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of the
FLORIDA
MUSEUM OF
NATURAL HISTORY
GEOGRAPHIC VARIATION OF LEAN BODY
MASS AND A MODEL OF ITS EFFECT ON
THE CAPACITY OF THE RACCOON TO
FATTEN AND FAST
John N. Mugaas and John Seidensticker
Biological Sciences, Volume 36, Number 3, pp. 85-107
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GEOGRAPHIC VARIATION OF LEAN BODY MASS
AND A MODEL OF ITS EFFECT ON THE CAPACITY
OF THE RACCOON TO FATTEN AND FAST
John N. Mugaas and John Seidensticker1
ABSTRACT
In the eastern United States, apparent lean body mass (ALBM) of raccoons (Procyon lotor)
increased from south to north, and appeared to follow Bergmann's rule: subtropical Key Vaca,
females = 2.0 kg, males = 2.4 kg; mild temperate southeastern United States, females = 3.2 kg,
males = 3.5 kg; harsh to severe temperate Michigan and Minnesota, females = 4.5 kg, males = 5.0
kg. We postulated that selection has favored large lean mass in the cold parts of the raccoon's range
because it provides greater fasting endurance. In mammals, as lean body mass (LBM) increases, the
potential to store energy as fat (Fs = LBM1.0) increases out of proportion to the cost of basal
metabolism (Hb = mass0.75). Thus, big fat raccoons should be able to fast for a longer period of
time than small fat ones. We modeled these relationships for raccoons. We found that each increase
in ALBM substantially increased the length of time they could fast. Since the increased fasting times
were necessary for their winter survival, the model supported our hypothesis. We also concluded that
the northern edge of their range is determined by the limits of their genetic potential to increase
ALBM. The amount of fat deposited in the fall also varied geographically: subtropical raccoons
achieved 14 to 17% apparent body fat (ABF), those from Florida to Virginia 19 to 42% ABF, and
those around the Great Lakes 31 to 50% ABF. Geographic variation in ABF suggests that seasonal
lipogenesis is coupled, via neuroendocrine mechanisms, to environmental cues that stimulate the
appropriate degree of fat deposition in each local area. The data also suggest that there may be
geographic differences in the capacity to fatten.
SDr. Mugas is Profesor of Physioogy in the Departmen of Physiology, Division of Functional Biology, West Virginia School of
Oslcopathic Medicine, Lewisburg WV 24901 USA; Dr. Seidensticlcr is Curator of Manmmls at the National Zoological Park,
Smilthonian Istitution, Washington DC 2008 USA.
MUGAAS, J. N., AND J. SEIDENSTICKER. 1993. Geographic variation of lean body mass and a
model of its effect on the capacity of the raccoon to fatten and fast. Bull. Florida Mus. Nat. Hist.,
Biol. Sci. 36(3):85-107.
86 BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
RESUME
El peso magro aparente (ALBM) de los mapaches (Procyon lotor) en el este de los Estados Unidos
aumenta de sur a norte y parece seguir la regla de Bergmann: en la zona subtropical de Key Vaca, las
hembras pesan 2.0 kg y los machos 2.4 kg; en el sudeste templado de los Estados Unidos, las hembras pesan
4.5 kg, los machos 5.0 kg. Postulamos que la selecci6n ha favorecido los pesos magros aparentes grandes en
las zonas frias del irea de distribuci6n del mapache, en raz6n que esta provee mayor resistencia al ayuno.
Los incrementos en el peso magro (LBM) en mamiferos son acompalnados por incrementos en la capacidad
para almacenar energia en forma de grasa (Fs=LBM .0) fuera de proporci6n con respect al costo del
metabolism basal (Hb=eso 075). Por tanto, los mapaches grandes y gordos deberian ser capaces de
ayunar por mis tiempo que aquellos gordos y peguefios. Nosotros modelamos esas relaciones para
mapaches. Encontramos que cada incremento en ALBM result en un incremento sustancial en el tiempos
que ellos pueden ayunar. Dado que el alargue en el tiempo de ayuno es necesario para la sobrevivencia
durante el invierno, el modelo apoya nuestra hip6tesis. Tambien concluimos que, el limited norte de su
distribuci6n es determinado por sus limits en el potential gen6tico para aumentar el ALBM. La cantidad de
grasa depositada durante el otoflo tambi6n vari6 geograficamente: los mapaches subtropicales alcanzaron
14% or 17% de gordura aparente (ABF), aquellos desde Florida a Virginia 19-42% ABF), y aquellos
alrededor de los Grandes Lagos 31-50% ABF. La variabilidad geogrifica en ABF sugiere que la lipog6nesis
estacional esta ligada, mediante mecanismos neuro endocrinos, a sefiales medioambientales que estimulan el
grado de deposici6n de grasa apropiado para cada Area. Los datos sugieren asimismo que, pueden existir
diferencias geogrificas en la capacidad para engordar.
TABLE OF CONTENTS
Introduction............................ ... ... ..................... ................ 86
Acknowledgements ................................................. ........................ 88
M materials and M ethods ............................................................................ 89
Geographic Variation in Apparent Lean Body Mass ................................ .......... 92
Results ...................................... ....................................... 92
Adults ............................. ........................................ 92
Juveniles................................ ................................... ... 94
Discussion ..... ......................................................................... .................. 95
Adults .................................................................... ................ 95
Adult raccoon response to warm climate....................... ....................... 96
Adult raccoon response to mild climate ............................ ................... .... 96
Adult raccoon response to harsh and severe climates.......................... ............ 97
Adult raccoon response to moderate climate ......................................................... 97
Response of juvenile raccoons to seasonal and geographic variation in climate .............. 98
Model of the Capacity for Fattening and Fasting ....................................................... 98
Total Body Mass, Apparent Lean Body Mass, and Apparent Body Fat Relationships.......... 98
Physiological Limits of Fattening ........................................ .................. 100
Days of Fasting ................................................................ 100
Lessons From the M odel........................................... ....................................... 101
Sexual dimorphism ................................................... ... ................... 101
Apparent lean body mass......................... ............ .............. 101
Fall fattening ............................... ........................................ .................. 103
Possible Genetic Basis of Geographic Variation in Raccoon Body Size ............................ 104
Literature Cited ....................................................... .............. .................. 105
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 87
INTRODUCTION
The North American raccoon, Procyon lotor (Fig. 1), has a distribution
that extends from 8*N in tropical Panama to 600N in Alberta, Canada (Lotze and
Anderson 1979; Kaufmann 1982). Geographic variation in the size of this widely
distributed species has been examined by several investigators. Johnson (1970)
reports that, east of the Mississippi River in the United States, adult male raccoons
from northern states are larger (6.1 to 8.0 kg) than those from southern states (2.4
to 5.2 kg). He speculates that their body mass follows Bergmann's rule.
Raccoons also display similar size-latitude correlations for head-body length
(McNab 1971), and, in the eastern United States, skull characteristics (Kennedy
and Lindsay 1984; Ritke and Kennedy 1988). In the western United States, skull
characteristics show a size-longitude, rather than a size-latitude relationship (Ritke
and Kennedy 1988).
Bergmann's rule states that "...races from cooler climates, in species of
warm-blooded vertebrates, tend to be larger than races of the same species living
in warmer climates..." (Mayr 1963:319). Many authors relate geographic
variation in size to adaptive differences in heat-exchange characteristics (surface to
volume ratio, insulative value of fur or feathers, etc.), and the assumption is that
selection favors size variation for thermoregulatory reasons (Mayr 1956, 1963;
Rensch 1959; Hamilton 1961; Bartholomew 1968; Brown and Lee 1969;
Kendeigh 1969; James 1970, 1991; Kleiber 1972; Calder 1974, 1984; Searcy
1980). Calder (1974) summarizes the size-related variation in heat-exchange
characteristics that influence thermoregulation: (1) the magnitude of the
thermoneutral zone varies directly with size, and (2) mass specific thermal
conductance varies inversely with size. In this scheme, the wider thermoneutral
zone and lower thermal conductivity of large size are a benefit in cold climates
because they slow down heat loss, while, for the opposite reasons, small size is a
benefit in warm climates. Some investigators contend that since there are
selection pressures other than temperature that also act on body size, it is difficult,
or even impossible, in some cases, to demonstrate that size variation functions
primarily as a thermoregulatory adaptation (Scholander 1955; Rosenzweig 1968;
McNab 1971; Searcy 1980; Kennedy and Lindsay 1984).
Another advantage to geographic variation in size, and one that may be
fundamentally more important than thermoregulatory superiority, is fasting
endurance, which varies directly with mass (Lindsey 1966; Rosenzweig 1968;
Calder 1974, 1984; Downhower 1976; Ketterson and Nolan 1976; Young 1976;
Adler 1984; Lindstedt and Boyce 1985; French 1986, 1988). Variation in size is
accompanied by variation in lean body mass (LBM). As LBM increases, the
ability to store energy as fat (Fs = LBM1.0, where Fs is fat storage; Calder 1984)
increases out of proportion to the rate of basal metabolism (Hb = m0.75, where
Hb is basal metabolism and m is total body mass). Consequently, large fat
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
Figure 1.-North American raccoon, Procyon lotor.
individuals can withstand a longer fast than small fat ones. This suggests the
following hypothesis. In the northern part of the raccoons'range, where they are
forced to retreat to a den and live without eating for several months each winter
(Stuewer 1943; Mech et al. 1968; Schneider et al. 1971), large lean body mass has
been selected for because it increases the length of time these individuals can fast.
The selection force in this case would be the length and severity of the winter
period.
To test this hypothesis we examined published and unpublished body-
mass data for raccoons from eight different latitudes in the eastern United States.
We were able to estimate apparent lean body mass (ALBM), and apparent amount
of fat deposited each fall (ABF) at each location. From the latitudinal differences
in these variables, and the differences in maximum fasting times associated with
variations in ALBM, we concluded that survival of raccoons in the northern part
of their range is dependent on selection for large LBM. Our data suggest there is
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 89
a genetic basis for geographic variation in LBM, and that the northern edge of the
raccoon's distribution may be determined by the limits of its genetic potential to
increase LBM.
ACKNOWLEDGMENTS
This investigation was supported by Friends of the National Zoo, National Zoological
Park, Smithsonian Institution, and the West Virginia School of Osteopathic Medicine. Essential
assistance and support were provided by M. Bush, J. Eisenberg, K. Halama, J. Hallett, A. J. T.
Johnsingh, D. Kleiman, K. Kranz, S. Lumpkin, M. A. O'Connell, J. B. McConville, G. Sanders, M.
Sunquist, and C. Wemmer. Their friendship and help is gratefully acknowledged. D. Brown, W. A.
Calder, HI, J. E. Eisenberg, B. K. McNab, and M. Sunquist provided critical comments on various
drafts of the manuscript. The Virginia Commission of Game and Inland Fisheries gave us permission
to use wild-caught raccoons in our study.
MATERIALS AND METHODS
Body masses of adult raccoons from 250 to 45 north latitude were taken from literature
(Table 1): Minnesota (Mech et al. 1968), southern Michigan (Stuewer 1943), southwestern
Tennessee (Moore and Kennedy 1985), east central and southwestern Alabama (Johnson 1970),
central Florida (Goldman 1950), and Key Vaca, Florida (Goldman 1950). Data for adult raccoons
from north central Virginia (38N) were obtained from a five-year investigation of the Posey Hollow
raccoon population at the National Zoological Park's Conservation and Research Center (CRC, Front
Royal, Virginia). Raccoons at CRC were live-trapped during the first 10 days of each month (May
1980 through December 1984) on a fixed trapping grid of 30 to 35 stations (one or two live-traps per
station). Within a few hours after capture all trapped animals were weighed, sexed, aged, and
categorized with respect to reproductive status, physical condition, and parasite load before being
released (Seidensticker et al. 1988; Hallett et al. 1991). Mass data of juvenile raccoons were obtained
for Minnesota (Mech et al. 1968) and for north central Virginia (Posey Hollow study, CRC).
At CRC, body mass was at its maximum (MBM) in late fall or early winter when raccoons
were fattest. During the winter months raccoons gradually depleted their fat stores, and, by spring,
their body masses were at their annual minimum. These animals were in poor condition and very
skinny in the spring, and this annual minimum was used to approximate their apparent lean body mass
(ALBM).. ALBM was constant from year to year, which indicated that body fat was depleted to about
the same extent each winter, and that ALBM was a stable feature of the population. Because of the
constancy in ALBM, we could estimate the approximate mass of body fat (ABF) deposited each fall,
and the resultant percent body fat, from the difference between MBM and ALBM. This same logic
was used to estimate MBM, ALBM, and ABF from mass data for raccoons from other geographic
areas (Table 1). Except for adult raccoons from Minnesota, where data are for individual animals, all
body mass values in Table 1 are averages for each geographic area.
Based on climate data from Kincer (1941), we classified the severity of winter for each of
these locations as either warm, mild, moderate, harsh, or severe (Table 2). Isotherm maps of average
January, and average annual temperatures for the eastern United States (Fig. 2) place these climate
categories, and the locations from which data were obtained, into perspective.
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL 36(3)
Table 1.-Annual Change in Procyon lotor body mass.
Annual Body Mass (kg) Body Fatb
Location (*N) Maximuma Minimuma Maximum (kg) %MBM Refh
ADULTS
Key Vaca (25*N)
Male
Female
Florida (280N)
2.8c 2.4c
1.8-2.7(5)
2.4c 2.0C
1.8-2.3(2)
Male 5.0d
4.5-5.4(5)
Female 4.1d
3.5-4.5(3)
Alabama, southwestern (320N)
Male 4.3
0.2 (56)
Female 3.8
1
Alabama, east central (340N)
Male
1
Female
1
Tennessee (36 N)
Male
Female
Virginia (38*N)
Male
Female
'
0.2 (30)
5.3
0.9 (12)
4.3
0.4 (17)
6.8e
5.5e
6.7
:0.9 (7)
5.6
0.8 (17)
3.6
0.2 (32)
3.2
0.3 (15)
3.5
0.4 (10)
3.2
0.2 (8)
4.0
0.5 (53)
3.6
0.6 (50)
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 91
Table 1 Continued.
Annual Body Mass (kg) Body Fatb
Location (N) Maximum" Minimuma Maximum (kg) %MBM Refh
Michigan (420N)
Female
Minnesotaf (45 N)
608, Female
170, Male
634, Female
JUVENILES
Virginia (38 N)
Male
Female
Minnesotag (45 N)
3.3
0.9 (19)
3.0
0.7 (22)
3.1
0.5 (12)
2.4
0.6 (22)
Sex? 4.3 2.1 2.2 51
0.9 (18) 0.3 (6)
aAnnual maximum body mass = MBM; annual minimum body mass = apparent lean body mass =
ALBM; in kg, with SD, or range of values, and (n) = number of individuals if known.
bAnnual maximum body fat in kg = apparent body fat = ABF; ABF = MBM ALBM; %MBM =
(ABF.100/MBM).
CData obtained in late winter and considered representative of ALBM. MBM = (0.18.ALBM) +
ALBM; based on fall mass gain of raccoons in southwestern Alabama (Johnson 1970).
dData obtained in winter and considered representative of MBM. ALBM estimated assuming 25%
ABF for males and 30% ABF for females (Johnson 1970; Zeveloff and Doerr 1985).
eTennessee raccoons from Fig. 1 in Moore and Kennedy (1985). Michigan raccoons from Table 3 in
Stuewer (1943).
fEach number is for an individual animal from the original study of Mech et al. 1968.
gCalculated from Fig. 1 ofMech et al. (1968).
hi. Goldman (1950); 2. Johnson (1970); 3. Mech et al. (1968); 4. Moore and Kennedy (1985); 5.
Stuewer (1943); Zeveloffand Doerr (1985); 7. This study.
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
AVERAGE JANUARY TEMPERATURE (oC)
AVERAGE ANNUAL TEMPERATURE (oC)
18.31 MN, SEVERE
,- M'. HASH
T vA MOnERATE
S TN. MODERATE
A L IRU.I.ILO
WARM O AL (i. MILD
KE MILO
SKEY VACA FL. NARM
Figure 2.-Locations from which body mass data for Procyon lotor were selected, and isotherms for
the eastern United States.
Table 2.-Climate data.a
Temperatures (*C)
Days Depth
Geographic With Frost- of Average
Area and Snow Free Frost Annual Average Average
Climate Coverb Days (inches) Minimum January Annual
Key Vaca (25 N)
Warm OC 365C 0C 4.4 21.0 23.9
Florida/Alabama (28 to 340N)
Mild <10 240-320 0-3 -9.4/-1.1 7.2/15.6 18.3/21.1
Tennessee/irginia (360 to 38*N)
Moderate 10-40 180-200 3-10 -17.7/-9.4 -1.1/4.4 12.7/15.6
Southern Michigan (420N)
Harsh 80-100 160 25-42 -26.1/-23.3 -6.7/-3.8 7.2/10.0
Minnesota (45 N)
Severe 100-120 140 60-72 -37.2/-31.7-12.2/-9.4 4.4/7.2
aExcept where indicated all data are talan from climate map for the United States (Kinoer 1941). Where two value are given for a
variable they premost the range of valuh from lowest to highest.
bOne inch or more of snow cover.
cFrom climate maps for Florida (Norton 1941).
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 93
GEOGRAPHIC VARIATION IN APPARENT LEAN BODY MASS
Results
Adults
At all latitudes males were heavier than females in both fall and spring
(Table 1). The difference in mass between sexes was greater in fall than in spring
(0.8 0.3 vs 0.5 0.24 kg; p < 0.025). This indicates that ALBM of males is,
on average, 0.5 kg greater than that of females, and that, in fall, males fatten
proportionately more than females.
In general, ALBM increases with increasing latitude (Y = 0.12.X 0.36;
r2 = 0.77, where Y is ALBM and X is latitude; Fig. 3), but it is obvious that the
relationship is not a smooth continuum. ALBM is stable across some latitudes but
changes across others. There are two regions of change in ALBM: one between
Key Vaca and the mainland, and the other somewhere between 35 and 40*N (Fig.
3). In each case, the transition zone separates a latitudinal change in ALBM of
more than 1 kg. This gives ALBM a steplike distribution from Key Vaca to
Minnesota, and produces three distinct, stable, size classes: Key Vaca, 25N,
females = 2.0 kg, males = 2.4 kg; southeastern United States, 28 to
a MN, SEVERE
MI, HARSH
S 5.5 VA, MODERATE
0 AL. (ec), MILD A1
) 0 AL, (sw), MILD 60
A FL, MILD
5 KEY VACA, FL, WARM
4.5- MN, JUVENILES 34
o [ VA, JUVENILES
0
DO
z 0
< 3.5 0
ULIJ A
.-J _
I-
z
cc 2.5 -
<
1.5
1 .5 ---- I --- --- I ---- I ---- I ---- I
20 25 30 35 40 45 50
LATITUDE (ON)
Figure 3.-Relationship between apparent lean body mass and latitude for Procyon lotor. The lowest
mass value for each pair of symbols represents females, and the highest value males. Symbols for
Minnesota are for individual animals. Symbols for adults from each climate type are surrounded by a
line.
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL 36(3)
34*N, females = 3.2 kg, males = 3.5 to 3.8 kg; and southern Great Lakes
region, 40 to 450N, males = 5.1 to 5.2 kg, females = 4.5 to 5.0 kg (Fig. 3). As
a species, therefore, raccoons display a wide range of variation in ALBM.
The amount of fat deposited in the fall appears to depend on the severity
of the climate to which they are exposed (Table 1, Fig. 4). Thus, ABF is variable
both within each climate zone and between climate zones; compare body fat of
raccoons from the mild (16 to 34% body fat) and moderate (36 to 42% body fat)
climates, and body fat of raccoons from the harsh (31 to 32% body fat) and severe
(47 to 50% body fat) climates. This indicates that fall fattening is not an all-or-
none phenomenon, but, rather, that it is a graded response, whose degree of
expression is determined by the physiological response to local climate. Within
any given geographic area, therefore, several environmental cues (changing
daylength, changing temperature, changing food abundance, changing food type,
to name a few), and the animal's metabolic response to them, as orchestrated by
its neuroendocrine system, determine how much body fat is actually deposited
during each fall period.
6 A MN, SEVERE 50
MI, HARSH
O VA, MODERATE
TN, MODERATE 170
5 0 AL, (ec), MILD
S0 AL, (sw), MILD
A FL, MILD 634 A608
KEY VACA, FL, WARM
4 E MN, JUVENILES
E VA, JUVENILES
m 3
2 130%
I-
120%
0II0III
0
0 1 2 3 4 5 6
APPARENT LEAN BODY MASS (kg)
Figure 4.-Relationship between apparent body fat and apparent lean body mass for Procyon lotor.
The lowest apparent lean body mass value for each pair of symbols represents females, and the
highest value males. Symbols for adults from each climate type are surrounded by a line. Symbols
for Minnesota are for individual animals. The diagonal isolipid lines represent 10, 20, 30, 40, and
50% body fat.
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 95
All raccoons may be able to fatten to 50% body fat, but only those in the
heaviest size class, living in the most severe climate (Minnesota), apparently
achieved this level of obesity (Fig. 4). This could simply mean that in areas south
of Minnesota conditions were not severe enough to stimulate maximum fattening.
Alternatively, different size classes may have different capacities to fatten. If the
latter is true, then there may be geographic differences between populations with
respect to their capabilities to fatten.
Juveniles
Young of the year from Minnesota and north central Virginia also
fattened in the fall, and reached MBM by early December. Between December
and early spring they depleted this fat reserve. The minimum mass achieved at the
end of their first winter was less than that of adult animals, and was considered to
be their ALBM. ABF was taken as the difference between this value and the
MBM attained during their first fall. During the spring and early summer,
following their first winter, these young animals increased their ALBM up to the
adult level, and maintained it there until they started to fatten during their second
fall. This pattern was evident in data from CRC as well as in Figure 2 of Mech et
al. (1968). Raccoons in Michigan (Stuewer 1943) and Alabama (Johnson 1970)
also do not attain adult mass until fall of their second year. Young of the year
from Minnesota had a lower ALBM (2.1 kg) than those from Virginia (females =
2.4 kg, males = 3.1 kg), but young of the year from Minnesota accumulated more
fat (2.2 kg = 51% body fat) than those from Virginia (females = 0.6 kg or 20%
body fat; males = 0.2 kg or 6% body fat; Table 1, Fig. 4). For young of the
year, therefore, the gain in ALBM and the degree of fattening both appear to be
linked to length and severity of local winter climate.
Discussion
Adults
Throughout the United States, raccoons fatten during fall, and they attain
their annual MBM in the interval from November to early January (Stuewer 1943;
Mech et al. 1968; Johnson 1970; Moore and Kennedy 1985; Zeveloff and Doerr
1985).
In climates with severe winter weather (Minnesota, for example; Mech et
al. 1968; Schneider et al. 1971), raccoons may remain in their dens for three
months or more in the interval from mid-November to mid-March. Since they
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
keep their body temperature (Tb) above 35C during this confinement
(Thorkelson 1972), their thermogenic requirement is, at the very least, equivalent
to their basal metabolic rate (Mugaas et al. 1993). All through this period of
forced fasting, their body fat serves as the primary energy source to maintain
endothermy. By end of winter, their body fat is depleted, and their mass is at its
minimum.
In milder climates, winter weather may confine raccoons to their dens for
short periods of time, only occasionally, or not at all (Sharp and Sharp 1956;
Johnson 1970; Zeveloff and Doerr 1985; Seidensticker et al. 1988). During
winter in these areas, food may be in short supply, difficult to find, and of poor
quality. Consequently, even though raccoons may be active nearly every night,
they may not find enough food to meet their energy requirements. In this case,
their body fat serves as an important energy supplement to their diet, and, as in
colder climates, it is depleted during the course of the winter (Johnson 1970;
Zeveloff and Doerr 1985). Fall fattening, therefore, appears to be an important
adaptation for the winter survival of this species in the United States.
Adult raccoon response to warm climate
Raccoons on Key Vaca (250N) live in the warmest climate (Fig. 2), and
have the smallest ALBM (2.0 to 2.4 kg; Table 1, Fig. 3). These are values for
individuals taken on the island in late winter (Goldman 1950), which is near the
time that raccoons on the adjacent mainland reach their annual minimum mass
(Johnson 1970; Zeveloff and Doerr 1985). Raccoons from the adjacent mild
continental climate zone have ALBMs that are 1.2 to 1.4 kg larger than those of
the Key Vaca raccoons (Fig. 3).
Unfortunately, little is known of the natural history of these diminutive
island raccoons, and the factors that have selected for their small size have not
been identified. However, small ALBM would reduce their muscle mass, and
total energy requirement (McNab 1978, 1986, 1989). This would provide these
animals with a thermoregulatory advantage in this hot humid climate (Calder
1974), and, compared to the heavier mainland raccoons, this would also reduce
their total food requirement. This latter point could be important if Key Vaca has
a lower level of productivity than the mainland. If food availability on this island
does undergo a small seasonal increase, there could be a period of fattening in Key
Vaca raccoons. If Key Vaca animals fattened to the same extent as those from
coastal southwestern Alabama, they would gain about 0.4 kg of fat each fall
(Table 1, Fig. 4). Even though these values are only approximations, we plotted
them on Fig. 4 to illustrate that this would provide these animals with a fairly
large caloric supplement to help them through lean times.
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 97
Adult raccoon response to mild climate
Data for raccoons from Florida and Alabama span the mild climate zone
(Figs. 2 and 3), and these are the smallest of the continental forms (Fig. 3).
Raccoons in this climate zone fatten less in fall than those from other continental
areas (Fig. 4). They also display a wide range of variability in their fattening
response, 16 to 34% body fat (Fig. 4), with raccoons from colder areas (east
central Alabama) fattening more than those from milder areas (southwestern
Alabama). Raccoons in this climate zone are active year-round (Johnson 1970;
Zeveloff and Doerr 1985), and the fat they store in fall must function primarily as
a caloric supplement to their normal winter diet. Within these latitudes, variations
in the requirement for winter energy supplementation are met simply with
differences in the amount of body fat stored during the fall.
Adult raccoon response to harsh and severe climates
Raccoons from harsh (southwestern Michigan), and severe (Minnesota)
climates are confined to their dens for several months each winter (Stuewer 1943;
Schneider et al. 1971), and fat stored in the fall is their only source of calories
during this period of fasting. Raccoons from these areas have ALBMs that are, on
average, 1.5 kg greater than those from Florida and Alabama (Table 1, Fig. 3).
In Minnesota, raccoons enter the winter with 47 to 50% body fat, while
on Stuewer's (1943) study area in Michigan they acquire only 31 to 32% body fat.
Winters on Stuewer's study area are milder than those in Minnesota, and raccoons
in that part of Michigan are only confined to their dens for about 60 days each
winter. Those at Cedar Creek, Minnesota, however, are usually obliged to den
continuously for 90 days or more (Mech et al. 1968; Schneider et al. 1971). In
other parts of Michigan, where winters are more severe, raccoons den for longer
periods of time (Stuewer 1943). We assume those animals meet that challenge by
fattening to a greater extent than raccoons on Stuewer's study area. Thus, even in
these colder climates, where selection favors an increase in ALBM, the degree of
fall fattening is still dependent on local climate conditions.
Adult raccoon response to moderate climate
Between 35 and 40N, ALBM increases with increasing latitude (Fig. 3).
These latitudes appear to form a transition zone between the large raccoons of the
southern Great Lakes region and the smaller forms in the southeast, making this a
region of phenotypic variability for ALBM. Within these latitudes, ALBM
changes by 0.19 kg/latitude (Y = 0.19.X 3.214; r2 = 0.75, where Y is ALBM
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL 36(3)
and X is latitude; solid line in Fig. 3). Separate regression equations for males
and females from these latitudes were not different from each other, or from the
equation for combined data (p > 0.1). Within this transition zone, therefore,
gene flow and selection for ALBM appear to have the same effect on both sexes.
This climate zone, situated at the mid-latitudes of the eastern United
States, has winter conditions that are intermediate to those north and south of it.
In some areas within this zone, winter weather may be so mild that raccoons are
active all during that season, in which case their fat stores would simply
supplement their diet. In other areas, such as southwestern Tennessee (Moore and
Kennedy 1985) and north central Virginia (Seidensticker et al. 1988), periodic
intervals of harsh winter weather confine raccoons to their dens for a few days or
weeks at a time. In these areas, fat stores serve as both a primary and a
supplementary source of calories.
Response of juvenile raccoons to seasonal and geographic variation in climate
In the eastern United States, most raccoon births occur in the interval
from March to mid-June (Lotze and Anderson 1979), but, in Florida, some may
be born as late as October (Kaufmann 1982). Weaning occurs between week 7
and month 4 (Lotze and Anderson 1979), and survival of the young during their
first winter, in any of the climate zones, depends on their ability to acquire
enough fat during fall to meet the energy requirements imposed on them by their
local climate. Because of the relationship between Fs and Hb, it is important that
young of the year first produce enough lean body mass during summer to support
the fat required to meet their winter energy requirement. Thus, these constraints
should exert a strong selective influence on the timing of births, particularly in
areas north of Florida (Table 2, Fig 2).
Young of the year from Virginia and Minnesota do not complete growth
of their lean body mass until after their first winter (Table 1, Fig. 3). As young
raccoons enter their first fall, growth of lean mass must be slowed, or even
suspended, in order to allow fattening to take place. In Minnesota, young of the
year (ALBM = 2.1 kg) entered their first winter with 51% body fat (Table 1,
Fig. 4), which suggests that, in the severe Minnesota climate, growth of lean body
mass was actually suspended in the fall in favor of fat deposition. In Virginia,
where winter climate is considerably milder than in Minnesota, ALBM was larger,
but ABF amounted to only 6 to 20% body fat (Table 1, Fig. 4). This appears to
be a contradiction to our hypothesis, but, in Virginia, young of the year do not
need as much fat for winter survival, and growth of lean body mass in the fall
must have simply been slowed, rather than suspended. This would give Virginia
juveniles a larger ALBM at the end of their first winter.
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS
S A MN, SEVERE
MI, HARSH
O VA, MODERATE
5 TN, MODERATE
0 AL, (ec)-MILD
0 AL, (sw)-MILD
A FL, MILD
KEY VACA,FL, WARM
6 8 10 12
APPARENT LEAN BODY MASS (kg)
TOTAL BODY MASS (kg)
Figure 5.-Model of fattening and fasting ability of Procyon lotor. Apparent body fat (ABF = kg),
and basal metabolism (Hbf = kg fat) are represented by the Y-axis. Apparent lean body mass
(ALBM = kg) and total body mass (TBM = kg) are represented by the X-axis. The heavy diagonal
line portrays 50% body fat. Each diagonal dashed line represents the relationship between ABF and
TBM. When ABF = 0, the diagonal dashed line intersects the X-axis, and TBM = ALBM.
Diagonal dashed lines, therefore, represent fattening trajectories for raccoons of various ALBM, and
they intersect the heavy solid diagonal line at 50% body fat. The relationship between Hbf and TBM,
as calculated from Eq. 1, is shown by the thin curved solid lines, which describe the mass of fat
required to fuel Hbf for 30, 60, 90, 120, or 150 days. Intersection of a fattening trajectory with a
thin curved line describes the mass of fat required for a raccoon of that ALBM to fast for that number
of days. Symbols represent fall fattening, and its related fasting potential for adult raccoons from
eastern United States.
MODEL OF THE CAPACITY FOR FATTENING AND FASTING
Total Body Mass, Apparent Lean Body Mass,
and Apparent Body Fat Relationships
In developing the model shown in Figure 5, the following assumptions
were made for adult raccoons. (1) TBM = ALBM + ABF, where TBM is total
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
body mass. (2) Changes in ALBM during the year are small compared to those
for ABF, so that ALBM is considered constant, and annual changes in TBM are
due to variations in ABF. (3) Body fat content is a function of LBM1.0 (Calder
1984); therefore, ABF will always be less than, or, at most, equal to ALBM.
Thus, in the spring, when ABF = 0, TBM = ALBM, but as the animal fattens
ABF and TBM both increase.
Temporal changes in ABF and TBM, for various ALBMs, are plotted on
Figure 5 as thin dashed diagonal lines, and we refer to these as fattening
trajectories. For example, one of the Minnesota raccoons (608) has an ALBM of
5.0 kg. In the spring when ABF = 0, and TBM = ALBM, we plot (TBM, ABF)
as (5.0, 0). In late summer and fall as the animal fattens, ABF and TBM both
increase. If the increases are plotted at various times, the points follow the thin
dashed line that runs diagonally upward from (5.0, 0) to (9.5, 4.5). The smallest
Minnesota raccoon (634) has an ALBM of 4.5 kg. Its fattening trajectory begins
at (4.5, 0), and follows a path that runs parallel to the other thin dashed lines. At
the end of fall fattening, ABF = 4.5 kg, and the final point on its trajectory is
(9.0, 4.5).
Physiological Limits of Fattening
Raccoons may be able to achieve 50% body fat, and this is shown in the
model by the heavy solid diagonal line (Fig. 5). For each ALBM, this degree of
obesity occurs at the point where its fattening trajectory intersects the heavy solid
diagonal line. In the above examples, the smallest raccoon from Minnesota
(ALBM = 4.5 kg) achieved maximal fattening (50% body fat), whereas the other
animal (ALBM = 5.0 kg) realized a little less than maximal fattening (47% body
fat; Fig. 5).
Days of Fasting
In winter, the lower critical temperature (Tic) of raccoons from Virginia
is 11oC, but when they are in their tree den it is reduced to -50C (Mugaas et al.
1993). The average January temperature for Front Royal, Virginia, is 1C
(Crockett 1972), therefore, even during this coldest month, thermal conditions in
their dens are, on average, within their thermoneutral zone, and their
thermoregulatory requirement is equivalent to their Hb. We do not have
metabolic data for raccoons from other areas, but it is reasonable to assume that
geographic variation in thermal conductance is such that the thermogenic demands
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 101
associated with winter denning are, in general, not much greater than Hb.
Consequently, we modeled fasting ability in terms of the mass of fat required to
fuel Hb. Equation 1 describes raccoon basal metabolism as kg of fat:
Hbf = (Hb.TBM0.75.tJ02)/Jf Eq. 1
where Hbf is basal metabolic rate (kg fat); Hb is measured basal metabolic rate
(16.44 L02.kg-0.75day-1, average for raccoons in winter; Mugaas et al. 1993);
TBMO.75 is metabolic body mass (kg); t is time (number of days); J02 is heat
equivalent for oxygen (0.020087 MJ/L); and Jf is heat equivalent for fat (39.329
MJ/kg). Equation 1 was used to estimate the mass of fat (kg) required to fuel
basal metabolism of raccoons ranging in TBM from 1 to 25 kg for 30, 60, 90,
120, and 150 days (thin curved lines, Fig. 5).
The fattening trajectory of each ALBM crosses the thin curved
metabolism lines, and their points of intersection describe ABF required to fuel
Hb for 30, 60, 90, 120, or 150 days. The smallest raccoon from Minnesota
(ALBM = 4.5 kg) requires 0.9, 2.1, and 3.7 kg ABF to fuel basal metabolism for
30, 60, and 90 days, respectively (Fig. 5). If this animal gained 5.8 kg of fat, it
could fuel Hb for 120 days (Fig. 5). However, since this represents 1.3 kg more
fat than it is capable of depositing (5.8 4.5 = 1.3), the model predicts that this
raccoon would not be able to fast for 120 days, and would not be able to survive
in areas that required it to do so.
Lessons From the Model
Sexual dimorphism
Male raccoons have a 0.5 kg larger ALBM than female raccoons. Males
also have a more massive skull (Kennedy and Lindsay 1984), and larger canines
(Grau et al. 1970). Kennedy and Lindsay (1984) argue that selection favors larger
size in male raccoons because it benefits them in competition for mates. Our
model predicts that the 0.5 kg difference in ALBM between sexes changes fasting
time within each size class by only 2.5 to 3.0 days. Selection for larger males, for
whatever reason, has not produced sexes with vastly different fasting potentials.
Because they are larger, however, males have a higher daily energy requirement
than females. In the fall, therefore, males would need more food than females to
achieve an equivalent degree of obesity, and potential for fasting. This suggests
that the increased food requirement associated with their larger size may be a
constraint that keeps male raccoons from becoming grossly bigger than females.
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
Apparent lean body mass
Each solid line on Figure 6 describes the relationship between days of
fasting, and ABF for a particular ALBM. Coordinates (ABF, days of fasting)
used to construct these lines were determined from an expanded version of Figure
5, and are the intersections of the fattening trajectories with the thin curved
metabolism lines for days of fasting. The large dots describe days of fasting
possible when starting with 50% body fat. The coordinates for each dot,
therefore, could be written as (ALBM, days of fasting), and the X-axis could
represent ALBM. Thus, the maximum days of fasting associated with any ALBM
can be estimated from Eq. 2, which describes a line drawn through the large dots:
Y = 69.2.X0.258 Eq. 2
where Y is days of fasting, and X is ALBM. As ALBM increases, the
incremental increase in days of fasting decreases (Fig. 6). Thus, while a 3-kg
increase in ALBM from 2 to 5 kg can yield a 24-day gain in fasting time, a similar
change in ALBM from 8 to 11 kg will produce only an 8.5-day increase. This
160 -
140 -
120 -
100 -
80 -
60
3 5 7 9
40 ,0, //
2 l0 . i i .I ,
APPARENT BODY FAT (kg)
Figure 6.-Relationship between days of fasting and apparent body fat for apparent lean body masses
from 1 to 15 kg (curved lines). Dots represent days of fasting at 50% body fat for each apparent lean
body mass.
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 103
indicates that, for raccoons, the strategy of increasing ALBM to increase fasting
time reaches a critical point of diminishing returns at about 8 kg.
Data from Key Vaca suggest that the smallest ALBM for raccoons is
about 2 kg. What is the largest ALBM? For the following reasons we think it is
around 8 kg. Lotze and Anderson (1979:2) state that the largest raccoons are
"from Idaho and vicinity," and Ritke and Kennedy (1988) report that, based on an
analysis of 22 skull characteristics from 128 sampling quadrants in the United
States, Mexico, and Central America, the largest raccoons are found in eastern
Washington, eastern Oregon, western Idaho, and California. However, neither
reference provides any body mass data for those areas. The largest raccoon body
masses reported in the literature are for animals taken by hunters and trappers in
northern Minnesota, Maine, and west central Illinois, and these animals had
TBM's between 12 and 14 kg (Goldman 1950; Whitney and Underwood 1952;
Sanderson 1983). These animals were taken in late fall; therefore, 40 to 50% of
their mass would have been fat. ALBM of a 12-kg raccoon would have varied
from 6.0 to 7.2 kg, and that of a 14-kg animal from 7.0 to 8.4 kg. Since raccoons
at the northern edge of their range may not need to fast for more than 120 days,
and since it would take a very large increase in ALBM to boost fasting time much
beyond 120 days (Fig. 6), 8 kg may closely approximate maximum ALBM for
this species. This implies that, for Procyon lotor, ALBM ranges from about 2 to
8 kg.
This model supports our hypothesis that geographic variation of ALBM
in raccoons is selected for by latitudinal differences in length and severity of
winter climate. In the parlance of adaptational biology (Prosser 1986), the wide
range of variation in ALBM has acclimatized this species to a wide range of
climates and habitats. In climates with short mild winters, selection favors
raccoons with small ALBM because it lowers their absolute energy requirement,
and improves their ability to dissipate heat. In the northern part of their range,
where winters are long, and fasting endurance is of critical importance, selection
favors raccoons with large ALBM. Even though large ALBM carries with it a
greater absolute energy cost (McNab 1971), this is obviously less of a
disadvantage in cold climates than the advantage it provides via its attendant
increase in fasting endurance. While we do not know what limits the southern
spread of the raccoon, this analysis suggests that the northern limit of their range
is physiologically determined by the size of their ALBM.
Fall fattening
In warm and mild climates, where the fattening response is minimal,
raccoons still deposit enough fat in fall to support a fast of nearly 30 days (Fig.
5). For raccoons in the continental United States, this may represent the minimum
BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 36(3)
response that occurs whenever environmental cues stimulate the fattening process.
If all raccoons have the physiological potential to attain 50% body fat, then the
difference between the degree of fattening actually achieved, and their potential
for fattening, may represent a physiological reserve that goes unutilized unless it is
activated by the appropriate environmental stimuli. This physiological reserve for
fat deposition may be an adaptation that would acclimatize each raccoon to winter
conditions over a wide geographic area (Prosser 1986).
But, not all raccoons may have the physiological potential to attain 50%
body fat, particularly those that live in areas where that degree of obesity is never
required for survival. If this is the case then we would expect to find small
raccoons from warm climates having less potential to fatten than larger raccoons
from mild and moderate climates, and those, in turn, to have less potential than
raccoons from harsh and severe climates. In this case the physiological reserve for
fat deposition would also vary geographically, and raccoons with small ALBM
would have a narrower range of acclimatization than those with large ALBM.
POSSIBLE GENETIC BASIS OF GEOGRAPHIC VARIATION
IN RACCOON BODY SIZE
In the eastern United States, raccoons can be divided into at least four
geographic populations that display distinctly different ALBMs and potentials for
maximum fasting (Figs. 3, 5, 6): (1) Florida Keys, (2) southeastern United
States, (3) transition zone, and (4) southern Great Lakes region. If ALBM is an
adaptation, selected for by winter climate on the basis of its survival value, then
each region of stable ALBM could represent an area of genetic homogeneity
maintained by ease of gene flow and a common selective pressure (Beck and
Kennedy 1980), while the transition zone could be a hybrid zone where
populations on either side of it meet, mate, and produce hybrids of intermediate
ALBM (Mayr 1963; Barton and Hewitt 1989). Variation in ALBM between
populations could be the result of (1) true genetic adaptation to local conditions,
or (2) environmental induction or modification during development, ie.,
phenotypic plasticity (Berven et al. 1979; Berven and Gill 1983; Steams 1989).
These genetic speculations need to be tested by a wide variety of studies
done both within and between the populations identified. These should include
data on breeding and pedigree, survival rates and reproductive success,
electrophoretic variability of gene products as well as nuclear and mitochondrial
DNA sequences, and chromosomal variation. This would provide information on
how ALBM, and the capacity to fatten are inherited, how they are genetically
coupled between populations, and how natural selection, gene flow, and genetic
drift function to maintain the observed patterns of variation (James 1991). One
MUGAAS & SEIDENSTICKER: GEOGRAPHIC VARIATION LEAN BODY MASS 105
could then determine the relative contribution of genetic differences, and
phenotypic plasticity to the observed geographic variation, and interpret correctly
the adaptive significance of the phenotypic variation (Berven et al. 1979; Barton
and Hewitt 1989; James 1991).
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captions, and literature citations. Figure legends and tables should be typed on separate
sheets. Also, please submit a copy of the complete text, tables, figure captions, and
literature cited on a floppy disk (software used does not matter, but we use Word for
Windows).
All illustrations are referred to as figures. They must comply with the following
standards: Photographs should be sharp, with good contrast, and printed on glossy
paper, or the originals submitted. If the background of photographs (especially those of
specimens) is not desired, amberlith should be cut out and used to cover the
background. Drawings should be made with dense black waterproof ink on quality
paper or illustration board. All figures should have a cover sheet. All lettering will be
medium weight, sans-serif type (e.g. Futura Medium, News Gothic) in cutout, dry
transfer, or lettering guide letters. Make allowance so that after reduction no lower case
letter will be less than 1 mm high (2 mm is preferred) nor any capital letter greater than
5 mm high. The maximum size for figures is 9" x 14" (twice BULLETIN page size);
figures should not be less than typepage width (4%"). With soft lead pencil, on the
back of each illustration, designate the top and identify each by author's name,
manuscript title, and figure number.
All manuscripts not submitted in BULLETIN format will be returned to the author
for retyping and/or formatting.
Manuscripts, all editorial matters, and requests for more detailed preparation instructions
should be addressed to:
Managing Editor of the BULLETIN
Florida Museum of Natural History
University of Florida
Gainesville FL 32611-2035
U.S.A.
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