RESEARCH ARTICLE
Female White-Footed Mice
(Peromyscus leucopus) Trade
Off Offspring Skeletal Quality
for Self-Maintenance When
Dietary Calcium Intake is Low
CHRISTINA M. SCHMIDT AND WENDY R. HOOD
Department of Biological Sciences, Auburn University, Auburn, Alabama
ABSTRACT
J. Exp. Zool.
00:1–7,
2016
During gestation and lactation in mammals, calcium and other minerals are transferred from
female to offspring to support skeletal ossification. To meet mineral requirements, females commonly mobilize mineral from their own skeleton to augment dietary intake. Because the fitness
costs of bone loss are expected to limit the amount of endogenous mineral that females transfer to their young, the amount of mineral allocated to offspring is predicted to be influenced by
the availability of mineral in the female’s diet. Calcium is the most abundant element in bone,
and exogenous calcium appears to be limiting for many species. Thus, we expected that females
would adjust mineral allocation to offspring relative to calcium abundance in the diet. We provided breeding female white-footed mice (Peromyscus leucopus) with a low-calcium (0.1% Ca)
or a standard diet (0.85% Ca) for approximately 1 year. Body mass and skeletal size of pups did
not differ between diets. Relative to pups from females on the standard diet, pups from females
on the low-calcium diet had less calcium and phosphorus in their femurs and humeri, less body
calcium content, reduced mass of their femurs and humeri, and had femurs with a reduced width.
Reproducing white-footed mice mobilize more bone when calcium intake is low; however, our
results suggest that this does not completely compensate for a reduction in calcium intake. Thus,
it appears that when calcium availability is low, female white-footed mice reduce the quantity
of mineral allocated per offspring as a means of maintaining their own skeletal condition. J. Exp.
C 2016 Wiley Periodicals, Inc.
Zool. 00:1–7, 2016. How to cite this article: Schmidt CM, Hood WR. 2016. Female white-footed mice (Peromyscus
leucopus) trade off offspring skeletal quality for self-maintenance when dietary calcium intake is low. J. Exp. Zool. 00:1–7.
INTRODUCTION
During a reproductive event, a parent must partition nutrients and other resources between self-maintenance and the current reproductive effort. Differential resource allocation between
these competing demands can affect parental body condition,
survival and future reproductive potential, as well as size of
offspring and/or number of offspring produced, offspring body
condition, and even offspring sex ratio when the probability
of reproductive success is condition dependent (Williams, ‘66;
Trivers and Willard, ‘73; Trivers, ‘74). Thus the quantity and
quality of nutrients available to developing young can affect
their phenotype, probability of survival, and future reproductive
success.
In addition to macromolecules that are required to generate
and support the development of offspring, vertebrates require a
substantial mineral to support skeletal ossification and growth.
Grant sponsor: The American Society of Mammalogists to CMS, Auburn
University to WRH, and the Society for Comparative and Integrative Biology
to CMS.
Correspondence to: Christina Schmidt, Wells College, 213 Stratton Hall, 170
Main St., Aurora, NY 13826.
E-mail: [email protected]
Received 11 September 2016; Revised 20 October 2016; Accepted 24
October 2016
DOI: 10.1002/jez.2051
Published online in Wiley Online Library (wileyonlinelibrary.com).
C
2016 WILEY PERIODICALS, INC.
2
Calcium, along with phosphorus, is the predominant mineral
allocated to this function (Kovacs, 2005), whereas other elements, such as magnesium and sodium, contribute to the skeleton in varying degrees (Spadaro et al., ‘70; Sarazin et al., 2000;
Cashman, 2006; Palacios, 2006). In mammals, calcium and other
minerals are transferred to offspring across the placenta in utero
and via milk from birth to weaning. These minerals are derived
from the maternal diet as well as from maternal endogenous
stores, and the amount of calcium available to invest in offspring
production is limited both by intrinsic and extrinsic factors.
Endogenous calcium is stored primarily in bones and is mobilized along with other bone minerals via resorptive activity.
Many mammals exhibit elevated bone resorptive activity during reproduction (Kovacs, 2005), and mobilized mineral can
then be allocated to offspring. For example, 19% of calcium
in milk produced by rats is derived from the maternal skeleton
when females are supplied with a high-quality diet ad libitum
(Brommage, ‘89). Yet when calcium intake is low, bone resorptive
activity intensifies (Boelter and Greenberg, ‘43; Brommage and
DeLuca, ‘85; Peng et al., ‘88; Schmidt and Hood, 2012; Schmidt
and Hood, 2013). Thus, exogenous calcium availability influences how much mineral a female mobilizes during a reproductive event.
Bone loss, the consequence of mobilizing mineral from the
skeleton without replacement, is considered to be a physiological
cost of reproduction (Speakman, 2008), and the likelihood of
sustaining a fracture increases as the relative concentration of
mineral in bone is diminished (Reilly and Burstein, ‘75; Broe
et al., 2000). As a result, there appears to be a limit to the amount
of mineral that can be resorbed, after which the costs incurred
by this activity are no longer offset by the benefit of a successful
reproductive event (see Hood, 2012).
Females may therefore respond to limited calcium intake by
curtailing calcium allocation to offspring production as a means
of protecting their own skeletal condition. A reduction in allocation may manifest as reducing the amount of calcium allocated
to each individual while maintaining number of offspring produced. This has been observed in guinea pigs (Cavia porcellus)
that do not alter litter size but do produce offspring with diminished bone mineral content when fed a vitamin D deficient
diet (which impairs calcium absorption) during gestation (Finch
et al., 2010). At the other end of the continuum, females may reduce the number of offspring produced and maintain the amount
of calcium allocated to each individual or forgo investing in
that reproductive event all together. When dietary calcium intake
is low, white-footed mice (Peromyscus leucopus) produce fewer
and smaller litters over their lifetime (Schmidt and Hood, 2012).
Similar to what has been observed in other mammal species
(Kovacs, 2005), white-footed mice experience a reduction in
bone mineral density during gestation and lactation that is exacerbated when dietary calcium intake is low (Schmidt and
Hood, 2012, 2013). Therefore, exogenous and endogenous calJ. Exp. Zool.
SCHMIDT AND HOOD
cium availability influences the amount of calcium that can be
allocated to the production of offspring.
White-footed mice consuming a low-calcium diet also produce strongly female-biased litters, which is predicted when allocation to reproduction is reduced in polygynous species in which
only males that are in good condition can secure mating opportunities (Trivers and Willard, ‘73; Ribble, 2003; Schmidt and
Hood, 2012). This suggests that elevated bone resorptive activity
does not sufficiently compensate for reduced calcium intake, and
that offspring produced by the low-calcium group are in poorer
condition than those offspring produced by females consuming
sufficient calcium.
With regard to calcium allocation, offspring condition can be
quantified by bone characteristics and relative mineral composition. As diminished bone mineral increases fracture risk, the
quality of offspring bone can have ecological impacts on both
their potential survival and reproductive success. To assess the
influence of maternal calcium intake on offspring quality when
litter size is maintained, we compared bone, mineral, and growth
characteristics of the offspring produced by white-footed mice
that had consumed a low-calcium diet over their course of their
lives with those that were produced by females consuming a
standard diet. If maternal bone resorption is limited in its capacity to compensate for a reduction in calcium intake, and litter
size is not adjusted, then we expect that individual offspring will
receive diminished support for skeletal development, resulting in
relatively poorer skeletal condition.
MATERIALS AND METHODS
We compared size, growth, mineral content, and bone morphology measurements of pups produced from a previous study in
which breeding females that were fed either a low-calcium or
standard diet (Schmidt and Hood, 2012, 2013). From this study,
48 pup carcasses were available for morphology and content
analyses: 32 pups were produced by females on the standard diet
(n = 8) and 16 pups were produced by females on the low diet
(n = 4). Litter size did not significantly differ between dietary
groups (see the Results section).
In the previous study, we bred 15 female white-footed mice
over the course of 75 weeks, providing them with four to five
mating opportunities with a randomly selected male. Females
were given ad libitum access to one of two custom manufactured
diets that varied only in calcium content (modified TD.08174,
Harlan Teklad, Madison, WI USA; see Schmidt and Hood, 2012
for content details). Seven of the females were fed a low-calcium
diet (0.10% Ca dry mass) and eight females were fed a standard diet (0.85% Ca dry mass) that contained the recommended
amount of calcium for reproducing mice (Harlan Teklad, personal communication); males were all provided with ad libitum access to the custom standard diet (Schmidt and Hood,
2012).
REPRODUCTIVE TRADE-OFFS AND MINERAL ALLOCATION
Dietary Intake and Reproductive Output
We measured weekly food consumption by females by calculating the difference between the mass of food added to the box
and the mass of food remaining after 7 days. Females were maintained as pairs for the first 2 weeks of gestation and then separated to monitor reproductive output of each individual. Therefore, individual food intake was only measured following the
second week. From these data, we estimated calcium intake over
the course of late gestation and lactation. Females were monitored daily to assess parturition date, and pups were weighed
and counted 7 days postpartum. We also measured tail length of
pups at these times as a metric for skeletal growth. We continued to measure and weigh pups every 7 days (day 14, 21, and
28). At day 28, we euthanized pups using CO2 and maintained
carcasses at –20°C until time of analyses. All procedures performed in this study were approved by the Auburn University
Institutional Animal Care and Use Committee (protocol number
2008-1365).
Bone Morphology and Composition
We excised the left femur and humerus from carcasses and
cleaned them by manually removing excess tissue, placing the
bones in an ultrasonic cleaner (Sper Scientific Ltd., Scottsdale,
AZ, USA) for 30 min to loosen remaining muscle fibers, and then
again manually removed any tissue that remained. Owing to
their fragility, several bones sustained fractures during the cleaning process; we photographed bones that remained intact with a
digital camera on a 1-mm2 grid under a dissecting microscope.
From the photographs, we measured the length of the bone shaft
and the width of the bone midshaft using ImageJ software (U.S.
National Institutes of Health, Bethesda, MD, USA); the grid in the
background was used for calibration. To obtain the fat-free dry
mass of individual bones, we extracted polar and nonpolar lipids
in a soxhlet apparatus for 12 hr using a 2:1 mixture of petroleum
ether and acetone. Following extraction, we dried the bones
overnight at 100°C (Binder drying oven FED 115-UL, Binder Inc.,
Great River, NY, USA). We recorded the fat-free mass of the bone
and then ashed the bones in a muffle furnace (Isotemp Muffle
Furnace, Fisher Scientific, Dubuque, IA, USA) at 500°C for 12 hr.
We weighed the remaining ash to obtain overall mineral content.
We then digested the ash in 70% nitric acid (trace metal grade) on
a heating block at 100°C for 1 hr. We diluted the digested samples by mass with nanopure water and quantified the calcium,
magnesium, phosphorous, and sodium (Ca, Mg, P, Na) contents
of each sample with an inductively coupled plasma optical emission spectrometer (ICP; Perkin Elmer Optima 7300DV, Waltham,
MA, USA). We added a known amount of internal silver (Ag)
standard to each sample to quantify recovery of minerals. Mean
recovery of the internal standard for ICP was 97.8 ± 0.3%.
We analyzed each sample in duplicate, and mineral concentrations were averaged across duplicates, and then used to calcu-
3
late total mass of each mineral present in each bone. From this,
we also calculated the calcium:phosphorus ratio (Ca:P) for each
bone.
Body Composition
After excising the femur and humerus, we dried carcasses at
100°C for 72 hr and then homogenized each sample. We redried
two subsamples (0.276 ± 0.001 g) of the homogenate, recorded
the dry mass, and performed fat extraction on the subsamples
as described above. We ashed, digested, and measured mineral
content by ICP of an additional two subsamples as described
above. Mean recovery of the internal standard for ICP was 96.8
± 0.2%. Since bone and attached tissues had been removed prior
to body analysis (i.e., incomplete carcasses were used in this
analysis), we calculated relative mineral concentration (% fat
free dry mass) instead of the total mass of ash and individual
minerals in the carcasses. As two large bones were absent from
the analysis, calculated mineral concentration is an underestimate of the proportion on mineral that is present in a complete
body.
Data Analysis
We compared the number of offspring produced per litter between dietary treatments using analysis of variance (ANOVA).
When females produced more than one litter, a female’s litter
size was based on the average of all litters produced. We used
general linear models (PROC GLM) to test the relationships between litter size and body mineral content of pups and pup bone
size, maternal food intake over time (from about the third week
of pregnancy until the third week of lactation), and whether maternal calcium intake interacted with these relationships. We also
used GLM to assess if mean maternal food intake over the course
of late gestation and lactation was related to mineral content of
pups and bone size measurements. We used a two-way nested
ANOVA, with pup nested within female, to compare body and
bone mineral composition of pups, body mass of pups at days 7,
14, 21 and 28, tail length of pups at days 7, 14, 21 and 28, and
pup bone size and bone mass between dietary treatment groups
(PROC GLM). We used a two-way nested multivariate analysis of
variance (MANOVA), with pup nested within female, to compare
Ca:P of pup bones between diets and bone types (PROC GLM).
We nested litter within female when more than one litter from
an individual female was included in the dataset (carcass composition and pup morphology). All reproductive output data (i.e.,
litter size, litter mass, pup mass, and tail length) used in these
analyses were limited to those pups in which body composition
analyses was performed, and thus does not represent lifetime
reproductive output as shown in Schmidt and Hood (2012). All
analyses were performed in SAS 9.2 (SAS Institute Inc., Cary,
NC, USA).
J. Exp. Zool.
4
SCHMIDT AND HOOD
week of reproduction did not interact (F4,46 = 1.79, P = 0.146).
There was no significant difference between diets for pup body
mass or tail length at day 7, 14, 21 or 28 (mass: F1,51 = 1.21,
P = 0.286; tail: F1,51 = 1.62, P = 0.220; Table 1) nor was there
a significant interaction between diet and day (mass: F3,51 =
0.01, P = 0.998; tail: F3,51 = 0.10, P = 0.959). Litter size had
no effect on body mineral composition or bone morphology and
composition of pups, nor did it interact with maternal diet to
affect these variables (F ࣘ 3.61, P ࣙ 0.124 in all models).
Table 1. Body mass and tail length of white-footed mouse pups
relative to maternal dietary treatment (low-calcium diet = Low,
standard-calcium diet = Std). Means are shown ± SE
Pup age
Body mass (g)
7 days
14 days
21 days
28 days
Tail length (mm)
7 days
14 days
21 days
28 days
Low (n = 5)
Std (n = 14)
3.99
5.92
8.43
11.07
±
±
±
±
0.15
0.30
0.35
0.46
5.29
7.49
9.73
11.98
±
±
±
±
0.74
0.74
0.76
0.71
20.87
38.32
49.84
57.43
±
±
±
±
1.10
0.80
1.04
0.81
18.44
31.46
41.73
46.57
±
±
±
±
1.13
2.79
4.35
4.96
Bone Morphology and Composition
Pups from females on the standard diet had significantly
wider femurs than those from females on the low-calcium diet
(Table 2). There were no other differences in femoral length or
in humeral length or width of pups between diets (Tables 2 and
3). Both weanling femurs and humeri were significantly heavier
in the standard group than they were for the low-calcium group
(Tables 2 and 3).
Females that consumed the standard calcium diet produced
pups with significantly higher percentage of ash in the humerus
than females that consumed the low-calcium diet (Table 3), and
total ash content was higher in both the humerus and the femur of pups produced by females on the standard diet (Tables 2
and 3).
Percent calcium and phosphorus in weanling femurs and
humeri did not vary between dietary treatments (Tables 2 and
3) although both bones exhibited a higher concentration of Na
and Mg in the low-calcium group (Tables 2 and 3). The total
RESULTS
Dietary Intake and Reproductive Output
There was no significant difference in litter size produced by
females on the low-calcium or standard diet (F1,15 = 0.40, P =
0.535, X̄ low = 2.5 ± 0.3 SE, X̄ standard = 2.7 ± 0.3 SE) for pups
included in this study. Mean weekly food intake did not differ
between females on the low-calcium and standard diet (F1,46 =
0.040, P = 0.846; X̄ low = 35.5 ± 0.8 SE, X̄ standard = 36.0 ±
3.9 SE). Food intake increased from gestation to late lactation in
both groups (F6,46 = 12.0, P < 0.0001), but dietary treatment and
Table 2. Femur morphology, mineral content (Ash), and mineral composition (% ash) of white-footed mouse pups relative to maternal
dietary treatment group (low-calcium diet = Low, standard-calcium diet = Std)
Std
F
df
P
10.6 ± 0.2
1.04 ± 0.03
8.30 ± 0.47
10.8 ± 0.2
1.16 ± 0.03
14.1 ± 0.9
0.69
7.02
23.2
1, 25
1, 25
1, 16
0.415
0.015
<0.001
44.5 ± 2.1
43.2 ± 1.2
35.9 ± 1.2
1.68 ± 0.10
7.77 ± 0.35
47.1
44.1
32.4
1.32
4.66
±
±
±
±
±
2.2
0.9
3.3
0.11
0.72
0.57
0.31
0.52
4.26
7.88
1, 16
1, 13
1, 13
1, 13
1, 13
0.461
0.589
0.482
0.060
0.014
6.73
2.97
2.30
0.082
0.284
1.26
±
±
±
±
±
±
0.57
0.28
0.22
0.006
0.034
0.02
14.0
7.66
8.35
1.52
0.02
6.39
1, 16
1, 13
1, 13
1, 12
1, 14
1, 12
0.002
0.017
0.014
0.243
0.901
0.028
Low
Morphology
Length (mm)
Width (mm)
Mass (mg)
Mineral composition
Ash (% of bone)
Ca (% of ash)
P (% of ash)
Mg (% of ash)
Na (% of ash)
Total mass in bone
Ash (mg)
Ca (mg)
P (mg)
Mg (mg)
Na (mg)
Ca:P (mg)
3.70 ± 0.29
1.70 ± 0.18
1.37 ± 0.15
0.070 ± 0.004
0.290 ± 0.013
1.173 ± 0.018
Means are shown ± SE. Significant effects are presented in bold.
J. Exp. Zool.
REPRODUCTIVE TRADE-OFFS AND MINERAL ALLOCATION
5
Table 3. Humerus morphology, mineral content (Ash) and mineral composition (% of ash) of white-footed mouse pups relative to maternal
dietary treatment group (low-calcium diet = Low, standard-calcium diet = Std)
Morphology
Length (mm)
Width (mm)
Mass (mg)
Mineral composition
Ash (% of bone)
Ca (% of ash)
P (% of ash)
Mg (% of ash)
Na (% of ash)
Total mass in bone
Ash (mg)
Ca (mg)
P (mg)
Mg (mg)
Na (mg)
Ca:P (mg)
Low
Std
F
df
P
8.38 ± 0.22
0.850 ± 0.042
6.20 ± 0.38
8.18 ± 0.07
0.866 ± 0.020
8.20 ± 0.67
0.68
0.26
5.83
1, 24
1, 24
1, 14
0.422
0.617
0.033
31.3
45.7
37.8
1.87
12.0
±
±
±
±
±
6.1
1.4
1.4
0.23
1.0
51.4
43.1
34.9
1.14
6.88
±
±
±
±
±
0. 9
0.7
0.5
0.21
0.61
9.20
2.40
3.46
5.26
19.2
1, 14
1, 10
1, 10
1, 10
1, 10
0.009
0.150
0.090
0.043
0.001
2.38
1.04
0.863
0.040
0.241
1.18
±
±
±
±
±
±
0.19
0.04
0.052
0.008
0.014
0.03
4.20
1.95
1.57
0.052
0.289
1.24
±
±
±
±
±
±
0.33
0.18
0.13
0.011
0.016
0.01
23.8
15.8
20.8
0.65
4.60
4.76
1, 16
1, 9
1, 10
1, 9
1, 11
1, 9
<0.001
0.004
0.001
0.442
0.058
0.061
Means are shown ± SE. Significant effects are presented in bold.
mass of calcium and phosphorus in the femur and humerus
was higher for pups of females consuming the standard diet
(Tables 2 and 3). Femurs from pups produced by females consuming the standard diet had a significantly higher Ca: P than
those from pups in the low-calcium group (Table 2), and humeri
followed that trend (Table 3). There was no difference between
treatments in the total amount of sodium or magnesium stored
in either bone (Tables 2 and 3).
Body Composition
There was no significant difference in percent ash content of
bodies of pups from females on either diet (Table 4). Pups produced by females on the low-calcium diet had lower body concentrations of calcium than those from females in the standard
diet group (Table 4). Phosphorus, magnesium, and sodium concentrations in the body did not vary between diets (Table 4).
DISCUSSION
Our results indicate that when female white-footed mice breed
under low dietary calcium conditions, they trade off offspring
skeletal condition for self-maintenance. Specifically, females
consuming a low-calcium diet that produce litters that are similar in body size and body mass to those from those consuming
the standard diet, but have lower concentrations of total body
calcium and a lower humeral and femoral calcium and phosphorus content than females consuming a calcium-replete diet.
Pups from the low-calcium group possessed lighter humeri
and lighter and thinner femurs than pups from the standard
Table 4. Mineral composition of the body (% fat free dry mass) in white-footed mouse pups relative to maternal dietary treatment group
(low-calcium diet = Low, standard-calcium diet = Std)
Mineral
Ash (% fat free dry mass)
Ca (% of ash)
P (% of ash)
Mg (% of ash)
Na (% of ash)
Low
8.82
24.3
28.6
1.34
4.42
±
±
±
±
±
Std
0.24
0.6
0.3
0.03
0.11
8.94
27.4
28.7
1.30
3.87
±
±
±
±
±
0.24
0.9
1.2
0.06
0.18
F
df
P
1.40
6.54
0.00
0.17
3.58
1, 27
1, 47
1, 47
1, 47
1, 47
0.250
0.015
0.949
0.683
0.067
Means are shown ± SE. Significant effects are presented in bold.
J. Exp. Zool.
6
group. Pups from the low-calcium group also had less total ash,
calcium, and phosphorus in both the femur and humerus than
did pups from the standard group, and total mineral content
of the humerus was also lower in the low-calcium group. Thus,
the interactions between these differences and bone morphology
and composition differed between the long bones of the fore and
hind limbs. Specifically, the femurs but not humeri of pups born
to low-calcium females displayed reduced width relative the femurs of weanling from the standard group, and the total mineral
content of the humeri but not femurs of pups in the low-calcium
group than was lower than that of the standard group. These
observations suggest that when calcium availability is limited,
bone dimensions are prioritized for the humerus and total bone
mineral content is prioritized for the femur (Tables 2 and 3).
The variation in response to maternal calcium intake between the femur and humerus of offspring could reflect a general difference in ossification rates of each bone (Chahoud and
Paumgartten, 2005), with ossification corresponding to the degree of mechanical strain than a specific bone may sustain
(Lerner et al., ‘98; Farnum et al., 2008). For example, thirteenlined ground squirrels (Spermophilus tridecemlineatus) and Eastern chipmunks (Tamias striatus) exert up to 30–50% more
vertical force on the forelimbs compared with the hind limbs
(Biewener, ‘83).
The relative calcium and phosphorus content of the mineral in
the femur and humerus did not differ between groups but Ca:P
in the femur and possibly the humerus was lower for pups from
the low-calcium group (Tables 2 and 3). It is clear that the body
prioritizes percent calcium and phosphorous in bone but it is less
clear why Ca:P of bone is reduced in the low-calcium pups. The
Ca:P in hydroxyapatite, the crystalline structure that gives bone
much of its strength, is 2.16 (by weight). During development,
Ca:P in bone increases as the relative amount of phosphorus
associated with collagen declines as cartilage is replaced with
hydroxyapatite (Dickerson, ‘62). We expect that relatively more
phosphorus remained associated with collagen in low-calcium
pups. Although the relative composition of both sodium and
magnesium was greater for the low-calcium group, there was
no difference in the total mass of either of these elements in
either bone.
We previously showed that female white-footed mice mobilize
more bone during reproduction when consuming a low-calcium
diet (Schmidt and Hood, 2012, 2013). This relationship has been
observed in several other species of mammals and suggests that
bone mobilization serves to compensate for inadequate intake of
dietary calcium. However, females in this study that consumed
the low-calcium diet still allocated less mineral to individual offspring, suggesting that there is a limit to the amount of mineral
that can be mobilized from bone.
The reduction in mineral content, bone mass, and bone width
observed in pups produced by females in the low-calcium group
may persist throughout life in a similar manner to what has been
J. Exp. Zool.
SCHMIDT AND HOOD
observed in human models (e.g., Javaid et al., 2006; Harvey et al.,
2010). Thus, the offspring of white-footed mice may experience
long-term consequences associated with calcium intake of their
females. In polygynous species, since males in good condition
are more likely to secure mates (Trivers and Willard, ‘73), lower
bone size, mass, and mineralization may put males at a reproductive disadvantage. Trivers and Willard (’73) predicted that if
a female is limited in her capacity to produce high-quality males,
it would be advantageous to invest in the production of female
offspring. This is supported by our previous observations that
females on the low-calcium diet produce strongly female-biased
litters (Schmidt and Hood, 2012). In this case, females were limited in their capacity to deliver calcium to their offspring, and
this limit was presumably generated by an interaction between
dietary calcium intake and the amount of calcium that was retained by the female to meet the demands self-maintenance. This
limitation may also reflect a threshold in the physiological capacity to store and/or mobilize skeletal calcium; further study is
necessary to determine which factors might influence the limits
to these functions.
When the availability of dietary calcium is limited and litter size is not reduced, white-footed mouse females reduce the
amount of calcium and other bone minerals allocated to individual offspring. This suggests that females limit the amount of
bone loss that they incur as a means of prioritizing their own
future reproductive potential over the fitness of their offspring;
however, this potential reduction in offspring quality may be
offset by an increase in the proportion of females produced in
a litter. Thus, in environments where dietary calcium may not
be consistently available, females have the capacity to respond
to fluctuations in availability as a means of maximizing reproductive success. This response feasibly occurs to some degree in
most mammals and is potentially attributable to all vertebrate
species.
ACKNOWLEDGMENTS
We thank T. Donaldson, T. Filhiol, C. Gentry, K. Jeffreys, P.
Monfore, C. Newton, M. Ramirez, C. Robinson, and A. Weems
for assistance with animals, M. Drake, K. Gardner, L. Harris, C.
Kuhn, M. Luger, M. K. Markham, and M. Reich for lab assistance,
J. Crossland for guidance on mouse husbandry, and G. Hill, A.
Skibiel, D. Raubenheimer, and H. Carey for comments and suggestions for the preparation and revision of this manuscript.
LITERATURE CITED
Biewener AA. 1983. Locomotory stresses in the limb bones of 2 small
mammals – the ground-squirrel and chipmunk. J Exp Biol 103:131–
154.
Boelter MDD, Greenberg DM. 1943. Effect of severe calcium deficiency on pregnancy and lactation in the rat. J Nutr 26:105–
121.
REPRODUCTIVE TRADE-OFFS AND MINERAL ALLOCATION
Broe KE, Hannan MT, Kiely DK, Cali CM, Cupples LA, Kiel DP. 2000. Predicting fractures using bone mineral density: a prospective study
of long-term care residents. Osteoporos Int 11:765–771.
Brommage R. 1989. Measurement of calcium and phosphorus fluxes
during lactation in the rat. J Nutr 119:428–438.
Brommage R, DeLuca HF. 1985. Regulation of bone mineral loss during lactation. Am J Physiol 248:E182–187.
Cashman KD. 2006. Milk minerals (including trace elements) and bone
health. Int Dairy J 16:1389–1398.
Chahoud I, Paumgartten FJR. 2005. Relationships between fetal body
weight of Wistar rats at term and the extent of skeletal ossification.
Braz J Med Biol Res 38:565–575.
Dickerson JWT. 1962. The effect of development on the composition
of a long bone of the pig, rat and fowl. Biochem J 82:47–55.
Farnum CE, Tinsley M, Hermanson JW. 2008. Forelimb versus hindlimb
skeletal development in the big brown bat, Eptesicus fuscus: functional divergence is reflected in chondrocytic performance in autopodial growth plates. Cells Tissues Organs 187:35–47.
Finch SL, Rauch F, Weiler HA. 2010. Postnatal vitamin D supplementation following maternal dietary vitamin D deficiency does not
affect bone mass in weanling guinea pigs. J Nutr 140:1574–1581.
Harvey NC, Mahon PA, Robinson SM, Nisbet CE, Javaid MK, Crozier
SR, Inskip HM, et al. 2010. Different indices of fetal growth predict
bone size and volumetric density at 4 years of age. J Bone Miner
Res 25:920–927.
Hood WR. 2012. A test of bone mobilization relative to reproductive
demand: skeletal quality is improved in cannibalistic females with
large litters. Physiol Biochem Zool 85:385–396.
Javaid MK, Crozier SR, Harvey NC, Gale CR, Dennison EM, Boucher BJ,
Arden NK, et al. 2006. Maternal vitamin D status during pregnancy
and childhood bone mass at age 9 years: a longitudinal study.
Lancet 367:36–43.
Kovacs CS. 2005. Calcium and bone metabolism during pregnancy
and lactation. J Mammary Gland Biol Neoplasia 10:105–118.
7
Lerner AL, Kuhn JL, Hollister SJ. 1998. Are regional variations in
bone growth related to mechanical stress and strain parameters?
J Biomech 31:327–335.
Palacios C. 2006. The role of nutrients in bone health, from A to Z.
Crit Rev Food Sci Nutr 46:621–628.
Peng TC, Garner SC, Kusy RP, Hirsch PF. 1988. Effect of number of
suckling pups and dietary calcium on bone mineral content and
mechanical properties of femurs of lactating rats. Bone Miner
3:293–304.
Reilly DT, Burstein AH. 1975. The elastic and ultimate properties of
compact bone tissue. J Biomech 8:393–396.
Ribble DO. 2003. The evolution of social and reproductive monogamy
in Peromyscus, evidence from Peromyscus californicus (the California mouse). In: Reichard U, Boesh C, editors. Monogamy: mating
strategies and partnerships in birds, humans, and other mammals.
Cambridge, UK: Cambridge University Press. p 81–92.
Sarazin M, Alexandre C, Thomas T. 2000. Influence on bone
metabolism of dietary trace elements, protein, fat, carbohydrates,
and vitamins. Joint Bone Spine 67:408–418.
Schmidt CM, Hood WR. 2012. Calcium availability influences litter
size and sex ratio in white-footed mice (Peromyscus leucopus). Plos
One 7:e41402.
Schmidt CM, Hood WR. 2013. Bone loss is a physiological cost of reproduction in white-footed mice (Peromyscus leucopus). Mammal
Biol 79:96–100.
Spadaro JA, Becker RO, Bachman CH. 1970. Distribution of trace
metal ions in bone and tendon. Calcif Tissue Res 6:49–54.
Speakman JR. 2008. The physiological costs of reproduction in small
mammals. Phil Trans R Soc B 363:375–398.
Trivers RL. 1974. Parent-offspring conflict. Am Zool 14:249–264.
Trivers RL, Willard DE. 1973. Natural selection of parental ability to
vary sex ratio of offspring. Science 179:90–92.
Williams GC. 1966. Natural selection, the costs of reproduction, and
a refinement of Lack’s principle. Am Nat 100:687–690.
J. Exp. Zool.
View publication stats