed using analysis of covariance (ANCOVA) based on recommendations by Packard and Boardman (1988): water as a percentage of lean mass; water as a percentage of total body mass; ash-free lean dry mass as a percentage of total body mass; ash as a percentage of lean dry mass; ash as a percentage of total body mass; total fat:lean dry mass (fat index); and total body fat as a percentage of total body mass. A square root of the arc sine transfonnation (Zar, 1984) was applied to all percentages before analyses. When ANOVA or ANCOVA revealed significant seasonal differences, seasonal means were compared using SAS Proc GLM and Duncan's multiple range test (SAS Institute Inc., 1993). Duncan's test was selected based on an evaluation by Canner and Swanson (1973). For all Duncan analyses, degrees of freedom (d./) were 5, 33 for females and 5, 42 for males and sample sizes (n) were 39 for females and 48 for males. Analyses of change in seasonal body composition were performed on adults only and were separated by sex. Subadults were analyzed only during early and late summer because they could
(9) (10) (16)
(13)
7.630 35.53 1.091 0.125
(1,13) (1,22) (1,19) (1,25)
P
0.016
< 0.001 0.124 0.727
not be distinguished in other seasons. ANOVA and Duncan's tests compared body mass and body composition between adults and subadults for early and late summer only. Results from the analysis of body components were used to infer how changes in different components contributed to changes in total body mass. Each component was analyzed using ANCaVA to detennine differences related to seasons, sex, reproductive status, and interactions among those three categories. All overwintering individuals were considered adults for those analyses. Changes in body components among seasons were investigated using SAS Proc GLM and Duncan's multiple range test (SAS Institute Inc., 1993). Absolute wet mass, absolute dry mass, relative wet mass, relative dry mass, and percent water for each component were tested for seasonal differences. Contribution of each component to the decline in body mass from late summer to early winter was detennined by calculating the percent change in total body mass due to the change in each component. The log of component mass was regressed on the log of total body mass to detennine allometric relationships of brain, liver. muscle of hind quarter, bone of hind quarter, and brown adipose tissue components to total body mass. Data on body components were separated by sex in these analyses. Statistical significance was set at P < 0.05 for all tests. RESULTS
Whole body composition.-Mean seasonal body mass differed between sexes in spring and early and late summer but not autumn and early and late winter (Table 1). Sexes differed seasonal in mean total body
ZUERCHER ET AL-BODY COMPOSITION OF VOLES
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oU-~~~~~~~~~L Late Spring Early Late Autumn Early Summer Winter Winter FIG. l.-Seasonal mean body mass of adult A) female and B) male Clethrionomys rutilus separated into constituents (fat, ash, ash-free lean dry mass, and water). Capital letters indicate Duncan's multiple comparisons of total body mass; lower-case letters indicate Duncan's multiple comparisons of the four constituents.
mass (females, F ~ 34.54, P < 0.0001; males, F ~ 17.07, P < 0.0001; Fig. I). Seasonal patterns in total body mass were different between sexes. Mean total body mass of females increased from late winter to a peak. in early'Summer, followed by a decline to early winter. Total body mass of females in early summer was roughly twice that of females in early winter. Total body mass of males increased from late winter to a peak in spring. Although not significantly different, average total body mass of males in early summer was lower than in spring, followed by a significant decline in total body
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mass (ca. 30%) from late summer to early winter, as in females. Total body masses of both sexes in early winter and late winter were not different but were distinguishable from body mass in all other seasons. Total body mass of subadults was lower than adults for both sexes during early and late summer (Table 2). Adult females weighed nearly twice that of subadult females, but adult males weighed ca. 20% more than subadult males. Seasonal differences in total body water were apparent for both sexes (females, F = 33.59, P < 0.0001; males, F ~ 18.11, P < 0.0001; Fig. 1). Duncan groupings showed the same pattern of seasonal change as in total body mass. Seasonal changes in total body mass were primarily a function of seasonal changes in total body water. Water as a percentage of lean mass also showed seasonal differences in sexes (females, F = 6.98, P < 0.0001; males, F ~ 9.41, P < 0.0001). The ANCOVA for water as a percentage of lean mass, and the remaining six whole body composition variables resulted in a non-significant interaction effect (body mass X season; P > 0.05). Patterns for seasonal change in water as a percentage of lean mass were similar to the patterns of change in total body mass, with peaks in early summer and lows in early winter (Table 3). To determine the percentage of loss in total body mass from late summer to early winter that was due to loss of water content in lean tissues, a comparison was made between actual body water content and potential body water content, had water as a percentage of lean mass remained constant. The difference between actual and potential body water content for females was 0.75 g, which accounted for only 4.5% of the decline in total body mass from summer to winter. The difference for males was 0.60 g, which accounted for only 9% of the decline in total body mass. Seasonal differences in percent water of total body mass for sexes differed (females, F = 7.81, P < 0.0001; males, F ~ 5.70, P < 0.0001; Fig. 2).
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TABLE 2.-Comparisons of body mass and body composition between adult and subadult Clethrionomys rutilus during summer. AFLDM = ash-free lean dry mass. Sample sizes for each class within season are in parentheses. Mean:!: SD provided. Body composition
Season-class
Body mass (g)
% water
% AFLDM
% ash
% fat
30.62 ± 4.91 12.13 ± 1.05**
74.68 ± 2.06 73.58 ± 0.82
20.38 ± 1.59 21.16 ± 0.92
4.05 ± 0.67 4.52 ± 0.67
4.23 ± 1.77 3.80 ::!: 1.26
20.97 ± 2.17 16.49 ± 1.23*
73.13 ± 0.79 72.43 ± 1.89
21.46 ± 0.13 21.47 ::!: 1.58
4.43 ± 0.68 4.75 ± 0.28
4.22 ± 1.75 5.88 ± 1.83
30.51 ± 4.15 15.46 ± 1.46**
73.89 ± 2.48 71.76 ± 1.84
18.48 ::!: 0.55 19.94 ± 1.55
7.19 ± 2.33 7.72 ± 1.31
2.37 ± 0.80 2.49 ± 1.07
21.27 ± 3.65 16.57 ± 1.95*
71.57 ± 0.91 71.85 ± 1.71
19.47 ± 0.54 19.71 ± 1.62
8.37 ± 1.32 7.68 ± 1.76
2.51 ± 0.46 3.26 ± 1.52
Early summer Females Adults (9)
Subadults (4) Males
Adults (6) Subadults (6)
Late summer Females Adults (5) Subadults (11)
Males Adults (4) Subadults (8)
* P < 0.05 and ** P < 0.01 for comparison of adults and subadults by sex within a season using a (-test.
Seasonal differences in ash-free lean dry mass (mostly protein) were significant for both sexes (females, F = 31.94, P < 0,0001; males, F = 10,98, P < 0,0001; Fig. 1). Seasonal patterns in ash-free lean dry mass for females were the same as for total body mass and total body water, with peaks in early summer and minima in winter. In males, mean ash-free lean dry masses also exhibited the same seasonal trends as total body mass and total body water, but there
was slightly more overlap in Duncan's multiple comparisons. Although differences occurred, ash-free lean dry mass as a percentage of total body mass remained fairly consistent among seasons for both sexes (females, F = 2.53, P < 0.0240; males, F = 3.08, P < 0.0053; Fig. 2). Seasonal differences occurred in total body ash for both sexes (females, F == 6.56, P < 0.0002; males, F = 4.53, P < 0.0002; Fig. 1). The seasonal pattern in total body
TABLE 3.-Summary of body composition of male and female Clethrionomys rutilus indicating seasonal changes in: water as a percentage of lean body mass (LBM), ash as a percent of lean dry mass (LDM), fat index (total body fat [TBF}:lean dry mass), brown adipose tissue (BAT) index (total BAT:lean dry mass). Sample sizes (n) were 39 for females and 48 for males. Water as % LBM
Ash as % LDM
Fat index
BAT index
Season
M
F
M
F
M
F
M
F
Late winter Spring Early summer Late summer Autumn Early winter p,
68.73 71.56 73.13 71.57 70.85 67.49 0.75 g of fat. Body mass and composition of subadults were compared, using a t-test, with those of adults only in sununer. Differences occurred in body mass but there were no differem:e s in body composition between adults and subadults within sex (Table 2). Contribution of different organs.-The ANCOV A for different organs resulted in nonsignificant interactions (body mass and season; P > D.OS). Most organs changed seasonally but remained proportionally the same (bead, kidneys. stomach, small intestine, cecum, large intestine, integument, subcutaneous white adipose tissue, and the remaining carcass). Seasonal differences occurred for sexes in wet mass of the muscle of the hind quarter (females, F = 16.95, P < 0.0001; males, F = 20.19, P < 0 .0001 ; Fig. 4) and percent water of the muscle of the hind quarter (females, F = 4.45, P = 0.0009; males, F = 8.43, P < 0.0001). Percent water was highest in early summer and lowest in early winter for both sexes. Relative wet mass of leg muscle showed seasonal differences (females, F = 2.37, P = 0.0342; males, F = 7.36, P < 0 .0001). Relative muscle mass was highest in spring and early summer for females and in spring only for males. Lowest relative muscle mass occurred in early winter for both sexes. The allometric equation relating log mass of hind quarter muscle to log body mass for females had an exponent tbat was not different from 1.0 (bo = 0 .9906, SE = 0.0739). The allometric equation for males had a slope significantly greater than 1.0 (bo = 1.3782. SE = 0.0929) . That indicated that seasonal changes in mu scle mass of males were proportionally greater than changes in total body mass; seasonal changes in muscle mass of females were proportional to changes in total body mass. Seasonal differences in wet mass of bone in hind quarters were apparent for females (F = 6.04, P = 0.0004) and males (F = 3.40, P = 0.0110; Fig. 4). However, there
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A) 1
o DMuscle WaterDMuscle Dry Mass • Bone Water 0 Bone Dry Mass B)
0.2 OUL~~~~~~~~ Late Spring Early Late A utumn Ea rl y WI nter Summer Winter
FiG. 4.-Mean water and dry matter content of hind quarter muscle and bone in adult A) female and B) male Clethrionomys rutilus by season. Lower-case letters indicate Duncan's multiple comparisons.
were no seasonal differences in percent water of bone for either sex. Sexes showed seasonal differences in relative wet mass of bone (females, F = 14.91, P < 0.0001; males, F = 2.99, P = 0.0082). The allometric equation relating log bone mass to log body mass had an exponent significantly < 1.0 (females, bo = 0.4453, SE = 0.0570; males, bo = 0.5274, SE = 0.0820), indicating that seasonal changes in bone mass were proportionally less than changes in total body mass. Of the four internal organs examined
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ZUERCHER ET AL.-BODY COMPOSITION OF VOLES
(liver, heart, kidneys, and spleen), only liver showed a decline in mass that accounted for >5% of the overall decline in total body mass in autumn. The liver also was the largest internal organ, regardless of season. Declines in liver mass from late summer to early winter accounted for 8% of the decline in overall total body mass for females and 7% for males. There were seasonal differences in wet mass of liver for females (F = 16.50, P < 0.0001) and males (F = 6.08, P = 0.0002). Percent water of liver showed no seasonal changes in either sex. Relative wet mass of liver differed among seasons in females (F = 2.26, P = 0.0426) but not in males (F = 1.62, P = 0.1324). Relative wet mass of liver was highest in early summer and lowest in late winter for females, but there was no clear seasonal pattern for males. The allometric equations relating log liver mass to log body mass were different between the sexes. The exponent of the allometric equa~ion for liver mass of females (b o = 1.5027, SE = 0.1198) was significantly > 1.0 and indicates that seasonal changes in liver mass were proportionally greater than changes in total body mass. The exponent of the allometric equation of liver mass of males was indistinguishable from 1.0 (b o = 0.9094, SE = 0.1230), indicating that seasonal changes in liver mass were proportional to changes in total body mass. The decline in mass of reproductive organs accounted for 15% of the decline in total body mass for females in autumn and 9% of the decline for males (Table 4). There were seasonal differences in wet mass of reproductive tracts for females (F = 55.38, P < 0.0001) and males (F = 23.36, P < 0.0001; Fig. 5A and 5B). Seasonal changes in mass of reproductive tract were accompanied by changes in percent water for females (F = 7.36, P < 0.0001) and males (F = 4.63, P < 0.0002). Percent water in reproductive tissues was highest in early summer for both sexes. Lowest percent water in reproductive tissues was in late winter for females and in early winter
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TABLE 4.-Relative contribution of different body components to the overall decline in body mass of Clethrionomys rutilus from late summer to early winter. The skeletomuscular components include head, hind quarter, muscle of the hind quarter, bone of the hind quarter, and the remaining carcass; the gastrointestinal components are the stomach. small and large intestines, and cecum; skin includes the pelage.
Percentage of change in total mass Body component
Females
Males
Skeletomuscular components Gastrointestinal components Reproductive tract Liver Skin
42 9 9 7 15
36 IS
15 8
7
for males. Seasonal differences in relative wet mass of reproductive tract also were observed (females, F = 3.54, P = 0.0039; males, F = 36.33, P < 0.0001). Wet mass of the brain differed among seasons for females (F = 18.16, P < 0.0001) and males (F = 18.76, P < 0.0001; Fig. 5C and 5D). Brain mass was highest in summer and lowest in winter for both sexes. Seasonal changes in brain mass were associated with changes in percent water of the brain (females, F = 2.58, P = 0.0231; males, F = 20.78, P < 0.0001). Percent water of the brain was highest in early summer for both sexes and lowest in early winter for females and late winter for males. Relative wet mass of brain also differed among seasons (females, F = 37.80, P < 0.0001; males, F = 18.66, P < 0.0001). Relative wet mass of brain, however, was highest in early winter (both sexes) and lowest in early summer for females and in spring for males. Thus, relative brain mass peaked at about the time when absolute brain mass was at a minimum. The allometric equation relating log brain mass to log body mass had an exponent significantly
