Journal of Mammalogy, 90(3):629–637, 2009
BLUBBER DEPOSITION DURING ONTOGENY IN
FREE-RANGING BOTTLENOSE DOLPHINS: BALANCING
DISPARATE ROLES OF INSULATION AND LOCOMOTION
SHAWN R. NOREN*
AND
RANDALL S. WELLS
Institute of Marine Science, Center for Ocean Health, University of California, Santa Cruz, CA 95060, USA (SRN)
Chicago Zoological Society, c/o Mote Marine Laboratory, Sarasota, FL 34236, USA (RSW)
Key words: blubber, buoyancy, cetacean, cost of descent, development, dolphin, marine mammal, morphology,
thermoregulation, surface-area-to-volume ratio
Marine mammals contain a specialized hypodermis called
blubber (Parry 1949), which is a layer of lipid-rich tissue between
the epidermis and the underlying muscles. Blubber is a critical
component of mammalian adaptations to the aquatic environment, as evidenced by the fact that it has evolved in parallel in
cetaceans (whales, dolphins, and porpoises) and pinnipeds (seals,
sea lions, and walrus). Marine mammals have a large investment
in the structure and maintenance of blubber, which can constitute
15–55% of body mass (e.g., McLellan et al. 2002; Ryg et al.
1993). Unlike pinnipeds, cetaceans do not have fur or hair, thus
the blubber layer provides the primary insulation for these animals by decreasing heat flow from the body core to the external
environment (Dunkin et al. 2005; Worthy and Edwards 1990).
Insulation via blubber may be particularly important in
young cetaceans. Cetaceans are born in water, which conducts
heat away from a body 25 times faster than air at the same
temperature (Parry 1949; Schmidt-Nielsen 1997; Scholander
et al. 1950). This heat loss is exacerbated by the relatively small
body size of young dolphins, which results in larger surface
area to volume ratios than in adult conspecifics, promoting heat
loss to the environment (Dunkin et al. 2005). This scaling
constraint may be further confounded by underdeveloped
thermoregulatory characteristics. For example, in most terrestrial mammals fat accumulates postparturition and continues to
increase with age (Adolph and Heggeness 1971), providing for
increased cold tolerance with age (Mount 1979). Similarly,
the insulative layer of cetaceans increases after birth; fetal and
neonatal dolphins (Dunkin et al. 2005; Struntz et al. 2004),
porpoises (Lockyer 1995), and whales (Blix and Steen 1979)
have thinner blubber than adult conspecifics. In addition, lipid
content increases steadily from fetal through juvenile (,1 year
old) life-history stages (Dunkin et al. 2005). As a result of
ontogenetic changes in blubber morphology and composition,
the thermal insulation of dolphin blubber, a measure of blubber
quality (i.e., conductivity) and quantity (i.e., thickness), increases 3-fold from fetal to subadult life-history stages (Dunkin
* Correspondent: snoren@biology.ucsc.edu
Ó 2009 American Society of Mammalogists
www.mammalogy.org
629
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Blubber is a critical component of thermoregulation for marine mammals, particularly for cetaceans. However,
the cost of overcoming blubber’s buoyant force during descent could constrain blubber deposition. One- to
12-year-old healthy, free-ranging common bottlenose dolphins (Tursiops truncatus) were studied in Sarasota
Bay, Florida, during summer (mean water temperature: 29.78C 6 0.1 SE) and winter (mean water temperature:
19.2 6 0.48C) to examine ontogenetic and seasonal trends in morphology and blubber deposition. Surface-areato-volume ratio decreased significantly with age. During summer, yearlings had significantly thicker blubber than
2- to 12-year-old animals but this difference diminished by winter because blubber deposition in response to the
colder water temperature was smaller in yearlings (2-mm increase) compared to 2- to 12-year-old animals (3- to
6-mm increase). During summer, buoyancy was highest in yearlings (6.24 N 6 0.41 SE), compared to a buoyant
force of 0.98 6 0.90 N (neutrally buoyant) for 12-year-old animals. Conversely, all dolphins converged upon
a similar buoyant force (8.01 6 0.56 N) in winter. The elevated buoyancy of yearlings in summer presumably
limits seasonal blubber adjustments, because all yearlings (regardless of season) converged upon a similar
calculated mass-specific cost of descent that was greater than all other age classes. Balancing energetic demands
of thermoregulation and locomotion may limit the flexibility of yearlings to adjust blubber deposition in response
to fluctuating water temperatures.
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JOURNAL OF MAMMALOGY
MATERIALS AND METHODS
Animals.— This research on wild dolphins met guidelines
approved by the American Society of Mammalogists (Gannon
et al. 2007), was approved by an institutional animal care and
use committee, and was conducted under a series of scientific
research permits issued by the National Marine Fisheries
Services. Wild common bottlenose dolphins (age: 1–12 years)
in a long-term, multigenerational resident community of approximately 150 individuals near Sarasota, Florida, were sampled as part of an ongoing health assessment program. The
long-term study of this population has spanned more than 5
generations (Wells 1991a, 2003). Individuals were identifiable
from dorsal fin markings, and ages of known dolphins were
determined from monitoring mothers and calves through time
(Scott et al. 1990). For older, unidentified animals, a tooth was
examined for age determination following the methods of
Hohn et al. (1989). The capture–release program involving this
population allowed for morphological measurements to be
taken while each dolphin was held temporarily for veterinary
examination and sampling; details of the program are described
elsewhere (Wells et al. 2004). Morphological measurements
included body mass, girths, segmental lengths, and blubber
thicknesses. These measurements were taken from 65 animals
over 10 winter field seasons (November, December, January, or
February, 1986–1989, 1993–1994, 2002–2005; mean water
temperature ¼ 19.28C 6 0.4 SE) and 258 animals over 21
summer field seasons (June or July, 1984–1995, 1997–2005;
mean water temperature ¼ 29.7 6 0.18C).
Morphology and measurements of blubber thickness.—
Morphological measurements were taken while the dolphins
were resting quietly in a boat. Body mass was determined using
a digital scale (Western Scale DF 2000 Indicator, Western
Scale Company Limited, Port Coquitlam, British Columbia,
Canada). Body length was measured as the straight-line body
length from rostrum to fluke notch, and blubber thickness was
measured at the thoracic–abdominal area (center of flank
directly below dorsal fin) using a portable ultrasound unit
(Scanoprobe II; Scanco, Ithaca, New York). These methods
were adapted from Wells (1991a, 1993). Interage and
interseason comparisons of blubber thickness were made at
the thoracic–abdominal site because this is the standard site
used to show variability among age and reproductive classes in
cetaceans (Koopman 1998). Additional measurements of girth
(taken at the nuchal crest, axilla, anterior dorsal fin, posterior
dorsal fin, and anus), blubber thickness (measured every 10 cm
along each girth on the left side of the body), and segmental
lengths (between each girth measurement along the entire body
length) were recorded from a subset of these animals (summer
n ¼ 73 and winter n ¼ 21) so that surface area, volume, and
blubber mass could be determined.
Calculations.— Surface area and total body volume, excluding appendages (fins and flukes), were determined by standard
geometric equations for a series of truncated cones following
methods of Gales and Burton (1987). It was not possible to
measure appendage surface area in the wild bottlenose
dolphins. Therefore, to account for appendage surface area,
measurements of flukes and fins were taken from a captive
bottlenose dolphin population (The Dolphin Experience, Grand
Bahama Island, Bahamas). The animals represented an age
range of 1.7 years to adult. We have observed that regardless of
age, extremity surface area represented approximately 19% of
total body surface area. Thus, surface area for the wild dolphins
was corrected to include extremity surface areas using this 19%
correction factor; the reported surface-area-to-volume ratios use
these corrected surface area values.
The blubber volume for each body segment was calculated
using the average blubber thickness for the body segment,
which was determined from the series of blubber thickness
measurements taken along each of the 2 girths defining the
body segment. The blubber volumes of each segment were then
summed to determine the total blubber volume for the dolphin,
following the methods of Gales and Burton (1987). Blubber
mass was calculated by multiplying total blubber volume by
the density of bottlenose dolphin blubber (0.969 g/cm3—Kipps
et al. 2002). The same value for blubber density was used for
all dolphins in this study because the density of bottlenose
dolphin blubber is similar across life-history stages (fetal
through adult—Dunkin 2004). The proportion of the body
composed of blubber was calculated by dividing blubber mass
by total body mass.
Buoyancy was calculated from adaptations of the equations
in Skrovan et al. (1999) and Webb et al. (1998):
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et al. 2005). Interestingly, the thermal conductivity of dolphin
blubber, which is independent of thickness, remains stable
across these stages (Dunkin et al. 2005). These results suggest
that neonatal blubber is not specialized to provide enhanced
insulation, but rather that fetal, neonatal, and juvenile lifehistory stages represent a period of continual blubber growth
(Dunkin et al. 2005).
The blubber layer also serves to streamline the body (Hamilton
et al. 2004; Pabst 2000), provide a metabolic energy storage site
(Aguilar and Borrell 1991; Koopman et al. 1996, 2002; Struntz
et al. 2004), provide some measure of protection from the full
effect of a predator bite or other trauma such as a boat strike
(Wells 1993; Wells et al. 2008), and contribute to buoyancy
(Dearolf et al. 2000; Kipps et al. 2002; McLellan et al. 2002;
Webb et al. 1998). Given that blubber is multifunctional and
dynamic, it is likely that multiple factors influence blubber
deposition patterns. We explored the effects of ontogeny (body
size) and thermal demands (season) on blubber deposition and
the influence of blubber deposition on buoyancy and the resulting cost of descent in common bottlenose dolphins (Tursiops
truncatus). We used a unique long-term data set across age
classes and seasons taken from live, presumably healthy, wild
dolphins in Sarasota Bay, Florida. Marine mammal species serve
as models for quantifying adaptation to extreme environments,
and immature animals can provide clues to limitations in
thermoregulatory characteristics. Ultimately, our study will
increase our understanding of the thermal lability of mammals.
Vol. 90, No. 3
June 2009
NOREN AND WELLS—BLUBBER DEPOSITION IN BOTTLENOSE DOLPHINS
631
BT ¼ ð1:0228 0:60V gÞ þ ð0:5568 MT AÞ
FIG. 1.—Surface-area-to-volume ratio (SA:VOL) in relation to
age for bottlenose dolphins (Tursiops truncatus). Points represent
means 6 1 SEM for summer (black circles) and winter (white circles).
Sample sizes are denoted in parentheses for age classes with .1
sample. Age and surface-area-to-volume ratio were significantly
correlated according to a nonlinear relationship. The solid and dashed
lines represent this relationship during summer (SA:VOL ¼ 20.07
age0.13, F ¼ 266.56, d.f. ¼ 1, 10, P , 0.0001) and winter (SA:VOL ¼
20.38 age0.16, F ¼ 80.767, d.f. ¼ 1, 9, P , 0.0001), respectively.
Slopes and intercepts were similar across seasons. See text for statistics.
parameters were used in these analyses so that each age class
contributed equally. The significance of these relationships was
determined using an F-test. The relationships for age versus
surface-area-to-volume ratio, proportion of blubber, and massspecific cost of descent were significant within both seasons;
thus, t-tests were used to compare the slopes and intercepts for
these relationships across seasons. Meanwhile, the relationship
for age versus buoyancy was only significant in summer;
therefore, within–age-class comparisons across seasons were
made using Student’s t-tests. Means are reported 61 SEM.
Sigma Plot and Sigma Stat (Jandel Scientific, San Rafael,
California) were used for all statistical analyses, and results
were deemed significant at P , 0.05, with the exception that
Student’s-t tests or Mann-Whitney rank sum tests for age-class
comparisons of thoracic–abdominal blubber thickness were
deemed significant at P , 0.10 because of limited sample sizes.
RESULTS
Surface-area-to-volume ratio showed a significant nonlinear
decrease with age both during summer and winter (Fig. 1). This
suggests that yearlings have a theoretically greater propensity
for heat loss compared to older age classes. The slopes (t ¼
1.554, d.f. ¼ 21, P ¼ 0.135) and intercepts (t ¼ 0.482, d.f. ¼
21, P ¼ 0.635) for these relationships were similar across
seasons.
A potential means of countering heat loss is to increase
blubber deposition. Blubber deposition in the thoracic–
abdominal region was significantly different across age classes
during summer (H ¼ 17.969, d.f. ¼ 11, P ¼ 0.082), but not
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þ ð0:6689 MT LÞ;
ð1Þ
where BT is total buoyancy (N), V is lung volume, g is
acceleration due to gravity (9.81 m/s2), MT is total body mass
(kg), A is percentage of adipose tissue, and L is percentage of
lean tissue. Unlike seals, dolphins dive on inspiration, thus the
equation takes into account the buoyancy of the diving
lung volume. According to Kooyman (1989), the diving lung
volume of marine mammals is approximately 60% of total
lung volume (V), where V ¼ 0.1MT0.96. For each dolphin, the
percentage composed of adipose tissue was assumed to be the
percentage of body mass composed of blubber based on
the total blubber volume calculation; the remainder of body
mass was assumed to represent lean tissue. The constant 1.0228
is the difference between the density of salt water and air at
208C and 1 atm. The constant 0.5568 assumes an adipose
density of 0.969 g/cm3 for bottlenose dolphins (Kipps et al.
2002). The constant 0.6689 is based on a lean density of
1.07 g/cm3 for seals (Nordøy and Blix 1985) because similar
data are not available for cetaceans.
The cost of descent, specifically the mass-specific locomotor
costs (J/kg) associated with overcoming buoyancy on a 10-m
dive, was calculated for each dolphin by multiplying the
distance traveled (m) by the dolphin’s buoyant force (N)
divided by body mass (kg; adapted from Skrovan et al. 1999).
A relatively shallow dive (10 m) was chosen for comparison
because immature dolphins have limited diving capacity
(Noren 2002, 2004; Noren et al. 2001, 2002, 2004) and thrust
for swimming (Noren et al. 2006) because of developmental
factors. In addition, we have observed that the average depth
utilized by Sarasota Bay dolphins is 3.8 m 6 2.9 SD, with
a maximum of only 9.97 m.
Statistics.—Thoracic–abdominal blubber thicknesses across
sexes within age class and season were compared using
Student’s t-tests or Mann-Whitney rank sum tests when
normality failed. The majority of comparisons showed no
significant differences between the sexes (exceptions were
7-year age class in summer: T ¼ 58.00, n ¼ 8, 13, P ¼ 0.032;
12-year age class in summer: t ¼ 3.528, d.f. ¼ 7, P ¼ 0.010;
5-year age class in winter: t ¼ 2.997, d.f. ¼ 4, P ¼ 0.040 ; 9- to
12-year age classes in winter had too few samples for cross-sex
comparisons), therefore, data across sexes within seasonal age
class were combined for all analyses. Age-class thoracic–
abdominal blubber thicknesses within season were compared
using 1-way analysis of variance (ANOVA) for summer and
1-way ANOVA on ranks for winter because normality failed;
subsequently Student’s t-tests or Mann-Whitney rank sum tests
were used for age-class comparisons within season. Within–
age-class comparisons of thoracic–abdominal blubber thickness across seasons also were compared using Student’s t-tests
or Mann-Whitney rank sum tests. Nonlinear regression
analyses were used to assess the relationship of age with
surface-area-to-volume ratio, proportion of blubber, buoyancy,
and mass-specific cost of descent independently for each
season (summer and winter); age-class means for these
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JOURNAL OF MAMMALOGY
Vol. 90, No. 3
TABLE 1.—Blubber deposition in the midthoracic region was significantly different across age classes measured during summer. Yearling age
class had significantly thicker blubber than 2- to 11-year-old animals. No other age-class comparisons were significant (NS).
1
2
3
4
5
6
7
8
9
10
11
12
1
—
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
t ¼ 2.814
d.f. ¼ 29
P ¼ 0.009
NS
—
NS
NS
NS
NS
NS
NS
NS
NS
NS
t ¼ 1.815
d.f. ¼ 29
P ¼ 0.080
NS
NS
—
NS
NS
NS
NS
NS
NS
NS
NS
T ¼275.5
n ¼ 10, 29
P ¼ 0.016
NS
NS
NS
—
NS
NS
NS
NS
NS
NS
NS
t ¼ 3.852
d.f. ¼ 21
P , 0.001
NS
NS
NS
NS
—
NS
NS
NS
NS
NS
NS
T ¼213.5
n ¼ 10, 21
P ¼ 0.025
NS
NS
NS
NS
NS
—
NS
NS
NS
NS
NS
t ¼ 3.678
d.f. ¼ 21
P ¼ 0.001
NS
NS
NS
NS
NS
NS
—
NS
NS
NS
NS
t ¼ 1.783
d.f. ¼ 18
P ¼ 0.091
NS
NS
NS
NS
NS
NS
NS
—
NS
NS
NS
t ¼ 1.975
d.f. ¼ 17
P ¼ 0.065
NS
NS
NS
NS
NS
NS
NS
NS
—
NS
NS
t ¼ 1.828
d.f. ¼ 16
P ¼ 0.086
NS
NS
NS
NS
NS
NS
NS
NS
NS
—
NS
NS
2
3
4
5
6
7
8
9
10
11
12
t ¼ 4.166
d.f. ¼ 36
P , 0.001
—
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
during winter (F ¼ 1.288, d.f. ¼ 11, 60, P ¼ 0.259). During
summer, yearlings had significantly thicker blubber than 2- to
11-year-old animals; no other age-class comparisons were significant (Table 1; Fig. 2). Blubber deposition also differed
seasonally; for all age classes blubber was thinner in summer
compared to winter (1 year old: t ¼ 3.025, d.f. ¼ 18,
P ¼ 0.007; 2 years old: T ¼ 150.000, n ¼ 5, 28, P ¼ 0.001;
3 years old: t ¼ 4.577, d.f. ¼ 23, P , 0.001; 4 years old: t ¼
3.697, d.f. ¼ 26, P ¼ 0.001; 5 years old: T ¼ 187.00, n ¼ 6,
29, P , 0.001; 6 years old: t ¼ 4.330, d.f. ¼ 16, P , 0.001;
7 years old: T ¼ 115.00, n ¼ 5, 21, P ¼ 0.002; 8 years old: T ¼
61.00, n ¼ 4, 13, P ¼ 0.005; 9 years old: t ¼ 3.194, d.f. ¼ 10,
P ¼ 0.010; 10 years old: t ¼ 1.852, d.f. ¼ 9, P ¼ 0.097;
FIG. 2.—Thoracic–abdominal blubber thickness in bottlenose
dolphins (Tursiops truncatus). Points represent means 6 1 SEM for
summer (black circles) and winter (white circles). Sample sizes are
denoted in parentheses. The asterisk denotes that during summer the
blubber of yearlings was significantly thicker than that of 2- to 11year-old animals; no other age-class differences were found. During
winter, all dolphins had similar blubber thicknesses regardless of age.
See Table 1 for statistics. Seasonal comparisons showed that for all
age classes, blubber was significantly thicker during winter compared
to summer. See text for statistics.
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
—
11 years old: t ¼ 6.047, d.f. ¼ 10, P , 0.001; 12 years old:
t ¼ 3.330, d.f. ¼ 14, P ¼ 0.005). In general, yearlings
demonstrated small seasonal adjustments in blubber, with only
a 2-mm increase in thoracic–abdominal blubber from summer
¼ 5 6 0.3 mm)
to winter compared to a 3- to 6-mm increase (X
found for 2- to 12-year-old animals.
Proportion of blubber, buoyancy, and mass-specific cost
of descent were correlated with age (Figs. 3–5). All variables
showed a significant nonlinear decrease with age, with the
exception of buoyancy during winter months. Thus, seasonal
comparisons of slopes and intercepts for proportion of blubber
FIG. 3.—Proportion of blubber (percentage of total body mass) in
relation to age for bottlenose dolphins (Tursiops truncatus). Points
represent means 6 1 SEM for summer (black circles) and winter
(white circles). Sample sizes are denoted in parentheses for age classes
with .1 sample. Age and proportion of blubber were significantly
correlated according to a nonlinear relationship. The solid and
dashed lines represent this relationship during summer (proportion
of blubber ¼ 20.09 age0.16, F ¼ 229.980, d.f. ¼ 1, 10, P , 0.0001)
and winter (proportion of blubber ¼ 22.93 age0.10, F ¼ 17.026,
d.f. ¼ 1, 9, P ¼ 0.003), respectively. Slopes and intercepts across
seasons were significantly different. See text for statistics.
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Age
June 2009
NOREN AND WELLS—BLUBBER DEPOSITION IN BOTTLENOSE DOLPHINS
and mass-specific cost of descent were conducted, whereas
a similar analysis for buoyancy was not possible. The slopes
(t ¼ 2.395, d.f. ¼ 21, P ¼ 0.026) and intercepts (t ¼ 2.964,
d.f. ¼ 21, P ¼ 0.007) for age versus proportion of blubber
across seasons were different, suggesting that the proportion of
blubber changes markedly with season (Fig. 3). Yearlings only
had a small seasonal increase in proportion of blubber from
summer to winter (1%) compared to the 2–5% increase found
for 2- to 12-year-old animals. A result of ontogenetic and
seasonal change in the proportion of blubber is change in
buoyancy. However, buoyancy was only significantly correlated with age for dolphins measured during summer months
(Fig. 4). All dolphins measured during winter months
¼ 8.01 6 0.56
converged upon a similar buoyant force (X
N) that surpassed the buoyant force of dolphins during summer
months as evident by intra–age-class seasonal comparisons
(1 year old: t ¼ 3.701, d.f. ¼ 7, P ¼ 0.008; 4 years old: t ¼
2.983, d.f. ¼ 7, P ¼ 0.020; 5 years old: t ¼ 4.069, d.f. ¼ 12,
P ¼ 0.002; 12 years old: t ¼ 2.668, d.f. ¼ 6, P ¼ 0.037;
limited winter samples precluded additional age-class comparisons; Fig. 4). Yearlings had the smallest seasonal difference in
buoyancy (Fig. 4), which is consistent with this age group’s
convergence upon a similar mass-specific cost of descent
across seasons (t ¼ 1.501, d.f. ¼ 7, P ¼ 0.177; mean yearling
mass-specific cost of descent ¼ 0.97 6 0.05 J; Fig. 5). In
addition, the intercepts for age- versus mass-specific cost of
descent across seasons were similar (t ¼ 1.785, d.f. ¼ 21,
P ¼ 0.089). Meanwhile, the slopes for these relationships were
different between summer and winter (t ¼ 3.352, d.f. ¼ 21,
FIG. 5.—Mass-specific cost of descent (COD) associated with
overcoming buoyancy for a 10-m dive. Points represent means 6 1
SEM for summer (black circles) and winter (white circles). Sample
sizes are denoted in parentheses for age classes with .1 sample. Age
and mass-specific cost of descent were significantly correlated
according to a nonlinear relationship. The solid and dashed lines
represent this relationship during summer (mass-specific COD ¼ 0.94
age1.16, F ¼ 65.398, d.f. ¼ 1, 10, P , 0.0001) and winter (massspecific COD ¼ 1.31 age0.44, F ¼ 22.315, d.f. ¼ 1, 9, P ¼ 0.001),
respectively. Slopes across seasons were different, whereas intercepts
across seasons were similar. See text for statistics.
P ¼ 0.003). These results indicate that with the exception of
yearlings, age-specific cost of descent varies with season.
DISCUSSION
As a group, cetaceans have relatively low surface area for
their body size compared to terrestrial mammals of similar size.
For bottlenose dolphins, we have observed that total body
surface area is only 67–87% of the surface area predicted by
Brody (1945) for terrestrial animals of similar body size (see
also Ridgway 1972). Similarly, Kasting et al. (1989) determined that the surface areas for beluga and killer whales
only represented 63–91% of that predicted by Brody (1945).
This adaptation in cetaceans likely minimizes heat loss from
the body to the highly conductive marine environment.
However, the same thermal advantage may not be afforded
to immature cetaceans. Compared to adults, young cetaceans
have small body sizes that result in relatively high surface area
to volume ratios (Fig. 1). Without appropriate insulation,
immature animals will theoretically lose comparatively more
body heat to the environment than adult conspecifics.
A thick blubber layer in cetaceans is the primary thermal
adaptation that insulates against heat loss to the water
(Kanwisher and Sundnes 1966). Therefore, dolphin calves
and yearlings may rely on intrinsically greater amounts of
blubber to decrease relatively high rates of heat loss to the
environment due to proportionately greater body surface area.
However, this does not appear to be the case for newborn
cetaceans. Although we were unable to measure the blubber
thicknesses of fetal and neonatal bottlenose dolphins, previous
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FIG. 4.—Buoyancy in relation to age for bottlenose dolphins
(Tursiops truncatus). Points represent means 6 1 SEM for summer
(black circles) and winter (white circles). Sample sizes are denoted in
parentheses for age classes with .1 sample. During summer, age and
buoyancy was significantly correlated according to the nonlinear
relationship shown by the solid line (buoyancy ¼ 7.12 age1.05, F ¼
18.376, d.f. ¼ 1, 10, P ¼ 0.002). In contrast, buoyancy was not
correlated with age during winter (F ¼ 0.431, d.f. ¼ 1, 9, P ¼ 0.528).
The dashed line represents the mean buoyancy across all age classes in
winter (8.01 6 0.56 N).
633
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JOURNAL OF MAMMALOGY
may be the confounding factor that alterations in blubber
thickness affect buoyancy.
In marine mammals, buoyancy is determined by the ratio of
adipose to lean body tissue and by the overall mass of the
animal. For animals of equal size, the animal with a higher ratio
of blubber to lean tissue is more buoyant, whereas for animals
of similar body composition, the smaller animal is more
buoyant (Beck et al. 2000; Biuw et al. 2003; Webb et al. 1998).
In consequence, individuals of the same species that are positively buoyant must expend more energy to maintain position
in the water column than individuals that are neutrally buoyant
(Lovvorn and Jones 1991), and it has been demonstrated that
diving bottlenose dolphins increase energy expenditure with
the work of overcoming buoyancy (Skrovan et al. 1999).
Regardless of season, the youngest, smallest age class (yearlings) had the highest proportion of blubber (Fig. 3) and
resulting buoyant force (Fig. 4). The minute seasonal blubber
adjustments in these young dolphins suggests that blubber
deposition may have been limited by a maximum buoyant force
(9.22 6 0.52 N; Fig. 4) and the associated locomotor costs to
overcome this force while diving to 10 m (1.02 6 0.05 J/kg;
Fig. 5). Given that yearlings are increasingly reliant on foraging
at depth for energy intake (Wells 1991b), limits to costs of
descent may be set by the inability of yearlings to produce large
amounts of thrust for swimming (Noren et al. 2006) due to
diminutive body size and underdeveloped locomotor muscles
(Dearolf et al. 2000; Noren 2002, 2004; Noren et al. 2001). In
contrast, positive buoyancy may be considered adaptive for
neonatal dolphins (Dearolf et al. 2000) to compensate for
extremely limited swimming capabilities (Noren et al. 2006),
because neonates do not forage, being completely reliant on
nursing for energy intake (Wells 1991b). Thus, yearlings may
have the most difficulty balancing the energetic demands of
thermoregulation and locomotion.
Admittedly, our calculations of buoyancy and the resulting
cost of descent only provide deductive estimates because these
values were not measured directly. We assumed that all tissue
that was not blubber was lean tissue, for which we used 1
density value; we did not take into account possible differences
in the densities of muscle, bone, and organ. If bone and organ
densities are higher than that of muscle then the buoyancy of
the animal would be decreased. Conversely, the positive
buoyancy of gaseous intestines also was not accounted for; this
would serve to increase the buoyancy of the animal (Kipps et
al. 2002). Thus, the magnitude of the actual buoyant force of
the dolphins may be different than the values presented in this
study. Nonetheless, given that the density of bottlenose dolphin
blubber is similar across life-history stages (fetal through
adult—Dunkin 2004), and that similar assumptions were used
for all animals in this study, the inter–age-class differences in
buoyant force we posit are likely to be real. Furthermore, it also
is important to note that only the costs of descent associated
with overcoming buoyancy were calculated in this study. While
diving, drag forces also act upward against dolphins as they
descend (Skrovan et al. 1999). Thus, the actual locomotor cost
incurred by a dolphin diving to 10 m is likely to be greater than
the values presented in this study.
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studies on carcasses demonstrated that average blubber
thicknesses of fetal and neonatal bottlenose dolphins were
only 5–6 mm and 11–12 mm, respectively (Dunkin et al. 2005;
Struntz et al. 2004). Blubber-thickness measurements of
neonates from other cetacean species demonstrate similar
results; newborn arctic whales (Blix and Steen 1979) and
harbor porpoises (Phocoena phocoena) 90 cm (Lockyer
1995) are equipped with relatively thin blubber layers
compared to adult conspecifics. This result also has been
found for newborn pinnipeds (for review see Blix and Steen
1979). However, pinniped pups are born and nurse on land,
allowing for physiological changes that enhance thermal
capacity to occur before weaning and before encountering the
thermal challenges of the marine environment (Blix and Steen
1979; Elsner et al. 1977). Cetaceans are born into the highly
conductive ocean and immediately experience the high thermal
demands of this environment. This thermal demand may drive
immature dolphins to quickly acquire enhanced blubber
thickness to decrease heat loss.
The present study demonstrates that by 1 year postpartum,
immature dolphins are able to maintain a greater proportion of
blubber than older conspecifics (Fig. 3). This result is similar to
that found for bottlenose dolphins (Dearolf et al. 2000), harbor
porpoises (Lockyer 1995; McLellan et al. 2002), and
franciscana dolphins (Pontoporia blainvillei—Caon et al.
2007) carcasses. Although the greater proportion of blubber
found in immature dolphins may be a result of constraints to
muscle growth (McLellan et al. 2002), the elevated blubber
levels in immature cetaceans does presumably limit excessive
heat loss from the body. In addition, 1-year-old bottlenose
dolphins acclimated to warm water (summer) maintained
thicker blubber at the thoracic–abdominal area comparable to
that by older age classes acclimated to warm water (Fig. 2).
This result is similar to that found for harbor porpoise carcasses
(Koopman 1998; Lockyer 1995), but differs from that found
for bottlenose dolphin carcasses, which demonstrated lower
(Struntz et al. 2004) or equivalent (Dunkin et al. 2005) blubber
thicknesses in 1-year-old animals (their juvenile age class)
compared to older age classes. The disparity among bottlenose
dolphin studies could be associated with the inability of the
previous studies to control for the season during which
carcasses were collected. In contrast to age-specific differences
in blubber thickness for summer-acclimated dolphins, winteracclimated dolphins had similar blubber thickness irrespective
of age (Fig. 2). This occurred because yearlings had the
smallest increase in blubber thickness (Fig. 2) and proportion
of blubber (Fig. 3) in response to the increased thermal
demands of colder water during winter. Thus, seasonal
adjustments in blubber thickness are greater for older dolphins
than for yearlings.
Initially this result seems counterintuitive; the increased
thermal gradient imposed by the colder water temperatures of
winter would favor vastly thicker blubber for increased
insulation in dolphins of all ages. This would be especially
important for the small-bodied yearlings. Thus, overall blubber
deposition, particularly in the youngest age classes, may be
limited by some other factor. One explanation for these results
Vol. 90, No. 3
June 2009
NOREN AND WELLS—BLUBBER DEPOSITION IN BOTTLENOSE DOLPHINS
the lower water temperatures of winter without compromising
locomotor demands. Conversely, yearlings needed to balance
thermoregulatory and locomotor demands. This trade-off
seemed to limit their blubber deposition in winter, likely
exacerbating relatively high rates of heat loss theoretically
associated with relatively high surface area-to-volume ratios.
Ultimately, this trade-off could constrain the lower temperature
limits of yearlings. Given that dolphin calves must maintain
proximity with their mothers for nourishment and protection,
they may find themselves in environments outside their thermal
neutral zones, the range of environmental temperatures over
which metabolic rate remains relatively constant and independent of ambient environmental temperatures (Mount 1979).
Similar to recent findings for Weddell seal (Leptonychotes
weddellii) pups (Noren et al. 2008), bottlenose dolphin calves
may rely on elevated heat production to safeguard against
heat loss.
ACKNOWLEDGMENTS
We thank the volunteers, staff, and graduate students based at Mote
Marine Laboratory working with the Chicago Zoological Society’s
Sarasota Dolphin Research Program for making the dolphin measurements possible. In particular, we thank S. Hofmann, W. McLellan, S.
Nowacek, A. Read, M. Scott, K. Urian, and G. Worthy for assistance
in collecting some morphological measurements. We also thank
Dr. Terrie Williams and an anonymous reviewer for their comments,
which have improved this manuscript. Data collection from the
dolphins was supported by the National Science Foundation, Office of
Naval Research, Earthwatch Institute, National Marine Fisheries
Service, Dolphin Quest, and International Whaling Commission. S. R.
Noren held an American Fellowship from the American Association
of University Women Educational Foundation during the preparation
of this manuscript.
LITERATURE CITED
ADOLPH, E. F., AND F. W. HEGGENESS. 1971. Age changes in body
water and fat in fetal and infant mammals. Growth 35:55–63.
AGUILAR, A., AND A. BORRELL. 1991. Heterogeneous distribution of
organochlorine contaminants in the blubber of baleen whales:
implications for sampling procedures. Marine Environmental Research 31:275–286.
BECK, C. A., W. D. BOWEN, AND S. J. IVERSON. 2000. Seasonal changes
in buoyancy and diving behaviour of adult grey seals. Journal of
Experimental Biology 203:2323–2330.
BIUW, M., B. MCCONNELL, C. J. A. BRADSHAW, H. BURTON, AND
M. FEDAK. 2003. Blubber and buoyancy: monitoring the body
condition of free-ranging seals using simple dive characteristics.
Journal of Experimental Biology 206:3405–3423.
BLIX, A. S., AND J. B. STEEN. 1979. Temperature regulation in newborn
polar homeotherms. Physiological Reviews 59:285–304.
BRODY, S. 1945. Bioenergetics and growth, with special reference to
the efficiency complex in domestic animals. Reinhold, New York.
CAON, G., C. BERNHARDT FIALHO, AND D. DANILEWICZ. 2007. Body fat
condition in franciscanas (Pontoporia blainvillei) in Rio Garnde do
Sul, southern Brazil. Journal of Mammalogy 88:1335–1341.
DEAROLF, J. L., W. A. MCLELLAN, R. M. DILLAMAN, D. FRIERSON, JR.,
AND D. A. PABST. 2000. Precocial development of axial locomotor
muscle in bottlenose dolphins (Tursiops truncatus). Journal of
Morphology 244:203–215.
Downloaded from https://academic.oup.com/jmammal/article/90/3/629/874589 by guest on 03 March 2024
Age-specific and seasonal differences in blubber deposition
also may be explained by another primary role of blubber for
marine mammals. In addition to thermal insulation and influences on buoyancy, the blubber layer serves in energy
storage (Nordøy and Blix 1985; Parry 1949). Seasonal fluctuations in prey abundance and age-related differences in
maternal milk energy intake or foraging efficiency, or both,
may affect blubber deposition. However, it is unlikely that the
observed differences in the wild dolphins were associated with
these factors, because we have observed similar patterns to
occur in captive bottlenose dolphins, where food is constantly
available and the diet of calves 6 months postpartum is
subsidized with fish. Captive yearlings residing in warm water
(28.9 6 0.118C) showed significantly greater thoracic–
abdominal blubber thickness (14 6 0.5 mm) than older
conspecifics (11 6 0.2 mm; t ¼ 6.329, d.f. ¼ 5, P ¼ 0.001).
Meanwhile, captive dolphins acclimated to colder water (20.6
6 08C) demonstrated similar thoracic–abdominal blubber
thicknesses irrespective of age class (yearlings ¼ 14 6 0.5
mm, and older dolphins ¼ 15 6 0.4 mm). Also similar to that
shown for wild dolphins, older, captive dolphins acclimated to
colder water had significantly thicker thoracic–abdominal
blubber than older, captive dolphins acclimated to warmer
water (t ¼ 7.303, d.f. ¼ 9, P , 0.001). Presumably the
thicker blubber of dolphins acclimated to colder water, both in
the wild (Fig. 3) and as we have observed for dolphins in
captivity, provides for the ability to maintain a greater thermal
gradient between the body core and the environment.
Ultimately, small body size and potential limitations to
blubber deposition in dolphin calves and yearlings may constrain thermal capacity, which may impact behavior. This has
been demonstrated previously in pinnipeds. For example,
hauling out for pupping in harbor seals (Phoca vitulina) in the
Moray Firth occurs during June and July because thermoregulatory costs for newborn pups and lactating mothers are
lowest during these warm months (Hind and Gurney 1998).
Limitations of thermal capacity in cetacean calves also may be
a potential factor influencing both the location and season
for calving. For example, movement of pregnant belugas
(Delphinapterus leucas) into warmer estuaries and lagoons for
calf delivery has been attributed to the lower thermal capacity
of the calf (Sergeant 1973). In addition, harbor porpoises off
the British Isles tend to give birth in June when water temperatures are highest, which favors the survivorship of the
neonates with thin, low-lipid-content blubber (Lockyer 1995).
A similar pattern has been noted for the calving of bottlenose
dolphins in Sarasota Bay, Florida. Peak calving occurs in
spring and early summer when water temperatures are approaching their warmest (Scott et al. 1990; Urian et al. 1996;
Wells et al. 1987). Finally, this may be an explanation for the
migration of some baleen whales from their cold polar feeding
grounds to lower latitudes to give birth and nurse.
In summary, the results of this study suggest that blubber
deposition within a species may be constrained by the buoyant
force of blubber, which increases the cost of descent during
diving. Because buoyancy is closely tied to body size, larger
dolphins were able to enhance blubber thickness in response to
635
636
JOURNAL OF MAMMALOGY
MCLELLAN, W. A., ET AL. 2002. Ontogenetic allometry and body
composition of the harbour porpoises (Phocoena phocoena) from the
western North Atlantic. Journal of Zoology (London) 257:457–471.
MOUNT, L. E. 1979. Adaptation to thermal environment: man and his
productive animals. University Park Press, Baltimore, Maryland.
NORDØY, E. S., AND A. S. BLIX. 1985. Energy sources in fasting grey
seal pups evaluated with computed tomography. American Journal
of Physiology 249:R471–R476.
NOREN, S. R. 2002. The ontogeny of diving in bottlenose dolphins
(Tursiops truncatus). Ph.D. dissertation, University of California,
Santa Cruz.
NOREN, S. R. 2004. Muscle acid buffering capacities in cetaceans:
influences of diving performance, swimming performance, body
size, and postpartum development. Marine Mammal Science
20:808–822.
NOREN, S. R., G. BIEDENBACH, AND E. F. EDWARDS. 2006. The
ontogeny of swim performance and mechanics in bottlenose
dolphins (Tursiops truncatus). Journal of Experimental Biology
209:4724–4731.
NOREN, S. R., V. CUCCURULLO, AND T. M. WILLIAMS. 2004.
The development of diving bradycardia in bottlenose dolphins
(Tursiops truncatus). Journal of Comparative Physiology, B.
Biochemical, Systemic, and Environmental Physiology 174:139–
147.
NOREN, S. R., G. LACAVE, R. S. WELLS, AND T. M. WILLIAMS. 2002.
The development of blood oxygen stores in bottlenose dolphins
(Tursiops truncatus): implications for diving capacity. Journal of
Zoology (London) 258:105–113.
NOREN, S. R., L. E. PEARSON, J. DAVIS, S. J. TRUMBLE, AND S. B.
KANATOUS. 2008. Different thermoregulatory strategies in nearly
weaned pup, yearling, and adult Weddell seals (Leptonychotes
weddelli). Physiological and Biochemical Zoology 81:868–879.
NOREN, S. R., T. M. WILLIAMS, D. A. PABST, W. A. MCLELLAN, AND
J. L. DEAROLF. 2001. The development of diving in marine
endotherms: preparing the skeletal muscles of dolphins, penguins,
and seals for activity during submergence. Journal of Comparative
Physiology, B. Biochemical, Systemic, and Environmental Physiology 171:127–134.
PABST, D. A. 2000. To bend a dolphin: convergence of force transmission designs in cetaceans and scombrid fishes. American Zoologist
40:146–155.
PARRY, D. A. 1949. The structure of whale blubber, and a discussion
of its thermal properties. Quarterly Journal of Microscopic Science
90:13–25.
RIDGWAY, S. H. 1972. Homeostasis in the aquatic environment.
Pp. 590–747 in Mammals of the sea (S. H. Ridgway, ed.). Thomas,
Springfield, Illinois.
RYG, M., C. LYDERSEN, L. Ø. KNUTSEN, A. BJØRGE, T. G. SMITH, AND
N. A. ØRISTLAND. 1993. Scaling of insulation in seals and whales.
Journal of Zoology (London) 230:193–206.
SCHMIDT-NIELSEN, K. 1997. Animal physiology: adaptation and
environment. Cambridge University Press, New York.
SCHOLANDER, P. F., V. WALTERS, R. HOCK, AND L. IRVING. 1950. Body
insulation of some arctic and tropical mammals and birds.
Biological Bulletin 99:259–269.
SCOTT, M. D., R. S. WELLS, AND A. B. IRVINE. 1990. A long-term study
of bottlenose dolphins on the west coast of Florida. Pp. 235–244 in
The bottlenose dolphin (S. Leatherwood and R. R. Reeves, eds.).
Academic Press, San Diego, California.
SERGEANT, D. E. 1973. Biology of white whale in western Hudson
Bay. Journal of the Fisheries Research Board of Canada 30:1065–
1090.
Downloaded from https://academic.oup.com/jmammal/article/90/3/629/874589 by guest on 03 March 2024
DUNKIN, R. C. 2004. Ontogenetic changes in the thermal and buoyant
properties of Atlantic bottlenose dolphin (Tursiops truncatus)
blubber. M.S. thesis, University of North Carolina, Wilmington.
DUNKIN, R. C., W. A. MCLELLAN, J. E. BLUM, AND D. A. PABST. 2005.
The ontogenetic changes in the thermal properties of blubber from
Atlantic bottlenose dolphins Tursiops truncatus. Journal of
Experimental Biology 208:1469–1480.
ELSNER, R., D. D. HAMMOND, D. M. DAVISON, AND R. WYBURN. 1977.
Temperature regulation in the newborn Weddell seal. Pp. 531–540
in Adaptations within Antarctic ecosystems (G. A. Llano, ed.).
Smithsonian Institution, Washington, D.C.
GALES, N. J., AND H. R. BURTON. 1987. Ultrasonic measurement of
blubber thickness of the southern elephant seal, Mirounga leonina
(Linn.). Australian Journal of Zoology 35:207–217.
GANNON, W. L., R. S. SIKES, AND THE ANIMAL CARE AND USE
COMMITTEE OF THE AMERICAN SOCIETY OF MAMMALOGISTS. 2007.
Guidelines of the American Society of Mammalogists for the use of
wild mammals in research. Journal of Mammalogy 88:809–823.
HAMILTON, J. L., W. A. MCLELLAN, AND D. A. PABST. 2004. Functional
morphology of tailstock blubber of the harbor porpoise (Phocoena
phocoena). Journal of Morphology 261:105–117.
HIND, A. T., AND W. S. C. GURNEY. 1998. Are there thermoregulatory
constraints on the timing of pupping in harbour seals? Canadian
Journal of Zoology 76:2245–2254.
HOHN, A. A., M. D. SCOTT, R. S. WELLS, J. C. SWEENEY, AND A. B.
IRVINE. 1989. Growth layers in teeth from known-age, free ranging
bottlenose dolphins. Marine Mammal Science 5:315–342.
KANWISHER, J., AND G. SUNDNES. 1966. Thermal regulation in
cetaceans. Pp. 397–409 in Whales, dolphins, and porpoises (K. S.
Norris, ed.). University of California Press, Berkeley.
KASTING, N. W., S. A. L. ADDERLEY, T. SAFFORD, AND K. G. HEWLETT.
1989. Thermoregulation in beluga (Delphinapterus leucas) and
killer (Orcinus orca) whales. Physiological Zoology 62:687–701.
KIPPS, E. K., W. A. MCLELLAN, S. A. ROMMEL, AND D. A. PABST. 2002.
Skin density and its influence on buoyancy in the manatee
(Trichechus manatus latirostris), harbor porpoise (Phocoena
phocoena), and bottlenose dolphin (Tursiops truncatus). Marine
Mammal Science 18:765–778.
KOOPMAN, H. N. 1998. Topographical distribution of the blubber of
harbor porpoises (Phocoena phocoena). Journal of Mammalogy
79:260–270.
KOOPMAN, H. N., S. J. IVERSON, AND D. E. GASKIN. 1996. Stratification
and age-related differences in blubber fatty acids of the male harbor
porpoise (Phocoena phocoena). Journal of Comparative Physiology, B. Biochemical, Systemic, and Environmental Physiology
165:628–639.
KOOPMAN, H. N., D. A. PABST, W. A. MCLELLAN, R. M. DILLAMAN,
AND A. J. READ. 2002. Changes in blubber distribution and
morphology associated with starvation in the harbor porpoise
(Phocoena phocoena): evidence for regional differences in blubber
structure and function. Physiological and Biochemical Zoology
75:489–512.
KOOYMAN, G. L. 1989. Diverse divers: physiology and behaviour.
Springer-Verlag, Berlin, Germany.
LOCKYER, C. 1995. Aspects of the biology of the harbour porpoises,
Phocoena phocoena, from British waters. Developments in Marine
Biology 4:443–457.
LOVVORN, J. R., AND D. R. JONES. 1991. Body mass, volume and
buoyancy of some aquatic birds and their relation to locomotor
strategies. Canadian Journal of Zoology 69:2888–2892.
Vol. 90, No. 3
June 2009
NOREN AND WELLS—BLUBBER DEPOSITION IN BOTTLENOSE DOLPHINS
WELLS, R. S. 2003. Dolphin social complexity: lessons from long-term
study and life history. Pp. 32–56 in Animal social complexity:
intelligence, culture, and individualized societies (F. B. M. de Waal
and P. L. Tyack, eds.). Harvard University Press, Cambridge,
Massachusetts.
WELLS, R. S., ET AL. 2008. Consequences of injuries on survival and
reproduction of common bottlenose dolphins (Tursiops truncatus)
along the west coast of Florida. Marine Mammal Science 24:774–
794.
WELLS, R. S., ET AL. 2004. Bottlenose dolphins as marine ecosystem
sentinels: developing a health monitoring system. EcoHealth 1:
246–254.
WELLS, R. S., M. D. SCOTT, AND A. B. IRVINE. 1987. The social structure
of free-ranging bottlenose dolphins. Pp. 247–305 in Current
mammalogy 1 (H. H. Genoways, ed.). Plenum Press, New York.
WORTHY, G., AND E. EDWARDS. 1990. Morphometric and biochemical
factors affecting heat loss in a small temperate cetacean (Phocoena
phocoena) and a small tropical cetacean (Stenella attenuata).
Physiological Zoology 63:432–442.
Submitted 29 April 2008. Accepted 11 September 2008.
Associate Editor was William F. Perrin.
Downloaded from https://academic.oup.com/jmammal/article/90/3/629/874589 by guest on 03 March 2024
SKROVAN, R. C., T. M. WILLIAMS, P. S. BERRY, P. W. MOORE, AND
R. W. DAVIS. 1999. The diving physiology of bottlenose dolphins
(Tursiops truncatus) II. Biomechanics and changes in buoyancy at
depth. Journal of Experimental Biology 202:2749–2761.
STRUNTZ, D. J., W. A. MCLELLAN, R. M. DILLAMAN, J. E. BLUM, J. R.
KUCKLICK, AND D. A. PABST. 2004. Blubber development in
bottlenose dolphins (Tursiops truncatus). Journal of Morphology
259:7–20.
URIAN, K. W., D. A. DUFFIELD, A. J. READ, R. S. WELLS, AND D. D.
SHELL. 1996. Seasonality of reproduction in bottlenose dolphins,
Tursiops truncatus. Journal of Mammalogy 77:394–403.
WEBB, P. M., D. E. CROCKER, S. B. BLACKWELL, D. P. COSTA, AND B. J.
LE BOEUF. 1998. Effects of buoyancy on the diving behavior of
northern elephant seals. Journal of Experimental Biology
201:2349–2358.
WELLS, R. S. 1991a. The role of long-term study in understanding the
social structure of a bottlenose dolphin community. Pp. 199–225 in
Dolphin societies: discoveries and puzzles (K. Pryor and K. S.
Norris, eds.). University of California Press, Berkeley.
WELLS, R. S. 1991b. Bringing up baby. Natural History August:
56–62.
WELLS, R. S. 1993. Why all the blubbering? Brookfield Zoo Bison
7:12–17.
637