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ARTICLE
Body mass, age, and reproductive influences on liver mass of
white-tailed deer (Odocoileus virginianus)
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Claire A. Parra, Adam Duarte, Ryan S. Luna, Daniel M. Wolcott, and Floyd W. Weckerly
Abstract: Previous research into the liver has mainly examined liver function and liver response to energy restriction. There
have been few investigations into how liver mass is coupled to body mass, body condition, age, and reproductive events like
lactation. Therefore, we examined the scaling relationship between body mass and liver mass and the influences of age, sex, body
condition (back fat), and lactation on liver mass to gain insight into liver-mass variation in white-tailed deer (Odocoileus virginianus
(Zimmermann, 1780)). Deer from two sites in Texas (89 males, 70 females) were sampled; one site was sampled before the mating
season (premating) and the other site was sampled during the mating season. There was an allometric relationship between body
mass and liver mass (scalars 0.59–0.80) at both sites. Also, sex and age were predictors of liver mass at one site, whereas lactation
was influential at the other location. Controlling for body mass, males had heavier livers than females during the mating but not
the premating season and age was positively related to liver mass. Our findings indicate that sex and lactation status were
coupled to liver masses, but the effect of these two factors differed between premating and mating seasons. It appears that
animals in situations where metabolic demands might be higher than current nutrient intake, such as mature males during the
rut or lactating females, have heavier liver masses when body mass is controlled.
Key words: Odocoileus virginianus, ruminant, scaling relationships, visceral organ, white-tailed deer.
Résumé : Les travaux antérieurs sur le foie se sont principalement intéressés à sa fonction et à sa réaction aux restrictions
énergétiques. Peu d’études se sont penchées sur la relation entre le poids du foie et le poids corporel, l’embonpoint, l’âge et les
évènements de reproduction, comme la lactation. Nous avons donc examiné la relation d’échelle entre le poids corporel et le
poids du foie, et l’influence de l’âge, du sexe, de l’embonpoint (gras du dos) et de la lactation sur le poids du foie pour mieux
comprendre les variations pondérales du foie chez le cerf de Virginie (Odocoileus virginianus (Zimmermann, 1780)). Des cerfs en
deux sites au Texas (89 mâles, 70 femelles) ont fait l’objet d’un échantillonnage, un site ayant été échantillonné avant la saison
des amours (avant l’accouplement) et l’autre, durant celle-ci. Une relation allométrique a été observée entre le poids corporel et
le poids du foie (facteurs d’échelle : 0,59–0,80) aux deux sites. Le sexe et l’âge se sont avérés être des variables explicatives du
poids du foie en un site, alors que la lactation avait une influence à l’autre site. En tenant compte du poids corporel, les mâles
avaient des foies plus lourds que les femelles durant la période d’accouplement, mais non avant cette dernière, et il y avait une
relation positive entre l’âge et le poids du foie. Nos résultats indiquent que le sexe et la lactation étaient reliés au poids du foie,
mais que l’effet de ces deux facteurs variait entre la période précédant l’accouplement et la période d’accouplement comme telle.
Il semble que, compte tenu du poids corporel, le poids du foie soit plus élevé chez les animaux se trouvant dans des situations
où les exigences métaboliques pourraient dépasser l’apport de nutriments, comme c’est le cas chez les mâles matures durant le
rut ou chez les femelles en lactation. [Traduit par la Rédaction]
Mots-clés : Odocoileus virginianus, ruminant, relation d’échelle, organe viscéral, cerf de Virginie.
Introduction
The liver is the largest visceral organ in the mammalian body
and performs numerous vital functions. The varied and numerous
functions performed by the liver suggest that many factors can
affect liver mass. Age, season, body condition, and lactation affect
body mass of mammals, which in turn influence the internal
organs (Thompson et al. 1973; Holter et al. 1977; Wheaton and
Brown 1983; Ramsey and Hagopian 2006; Therrien et al. 2008;
Hewison et al. 2011). The combined effects of these various factors
on the relative mass of ruminant livers, as well as the intraspecific
scaling relationship of body mass to liver mass, are largely unexplored.
Nutrition and body condition have a direct effect on male and
female livers in ruminants (Johnson et al. 1990). Sheep with high
dietary intake had greater energy expenditure in the liver com-
pared with fasting individuals (Johnson et al. 1990). Emaciated
deer showed significantly smaller mean size of hepatocyte nuclei
compared with those that were not emaciated. Similar findings
were found in a study of food-restricted deer that showed significant reduction in body mass and liver cell size and number (Wolkers
et al. 1994)., which might indicate reduced energy expenditure
(Ramsey and Hagopian 2006).
There has been no investigation into covariation or scaling relationships between body mass and liver mass in an ungulate
species. Estimating the scaling relationship of visceral organs is
useful to understand the metabolic requirements of small and
large animals. The scaling of organ masses of homeothermic animals is most likely constrained by their energy requirements.
Since the metabolic rate presumably scales to the 0.75 power
of body mass in ruminants, liver mass might also scale to the
0.75 power of body mass. Among species, Prothero (1982) reported
Received 22 August 2013. Accepted 17 February 2014.
C.A. Parra, A. Duarte, R.S. Luna, D.M. Wolcott, and F.W. Weckerly. Department of Biology, Texas State University – San Marcos, San Marcos,
TX 78666, USA.
Corresponding author: Floyd W. Weckerly (e-mail: [email protected]).
Can. J. Zool. 92: 273–278 (2014) dx.doi.org/10.1139/cjz-2013-0201
Published at www.nrcresearchpress.com/cjz on 18 February 2014.
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274
liver mass scaled to the 0.89 power of body mass across adult
mammals and was found to be independent of sex and habitat.
More recently, Lindstedt and Schaeffer (2002) and Franz et al.
(2009) reported that liver mass scaled to the 0.92–1.0 power of
body mass.
In addition to body mass and condition, age and reproduction
might influence liver size in mammals. In white-tailed deer
(Odocoileus virginianus (Zimmermann, 1780)), fatty acid content of
the liver increased with age (Rule and McCormick 1998). In female
rats, older and multiparious individuals had larger livers than
nulliparous animals, even after reproductive senescence (Kennedy
et al. 1958). Yet, neither of these two studies considered body mass
of the animals in the analyses. Lactation is the most energetically
demanding activity in the lifetime of most female mammals
(Ericsson et al. 2001; Naya et al. 2008) and affects female body
condition and size (Hewison et al. 2011). During the peak of lactation, female ruminants cannot ingest enough metabolizable energy to meet the demands of producing milk (Van Soest 1994;
Barboza et al. 2009). Lactating females have been observed with
decreased body mass and condition, as well as decreased fecundity in the next year, suggesting a trade-off between body maintenance and reproductive output (Mitchell et al. 1976; Therrien
et al. 2008). In relation to body mass, lactating Holstein cows had
heavier livers than did steers to meet greater workload demands
(Johnson et al. 1990).
We hypothesized that change in liver mass in relation to body
mass will be influenced by metabolic requirements associated
with reproduction. When females are lactating or when males
display hypophagia during the mating season, liver mass should
be greater in relation to body mass. A greater relative liver mass is
needed to meet the increased workload demands associated with
elevated food intake or it is a consequence of when metabolic
requirements are not matched by food intake (Barboza et al. 2004,
2009). When metabolic requirements exceed intake of metabolizable energy, mobilization of adipose tissue can result in fatty acid
accretion in the liver (Emery et al. 1992; Gross et al. 2013).
Herein, we estimated liver-mass variation in white-tailed deer
during premating and mating seasons to test predictions of our
hypothesis. Two months before the mating season (premating),
females with young should still be lactating and nonlactating
females and males should have food intakes to meet metabolic
requirements. In contrast, during the mating season females that
lactated in the premating season should have had time to replenish reserves and males actively engaged in the physical demands
of mating eat less (Carranza et al. 2004). We tested the following
predictions about liver masses when body mass was statistically
controlled.
In the premating season, we predict that (i) lactating females
should have larger livers than nonlactating females and (ii) masses
of nonlactating females and males should be similar. In the mating season, (i) older, sexually active males should have larger livers
than females and younger, sexually inactive males; (ii) both females that weaned young or did not wean young should have
livers with similar masses; and (iii) there will be no difference in
liver masses between young, sexually inactive males and females.
To assess the influence of age, lactation, and reproductive activities, we controlled for body size by estimating the scaling relationship between body mass and liver mass. In both the premating and
mating seasons, the scaling relationship between body mass and
liver mass should have a scalar of 0.75.
Materials and methods
Study areas
White-tailed deer were collected from two study areas. One
study area, Kerr Wildlife Management Area (WMA), was located in
Kerr County, Texas, USA. Mean annual precipitation was 69.7 cm.
Mean temperature on Kerr WMA was 7 °C in January and 27 °C in
Can. J. Zool. Vol. 92, 2014
July, with a mean annual temperature of 18 °C. The deer in this
study were confined to a 0.27 ha breeding pen or a 1.62 ha rearing
pen enclosed by a 2.5 m high game fence. Their primary diet was
ad libitum access to food pellets in addition to about 1 kg (dry mass)
of alfalfa provided per animal per week. Kerr Wildlife deer pellets
consisted of 16% minimum crude protein and 18.5% acid detergent
fiber (Lockwood et al. 2007).
The other study area was a 3238 ha private ranch in Jim Hogg
County, Texas, USA, in the brush-country region of south Texas
(hereafter, the south Texas ranch). The deer at this study site were
free-ranging. Mean annual precipitation was 60.5 cm. Mean temperature was 14 °C in January and 30 °C in July, with a mean
annual temperature of 23 °C. The dominant vegetation on the
south Texas ranch included honey mesquite (Prosopis glandulosa
Torr.), cenizo (Leucophyllum frutescens (Berland.) I.M. Johnst.), retama
(Parkinsonia aculeata L.), western ragweed (Ambrosia psilostachya DC.),
tanglehead (Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult.),
woolly croton (Croton capitatus Michx.), Hooker’s palafoxia (Palafoxia
hookeriana Torr. & A. Gray), prickly pear (genus Opuntia P. Mill.), sand
bur (Cenchrus spinifex Cav.), little bluestem (Schizachyrium scoparium
(Michx.) Nash), king ranch bluestem (Bothriochloa ischaemum (L.) Keng),
and Johnson grass (Sorghum halepense (L.) Pers.).
Specimen and data collection
Collection procedures followed the Institutional Animal Care
and Use protocol from Texas State University (#00933_09_0603141BF15D). Data at Kerr WMA were collected near the peak
of the mating season in central Texas, late November or early
December of 2011 and 2012 (Robinson et al. 1965). Deer were dispatched using a high-powered rifle and brought to a central staging area where they were processed within 30 min post mortem.
Live mass, minus blood loss, was recorded to the nearest 0.1 kg
after which visceral organs were removed. The rumen–reticulum
was first separated from the liver and spleen. The rumen was
severed from the rest of the digestive tract at the reticulo-omasal
junction and 5 cm above the junction of the esophagus and reticulum and then weighed with contents to the nearest 0.1 kg
(Weckerly et al. 2003; Ramzinski and Weckerly 2007; Duarte et al.
2011). After which the rumen was inverted to remove contents,
rinsed with tap water, and reweighed. The mass of the rumen–
reticulum contents was determined to the nearest 0.1 kg by subtracting the rumen–reticulum organ mass from the mass of the
rumen–reticulum plus its contents. The liver was also weighed to
the nearest 0.1 kg. Sex and number of young that females weaned
that year were also noted. A juvenile was considered weaned if it
survived to 90 days of age. The number of young born and the
number of young weaned was determined from daily observations of every female. Animals in the deer pens were ear-tagged so
that identification of every individual was possible. Back fat,
which is a measurable indication of body condition, was determined by making an incision adjacent to the L4–L5 vertebrae and
measuring the thickness of the fat with a ruler to the nearest
0.1 cm (Luna et al. 2012). Age was known for every animal from
Kerr WMA because birth date of each deer was known.
Free-ranging deer at the south Texas ranch were collected on
16–17 October 2010. The collection occurred before the mating
season, which begins in late December (Webb et al. 2007). The
deer were net-gunned, restrained, and brought to a central area
where they were dispatched and processed in a similar manner as
the specimens at Kerr WMA. Deer sampled at the south Texas
ranch had their age estimated based on tooth wear and replacement (Severinghaus 1949) and only animals ≥1.5 years were included in the study. Lactation was determined by whether the
udder contained milk.
Statistical analyses
An information–theoretic model selection approach was used
at both sites to estimate liver-mass relationships (Burnham and
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275
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Table 1. Model selection summary for the nine
models estimated from the data collected at
Kerr Wildlife Management Area, Texas.
Model predictors
⌬
nPar
r2
BM, AG, SX
BM, AG, SX, BF
BM, AG, SX, 1W/2W
0.00
1.97
3.73
5
6
7
0.80
0.80
0.80
Note: Predictor labels are body mass minus rumen–
reticulum contents (BM), age (AG), sex (SX), one
or two weaned young recruited (1W/2W), and back fat
(BF). The remaining six models are continued in the
order of increasing ⌬: BM, AG, SX, BF, 1W, 2W; BM, BF;
BM, SX, BF; BM, SX; BM, SX, 1W/2W, BF; BM, SX,
1W/2W. The change (⌬) in Akaike’s information criterion corrected for small sample size (AICc) between a
model and the model with the smallest AICc, number
of parameters estimated (nPar), and adjusted r2 are
provided for the three models with a ⌬ ≤ 5.0.
Table 2. Model selection summary for the six
models estimated from the data collected at
the south Texas Ranch.
Model predictors
⌬
nPar
r2
BM, SX, LAC
BM, SX, LAC, BF
0.00
1.56
5
6
0.73
0.73
Note: Predictor labels are body mass minus rumen–
reticulum contents (BM), sex (SX), female was
lactating (LAC), and back fat (BF). The remaining four
models are continued in the order of increasing D:
BM, BF; BM, SX, BF; BM; BM, SX. The change (⌬) in
Akaike’s information criterion corrected for small
sample size (AICc) between a model and the model
with the smallest AICc, number of parameters estimated (nPar), and adjusted r2 are provided for the two
models with a ⌬ ≤ 5.0.
Anderson 2002). In deer, the rumen–reticulum comprises >60% of
the gut contents and its contents can be quite variable (Ramzinski
and Weckerly 2007; Barboza et al. 2009; Luna et al. 2012). Therefore, rumen-content mass was subtracted from the body mass to
lessen the influence of variation in mass of gut contents on the
scaling relationship. We also subtracted liver mass from body
mass minus the mass of the rumen–reticulum contents (henceforth body mass) so that our response variable was not part of our
predictor variable, body mass (Christians 1999). At Kerr WMA, the
model-building goals were to (i) estimate the scaling relationship
between body mass and liver mass and (ii) estimate the relationship, or lack thereof, of age with liver mass while considering
influences from back fat, sex, and number of young recruited by
females. At the south Texas ranch, the first goal was the same as at
Kerr WMA and the second goal was to assess influences from back
fat, sex, and whether or not females were lactating. Regressions
were estimated using least-squares procedures (Sokal and Rohlf
2012). Body mass and liver mass were given a natural logarithmic
transformation to accommodate possible nonlinear relationships
and meet the assumption of homoscedasticity (Luna et al. 2012).
Age and back fat were continuous, whereas sex and lactation
were categorical. We selected a model at each site considering the
change in Akaike’s information criterion corrected for small sample size (AICc) between a model and the model with the smallest
AICc (⌬) and the adjusted r2. Because there were two competing
models (⌬ < 2) in each of the model selection analyses, we modelaveraged parameter estimates (Burnham and Anderson 2002).
Results
There were a total of 159 animals, 46 males and 26 females
taken from Kerr WMA and 43 males and 44 females taken from
Table 3. Model-averaged parameter estimates and
95% confidence intervals for the model with the smallest
Akaike’s information criterion corrected for small sample size (AICc) that summarized the data collected from
Kerr Wildlife Management Area, Texas.
95% confidence intervals
Parameter
Estimate
Lower bound
Upper bound
Intercept
Body mass*
Age
Sex
0.038
0.795
0.038
0.178
0.021
0.610
0.019
0.054
0.069
0.980
0.058
0.304
Note: Parameter estimates were averaged for competing models (AICc < 2). Females were the reference category for sex. The
intercept and confidence intervals are in kilograms.
*Body mass minus rumen–reticulum contents and liver mass.
the south Texas ranch. Back-fat measurements of deer from Kerr
WMA ranged from 0 to 2.22 cm (mean = 0.59 cm, SE = 0.11 cm) for
males and from 0 to 1.59 cm (mean = 0.44 cm, SE = 0.08 cm) for
females. Ages ranged from 1.5 to 6.5 years (mean = 2.65 years, SE =
0.31 years) for males and from 5.5 to 10.5 years (mean = 6.81 years,
SE = 0.31 years) for females. At the south Texas ranch, back-fat
measurements ranged from 0.16 to 5.08 cm (mean = 3.49 cm, SE =
0.19 cm) for males and from 0.16 to 3.81 cm (mean = 1.22 cm, SE =
0.15 cm) for females.
The model selection analysis of Kerr WMA data indicated that
there were two competing models (Table 1). These two models had
parameters for the natural logarithm of body mass, age, sex, and
back fat. The competing models had an adjusted r2 value of 0.80.
For the south Texas ranch data, there were two competing models
(Table 2). These models had parameters for body mass, sex, whether
females were lactating, and back fat. The competing models had
an adjusted r2 value of 0.73.
We only report parameter estimates at both sites for the model
with the smallest AICc (Tables 3, 4) because the additional covariate (back fat) in the other competing models was not statistically
significant. For deer from both sites, there was an allometric relationship between body mass and liver mass because confidence
intervals of the coefficients were <1.0. Although the scalar estimated from the south Texas ranch data was smaller (0.59) than the
scalar estimated from the Kerr WMA data (0.80), there was considerable overlap in confidence intervals and thus scalar estimates
from the two regressions were considered similar.
At Kerr WMA where samples were collected in the mating
season, liver and body masses were greater in older, male deer
(Figs. 1A, 1B). Furthermore, controlling for age, males had larger
livers than females, but there were no differences in masses of
livers between females that weaned or did not wean young. Although both age and sex were statistically significant coefficients
(Table 3), the increase in liver mass with age was only about 4%
(ratio of back-transformed sum of intercept and age coefficients
and back-transformed intercept coefficient), whereas male livers
were about 19% heavier than female livers.
At the south Texas ranch, where samples were collected 2 months
before the mating season, lactating females had heavier livers
than nonlactating females (Table 4). Liver masses of lactating females were about 17% heavier than nonlactating females. We did
not detect statistically significant differences between males and
females in liver masses.
Discussion
Our hypothesis made predictions about liver mass (in relation
to body mass) before and during the mating season based on
presumed liver tissue workload and accretion of fatty acids into
the liver when adipose tissue is mobilized. Our findings supported
our hypothesis. In the premating season, lactating females had
larger livers than nonlactating female, whereas males and nonPublished by NRC Research Press
276
Can. J. Zool. Vol. 92, 2014
Table 4. Model-averaged parameter estimates and
95% confidence intervals for the model with the smallest
Akaike’s information criterion corrected for small sample size (AICc) summarizing the data collected from the
south Texas ranch.
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95% confidence intervals
Parameter
Estimate
Lower bound
Upper bound
Intercept
Body mass*
Sex
Lactation
0.010
0.585
0.119
0.154
0.043
0.373
−0.015
0.075
0.230
0.783
0.253
0.233
Note: Parameter estimates were averaged for competing models (AICc < 2). Females were the reference category for sex as were
females that were not lactating. The intercept and confidence
intervals are in kilograms.
*Body mass minus rumen–reticulum contents and liver mass.
lactating females had similar-sized livers. In the mating season,
older males, which were presumably sexually active, had the
heaviest livers (Fig. 1A). Also, in the mating season, there were no
differences in liver masses between females that had weaned or
not weaned young. These females, moreover, had liver masses,
controlling for body mass, that were similar to young males who
presumably were sexually inactive (Fig. 1A). Lastly, on a body-massspecific basis, smaller deer had heavier livers because the scalar
between body mass and liver mass was about 0.75.
Our hypothesis proposes two mechanisms for how the liver can
be heavier in relation to body mass. One means is by greater
workload and increased metabolic functioning (Johnson et al.
1990; Wolkers et al. 1994; Bollo et al. 1999; Ribeiro et al. 2001). The
other mechanism is from greater concentration of fatty acids in
the liver due to adipose tissue mobilization (Emery et al. 1992;
Gross et al. 2013). When food intake is elevated, such as when
recovering from the enormous energetic demands of lactation, it
seems that greater liver workload would be the most likely explanation for heavier livers (Gerhart et al. 1996; Barboza and Bowyer
2000; Weckerly 2013). In contrast, during the mating season, sexually active males usually reduce food intake and devote tremendous physical activity to acquiring mating opportunities. The
resultant negative energy balance should require the mobilization of adipose tissue (Barboza et al. 2004). For sexually active
males in the mating season, it seems more likely that heavier
livers are from increased fatty acid accretion. Leader-Williams and
Ricketts (1982) also found that male reindeer (Rangifer tarandus
(L., 1758)) had heavier livers than female reindeer during the mating season.
Back fat, a measure of animal condition, was a predictor in
competing models at both the Kerr WMA and the south Texas
ranch; yet, this predictor was not statistically significant. The lack
of an effect from back fat on liver mass is probably because the
variable reflects the state of back fat but not whether back fat is
being depleted or deposited (Monteith et al. 2013). Liver mass
should be directly affected by the dynamical change of animal
condition, which is not necessarily captured by a measure of the
state of animal condition. A study of caribou (Rangifer tarandus
(L., 1758)) strongly suggested that body condition was related to
liver mass (Gerhart et al. 1996). That study, however, did not also
include body mass as a predictor like we did. Usually, body mass
and fat measurements are correlated to some degree, and so if
only one of these variables is included in an analysis, it will probably be correlated with liver mass (Gerhart et al. 1996).
Our finding that liver masses of Kerr WMA females were not
influenced by whether they reared 0, 1, or 2 young that year was
probably influenced by their high nutritional plane. The highquality diet these animals consumed probably allowed recovery
from the demands of lactation before the mating season. In a
free-range setting where animals are more exposed to environ-
Fig. 1. Scatterplots of body mass (minus masses of the
rumen–reticulum contents and liver) and liver mass of white-tailed
deer (Odocoileus virginianus) for data collected from (A) Kerr Wildlife
Management Area and (B) the south Texas ranch. Regressions were
obtained from model-averaged parameter estimates of competing
models. For scatterplot A, open circles and the solid line are for
males 4.5–6.5 years of age, whereas dashes and the broken line are
for males 1.5–2.5 years of age; gray crosses and the gray solid line
are for females. Regressions for each age range of males used means
of ages of deer in each age range. Age was not considered for
females because there was no correlation between age and body
mass (r[24] = –0.10, P = 0.603). The similarities in regressions for
females and males 1.5–2.5 years of age were because of age
disparities. For scatterplot B, open circles and the solid line are for
males, whereas gray symbols and gray lines are for females. Gray
crosses and the gray solid line are for females that were lactating,
whereas gray open triangles and the gray broken line are for
females that were not lactating.
mental heterogeneity, the state of the food supply in autumn might
not allow lactating animals to deposit body reserves by the mating
season to the extent of animals that are supplied an ad libitum
diet that is rapidly digested and high in protein (Lockwood et al.
2007). Consequently, differences in liver masses between females
that had or had not lactated prior to the mating season might be
more frequent in free-ranging animals.
One finding that we had no explanation for was a positive relationship between age and liver mass in female deer (Table 3). The
increase in liver mass with age of females is consistent with the
finding that livers in older deer have increased fatty acid content
(Rule and McCormick 1998). It should be noted, however, that our
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Parra et al.
data from Kerr WMA lacked younger and older females, as well as
older males. Female deer can live longer than 12 years and males
as old or older than 8 years do occur (McCullough 1979; Asmus and
Weckerly 2011). We did not have replicate samples from females
younger than 5 years and older than 10 years and we had no males
older than 6 years. The merit of including animals across the
spectrum of age categories is to provide a more robust assessment
of the relationship between age and liver mass. Thus, in the future
it would be useful to assess the influence of age on liver masses in
a sample of deer that had numerous young, prime-aged, and old
deer.
The allometric scaling relationship between body mass and
liver mass is similar to what has been found in humans (Müller
et al. 2001). Moreover, in humans, liver mass has a strong correlation with metabolic rate. Although intraspecific scaling relationships can vary for liver, brain, kidneys, and heart, all of the scalars
were still less than 1.0, indicating that intraspecific metabolic rate
also has a scalar less than 1.0 (Wang et al. 2001).
The interspecific body mass – liver mass scalar observed in previous studies (0.92–1.0) is higher than the intraspecific scalars estimated herein (0.58–0.80). On an interspecific basis, mammals
apparently show a larger unit increase in liver mass with every
one unit increase in body mass (Prothero 1982; Lindstedt and
Schaeffer 2002; Franz et al. 2009). The reasons for the disparity are
difficult to ascertain. Confidence intervals are rarely reported for
scaling estimates. There might be broad overlap in confidence
intervals between intra- and inter-specific scalars, indicating no
real difference. It is also possible that intra- and inter-specific scaling
relationships are affected by differing sets of factors that result in
differing scalars. We found that intraspecific body mass – liver
mass relationships could be affected by age, sex, and whether or
not females were lactating. On the other hand, interspecific scalars are probably affected by heterogeneity from ancestry, dietary
niche, and reproductive strategies (McNab 2008; Isaac and Carbone
2010). It should also be mentioned that in most interspecific studies the entire body mass is used as the predictor, whereas we
removed the masses of the rumen–reticulum contents and liver
from body mass.
Our study examined predictions to explain variation in liver
mass considering metabolic requirements in relation to intake of
metabolizable energy during a time when food should be plentiful. We detected a positive relationship between age and liver size
and the scalars of body mass to liver mass in white-tailed deer
ranged from 0.59 to 0.80. Furthermore, sex and lactation status
were coupled to liver masses, but the effect of these two factors
differed between premating and mating seasons. The most rigorous evaluation of our hypothesis would have been measuring liver
masses of the same animals at different ages and reproductive
states; nonetheless, our study does establish a framework for further study. Finally, much of the previous research on liver size has
addressed liver response to energy restriction (e.g., Ramsey and
Hagopian 2006). Herein, we revealed connections between age,
body mass, reproductive state, and heavier livers.
Acknowledgements
We thank Texas Parks and Wildlife and Kerr Wildlife Management Area, as well as the south Texas ranch, for allowing us to
collect data and for their assistance. We also thank K. Cummings,
J. Duarte, A. Julian, J. Lattanzio, A. Munters, and V. Pina for helping
in data collection. Funding was provided by the Houston Safari
Club.
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