This document discusses estimating the energy requirements for maintenance and growth of wild mammals, birds, and reptiles kept in captivity. It reviews existing data on basal metabolic rates and maintenance energy requirements for some species. The document develops a model to estimate lower limits of energy requirements during growth, taking into account factors like variation in time to reach maturity and energy density of tissue deposited. Having estimates of energy needs can help with issues like treating obesity or providing nutritional support when animals lose appetite. Overall, the document aims to provide guidance for estimating energy requirements when specific data is lacking for non-domesticated species in captive settings.

1. Energy Requirements
Energy Requirements for Maintenance and Growth of
Wild Mammals, Birds and Reptiles in Captivity1
JAMES K KIRKWOOD2
Department of Veterinary Science, Institute of Zoology, Zoological Society of London,
Regent's Park, London NW1 4RY, United Kingdom
ABSTRACT A wide range of wild animals are main
tained in captivity as pets and an increasing number
are likely to become dependent on captive breeding for
conservation. Generally, these animals are fed ad libi
tum and a knowledge of their energy requirements is
not essential. However, estimates of energy require
ments are helpful in several situations: treating obesity,
providing nutritional support to anorexic animals and
feeding neonates. Data on basal metabolic rates (BMR)
are available for ~5% of avian and 17% of mammalian
species, and the maintenance requirement can be es
timated at twice BMR. Estimates for other species can
be based on allometric equations relating energy ex
penditure to body weight in the species that have been
studied. Although between species time taken to grow
increases with adult mass, wide variation remains after
the effect of adult mass is considered. A model is de
veloped which illustrates the impact of variation in time
taken to grow on daily growth rate (per metabolic mass)
at all stages of maturity. This model may assist in es
timating the lower limit to energy requirements during
growth. J. Nutr. 121: S29-S34, 1991.
INDEXING KEY WORDS:
•symposium
•energetics «
•mammals
growth
•birds •vertebrates
Loss of habitat and changes in environmental qual
ity have lead to drastic declines in the populations of
some species of wild animals. For an increasing num
ber of species, survival will depend upon active inter
vention, either by managing populations in the wild
or by captive breeding.
Nutrition is a fundamental part of captive man
agement and devising appropriate diets for the great
variety of species maintained in captivity presents
a major challenge. There is very little specific infor
mation on the nutrient requirements of nondomes-
ticated animals. Most wild animals kept in zoos or
as pets are fed ad libitum, and it is not usually nec
essary to know their energy requirements. Ad libi-
0022-3166/91 $3.00 ©1991 American Institute of Nutrition.
turn feeding is generally a suitable system because
most animals are better able to judge their require
ments than their keepers. However, there are cir
cumstances when estimates of energy requirements
are useful, for example, when treating or preventing
obesity, when treating inanition or anorexia and in
the artificial rearing of neonates of some species. The
aim of this paper is to review the bases for estimating
the energy requirements of reptiles, birds and mam
mals for maintenance and growth.
Although objective criteria for assessment of obe
sity in wild animals are lacking, it is clear that in
dividuals of some species can become abnormally
fat in captivity. Animals in captivity tend to main
tain higher body weights and fat contents than in
the wild. The point at which fatness is judged to be
pathological is subjective. When an individual is
manifestly grossly fatter than typical for the species,
it is useful to have an estimate of maintenance en
ergy requirement to devise a ration likely to promote
weight loss.
Anorexia can be due to systemic illness, dental or
other oral diseases or injuries causing pain, or pro
vision of unfamiliar or inappropriate diets. It can
also be a normal part of the biology and associated
with season or reproductive status. When the cause
is pathological, in addition to treating the cause of
the anorexia, it is important to have an estimate of
energy requirements so that nutritional support can
be given, if necessary by stomach tube or intrave
nously. Anorexia is a common complication of dis
eases of reptiles. Inanition due to provision of in
sufficient food occurs sometimes because of igno-
1Presented as part of the Waltham International Symposium on
Nutrition of Small Companion Animals, at University of California,
Davis, CA 95616, on September 4-8, 1990. Guest editors for the
symposium were James G. Morris, D'Ann C. Finley and Quinton
R. Rogers.
1To whom correspondence should be addressed: Department of
Veterinary Science, Institute of Zoology, Zoological Society of Lon
don, Regent's Park, London NW1 4RY.
S29
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2. S30 KIRKWOOD
TABLE 1
Maintenance or average daily metabolizable energy
requirements for some groups of animals
in relation to body mass
GroupCaptive
snakes'Passerine
birds,30°CNon-passerines,
30°CCaptive
raptorsCaptive
eutherianmammalsCaptive
eutherianmammalsRodents,
captiveADMR1Insectivores,
captiveADMR2Captive
primatesCaptive
prosimiansEnergy
Requirementk]/d-45
M075481
M°«41
4M075456
M°•"•607
M°•"586
M°•"356
M°"280
M043406
M°"264-573
M°"SourceGili
&Kirkwood,unpublisheddataRef.
10Ref.
10Ref.
11Ref.
12Ref.
6Ref.
13Ref.
13Ref.
14Ref.
15
1Snakes maintained at temperatures close to 28°C.
2Average daily metabolic rate.
ranee of requirements and failure to recognize the
effects.
It is quite often necessary to artificially rear wild
animals born in captivity either because mother or
baby are ill or because of inadequate maternal behav
ior. The need also arises when eggs are removed for
artificial incubation to increase clutch size and pro
ductivity (1). In some species, clutch or litter sizes of
ten exceed the number that the mother can rear. For
example, common marmosets often have triplet litters
but can rear only two babies (2), giant pandas fre
quently have twins but rear only one (3) and some
eagles incubate two eggs but rear only one hatchling
(4). In these cases the surplus neonates can be hand-
reared. It can be difficult to judge how much food to
provide and an estimate of what the requirements are
likely to be is useful so that gross over- or underfeeding
is avoided.
Estimates of energy requirements can therefore be
useful, but can useful estimates be made? Intra- and
interspecies variation makes the precise prediction of
the energy requirements of an individual impossible,
but enough is known about the influence of body size
and taxonomic position for it to be possible to make
adequate first approximations. Intakes in excess of
~ 1700 kj/d per metabolic mass (M in kg raised to the
I power) are very unlikely under any circumstances
since there seems to be a limit to intake at about this
level (5). In this paper, information relevant to the es
timation of energy requirements for maintenance is
reviewed, and a model is developed that predicts a
lower limit to the energy requirements for growth in
relation to stage of maturity and time taken to grow.
METABOLIZABLE ENERGY REQUIREMENTS
FOR MAINTENANCE
The metabolizable energy requirements for main
tenance of adult animals in energy balance and kept
in a comfortable thermal environment approximate
twice the basal metabolic rate (BMR) (6) and recom
mendations for maintenance are often based on this
(7).This is a useful guide in practice because, although
energy requirements for maintenance or captive ex
istence have been measured in relatively few species,
data on BMR are available for many. For example,
Bennett and Harvey (8) collected data on resting met
abolic rate for 399 species of birds from the literature
for a review of the effects of body size and other fac
tors, and Heusner (9) studied data from 685 mammal
species.
Allometric equations relating maintenance energy
requirements or the average daily metabolism in cap
tivity in relation to body mass for several groups of
animals are shown in Table 1. The BMRs of mono-
tremésand marsupials are lower than typical for eu
therian mammals (16, 17) and their maintenance re
quirements are likely to be correspondingly low. The
mean maintenance energy requirement per metabolic
mass of three species of macropod marsupials for
which data are cited by Loudon (18)was 427 kj/d. The
prosimians also tend to have lower metabolic rates
than the mean for eutherian mammals (15, 19).
Average daily metabolic rates have been measured
in free-living individuals of quite a wide range of spe
cies and some allometric equations are listed in Table
2. Where the captive conditions mimic the wild en
vironment closely, estimates of requirements based
on these equations may be more appropriate.
ENERGY REQUIREMENTS DURING GROWTH
During growth, energy is required both for mainte
nance functions and also for deposition of new tissue.
The amount by which the total daily requirement ex
ceeds the maintenance requirement depends upon the
rate of tissue deposition and the composition of tissue
TABLE 2
Average daily metabolic rate ¡ADMR]of free-living animals
Species ADMR Source
Free-living lizards
Free-living birds
Free-living birds
Free-living rodents
ki/d
54 Ma>0
854 M0<1
920 M0<1
753 M067
Ref. 20
Ref. 21
Ref. 8
Ref. 22
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3. ENERGY REQUIREMENTS OF CAPTIVE WILD ANIMALS S31
deposited, and these vary with stage of growth and
between species. To assist in understanding the way
in which these variables influence energy requirements
during growth, and thus in predicting energy require
ments, it is helpful to develop a model. The model
described here is similar to that developed previously
(23)but the impact of different scaling procedures and
of variation in the energy density of tissue deposited
are also examined in this case.
The basic assumptions and development of the
model are as follows:
Assumption 1. That the pattern of growth in body
mass follows a Gompertz curve
M = Mae-e""'"'' (1)
where M = mass (kg),Ma = adult mass (kg),B - growth
rate constant (1/d), t = age (d) and t' = age at the in
flexion point of the sigino id curve (d).
Various mathematical functions approximate the
typically sigmoid pattern of weight gain of vertebrates.
In addition to the Gompertz equation shown above,
the logistic and Von Bertalanffy equations have often
been used to summarize data and as models (24-26).
These curves differ in the position of their inflexion
points. The inflexion of the logistic curve occurs at
0.5 of the asymptote, that of the Gompertz at 0.37
and that of the Von Bertalanffy at 0.3 of the asymptote.
Assumption 2. That between species the growth
rate constant (B)is inversely related to adult body mass
raised to an exponent of ~0.25:
= xM -0.25
(2)
Comparing time taken to grow among domesticated
mammals, using data from Brody (27), Taylor (28)
found that it increased with adult body mass raised to
the 0.27 power between species. Ricklefs (24) found
the same exponent when investigating the relationship
between time taken to grow and adult mass in birds.
Since then others, for example, Case (29) and Calder
(30)have presented further evidence that time to grow
tends to increase with about the quarter power of adult
mass (and thus that the growth rate constant B is in
versely related to M0025as shown in Eq. 2). However,
the value found for the exponent in allometric rela
tionships is dependent on the species included within
the sample and on the type of regression analysis used
(31),and other values have been reported. For example,
Zullinger et al. (26) derived an exponent of 0.30 in
their analysis of growth rates among mammals and
Ricklefs (32) derived an exponent of 0.34 in his anal
ysis of growth rates in birds.
Kirkwood (23) proposed using the time taken to
grow from 25 to 75% of adult weight (125-75d) as an
index of time taken to grow when undertaking com
parisons between species. The times at which these
proportions of adult size are reached can be quite ac
curately pinpointed because growth rate is relatively
high at these stages of growth, so tis_75can be easily
measured from growth curves.
If Õ25-75is dependent on Ma°25between species, then
comparisons of time taken to grow can be made be
tween species of different adult masses by comparing
values of tïs_75/Ma°-2S.Kirkwood and Webster (33) la
beled this quantity 0. It is a body-mass-independent
index of time taken to mature. Values of 6 are shown
for a variety of species in Table 3. These show that
considerable variation in time taken to grow remains
after taking body mass into account. Birds generally
take a shorter time to grow than mammals, and mam
mals generally take a shorter time to grow than rep
tiles. Marsupials tend to grow more slowly than av
erage for eutherian mammals, but there is wide vari
ation among eutherians. The Old World monkeys and
apes (especially humans) have very prolonged growth
periods.
Assumption 3. That the energy cost of each unit
weight gain is 8400 kj/kg throughout growth. In prac
tice, the energy cost of weight gain cannot be much
less than this but is often considerably more.
Taking Assumption 1 it follows that growth rate
(GR, kg/d) is described by the differential of the Gom
pertz equation:
TABLE 3
Values for 6(abodymass-independent index of time taken to
grow) for a variety of species of vertebrates
Species B Source
ReptilesMediterranean
tortoiseRussell's
viperBirdsFairy
penguinSalvin's
prionDouble-crested
cormorantAndean
condorKestrelDomestic
broilerfowlJapanese
quailNicobar
pigeonScarlet
macawAfrican
greyparrotScarlet
cock-of-thè-rockStarlingMammalsQueensland
koalaMasked
shrewRufous
elephantshrewLesser
mouselemurCommon
marmosetRhesus
macaqueHumanGuinea
pigMongolian
gerbilCatDog
(Labradorbitch)Giant
pandaFriesian
cow211064018261029113525211831211118528674823461912609395105408
34Ref.
35Ref.
36Ref.
37Ref.
38Ref.
39Ref.
40Ref.
41Ref.
42Ref.
43Ref.
44Ref.
44Ref.
45Ref.
46Ref.
47Ref.
48Ref.
49Ref.
50Ref.
51Ref.
52Ref.
53Ref.
54Ref.
55Ref.
56Ref.
57Ref.
58Ref.
59
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4. S32 KIRKWOOD
GR = -BM In (u) (3)
where stage of maturity, u = M/Ma.
It is a property of the Gompertz curve that
B = 1.57/125-75 (4)
and therefore:
GR = (-1.57/t25.75)Mln(u) (5)
We have defined 0 as t2S>,75/Maoli,and it follows that:
GR = (-1.57/0M025u°25)MIn (u) (6)
this simplifies to:
GR = (-1.57/0)M°75u025In (u) (7)
and therefore the growth rate per metabolic mass (GR/
M075) is related to time taken to grow and stage of
maturity as follows:
GR/M°7S= (-1.57/0)u°2SIn (u) (8)
The form of this expression, relating growth rate per
metabolic mass to stage of growth as 0varies, is shown
in Figure 1A. It is dependent on the value of the ex
ponent, and if Õ25-75is scaled by dividing by M°33(in
stead of M°25)the equation becomes:
GR/M0«7= (-1.57/0)u033 In (u) (9)
This equation is illustrated in Figure IB. If the model
is based on the logistic curve instead of the Gompertz
curve, the equations corresponding to equations 8 and
9 are:
GR/M075 = (2.2/0)u°25(l -u) (10)
and
GR/M067 = (2.2/0)u033 (1 - u) (11)
These equations are illustrated, solved for 0 values of
20 and 100, in Figures 1C, D.
The energy required to sustain the rate of tissue
deposition (Pg, kj/d per metabolic mass) can be esti
mated as:
Pg/M075 = -1.57c/0)u°25In (u) (12)
where c is the energy cost per unit mass gain.
The total energy requirement can then be estimated
by adding an approximation of the maintenance com
ponent of the energy budget. This can be based on the
maintenance requirements of adults or, if no specific
information is available, on allometric equations such
as those in Table 1.
The pattern of energy requirement for growth that
this model predicts in relation to stage of growth and
time taken to grow is shown in Figure IE. In Figure
IE, equation 12 is solved for when 0 = 20 and c = 8400
kj/kg. This model and the variants shown in Figure 1
indicate that growth rate and energy requirements rel
ative to metabolic mass are likely to be highest in the
early stages of growth and to decline as adult mass is
10O
40
0-2 0-4 0-6 0-8 1-0
80
60
40
30
0-2 0-4 0-6 0-8 1-0
d
0-2 0-4 0-6 0-8 1-0 0-2 0-4 0-6 0-8 1-0
f
= 0-2 0«0-6 0-8 1-0 O-2 0-4 0-6 0-8 1-0
Stage of maturity
FIGURE 1 Predicted growth rates and energy require
ments of homeothermic animals in relation to stage of ma
turity (u, proportion of adult mass attained) and a body-
mass-independent index of time taken to grow (6, see text).
The upper and lower lines are solutions to the model (Eq. 8)
when 8 values are 20 and 100, respectively. (A) Predicted
growth rate per metabolic mass when growth follows a
Gompertz curve and when 8 is 20 (upper line) or 100 (lower
line). (B) As for 1A but assuming growth follows a logistic
curve. (C) As for 1A but assuming growth rate scales with
M°67(rather than M°75| and that time taken to grow scales
with M033 (rather than M°"). (D) As for 1C but assuming
growth follows a logistic curve. (£) Predicted minimum en
ergy requirement of homeothermic animals in relation to
stage of maturity and time taken to grow. In this case 6 is
20. The solid line is derived assuming the energy cost of
tissue deposition is constant throughout growth at 8400 kj/
kg, the broken line is derived assuming that the energy cost
of tissue deposition increases with stage of maturity at (8400
+ 8400 u) kj/kg. The maintenance requirement is assumed
to be 500 kj/d per kg075 throughout growth. (F) As for IE
but assuming growth follows a logistic curve.
approached. They also illustrate the impact of varia
tion in 0 on growth rate and energy requirements in
relation to metabolic mass during the growth period.
The energy requirements that the models predict can
be considered as minimum estimates.
In reality the situation is complicated by several
factors and I will mention some here. First, growth
does not exactly follow Gompertz or logistic curves.
However, these often provide remarkably good fits and
differences are unlikely to change the shape of the
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5. ENERGY REQUIREMENTS OF CAPTIVE WILD ANIMALS S33
model greatly. Second, the energy density of tissue
deposited usually increases with stage of maturity (u)
due to an increase in proportion of fat to protein de
posited. In wild animals the energy density of weight
gain typically increases from ~6300 kj/kg in the early
stages of growth to ~ 12,600 kj/g in the late stages
[see Figs 10.4-10.8 in Robbins (6)]but is higher than
this in some species (e.g., seals and storm petrels) and
in domestic animals [e.g., cattle, (60)]. If we assume
that the efficiency of deposition of energy available
for growth is 0.75, then the energy required for each
gram weight gain will typically increase from 8400 to
16,800 kj as adult mass is approached. Assuming the
energy cost of weight gain (c, kj/kg) increases with u
as follows:
c = 8400 + 8400u (13)
then Pg can be calculated:
Pg/M°75= (8400 + 8400u)
- 1.57c/0u°25ln(u) (14)
Taking the increasing cost of tissue deposition with
stage of maturity into account in this way skews the
predicted energy requirements to the right (Figs. IE,
F). Third, the maintenance component of the energy
budget during growth varies between species (see
above) and is unlikely to remain constant throughout
growth. In some altricial species it appears to be below
adult level (per metabolic mass) before developing en-
dothermy (33). It may increase during growth as ex
penditure on activity increases and there is evidence
that it is linked with growth intensity (60). Rapid
growth demands a relatively high food intake and thus
a relatively large digestive system (61) and there is ev
idence that this metabolically active tissue is relatively
expensive to maintain (62).
For the reasons outlined above, equation 20 is likely
to underestimate the actual requirements, and com
parison of the predictions with energy budget data
provided by Kirkwood and Webster (33) suggests that
it fairly consistently does so. However, no generaliza
tions can be made about the costs of other components
of the energy budget so they cannot be taken into ac
count.
In conclusion, the energy requirement of an indi
vidual cannot be predicted precisely but approximate
estimates of energy requirements usually suffice when
they are needed in practice. Adequate estimates of
maintenance requirements can be generated from
BMR data or from interspecies allometric relation
ships.
The energy requirements for growth are more dif
ficult to estimate. The models generated here approx
imate the patterns of growth in homeotherms quite
well but usually underestimate total daily energy in
takes. They can provide some guidance about the
magnitude of the impact of variation in time taken to
grow on weight gain at various stages of growth, pre
dictions of the lower limit of energy requirements
during growth, and an estimate of how energy re
quirements for tissue deposition may change during
growth.
ACKNOWLEDGMENT
I am most grateful to Moya Foreman for preparing
this manuscript.
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