Biology Reference
In-Depth Information
experiments with laboratory animals (
Ambrose and Norr, 1993; Tieszen and Fagre, 1993;
Howland et al., 2003
). As a result, it is well known that
d
13
C data derived from biological
apatite are more reflective of an individual's whole diet (or dietary energy) while
d
13
C
data gleaned from bone collagen are more reflective of protein sources (
Harrison and Katzen-
berg, 2003; Tykot, 2006
). Moreover,
Ambrose and Norr (1993)
demonstrated that
d
13
C values
differed by 9.4
between collagen (
d
13
C
CO
) and biological apatite (
d
13
C
AP
), primarily due to
the complex physiological differences by which these tissues take up carbon. Such dissimi-
larities had been suggested by earlier scholars (
Krueger and Sullivan, 1984
;
Lee-Thorp
et al., 1989
) and have ultimately demonstrated that skeletal biologists can reconstruct
a more nuanced dietary interpretation by analyzing differences between
d
13
C
CO
and
d
13
C
AP
, otherwise known as
dietary spacing
(
Ambrose et al., 1997
).
2
Recent work by
Kellner
and Schoeninger (2007)
and
Froehle et al. (2012)
produced several dietary models that further
indicate analyzing
d
13
C
CO
and
d
13
C
AP
together allow for further elucidation of C
3
and C
4
dietary strategies.
&
Nitrogen
Shortly after the first studies on carbon isotopes were published, other researchers began
publishing work on nitrogen isotopes (
d
15
N) (
DeNiro and Epstein, 1981
). Initially, these
studies were initiated to investigate
trophic level
distinctions within the food chain, with
particular emphasis on marine systems (
DeNiro and Schoeninger, 1983; Schoeninger and
DeNiro, 1983, 1984
). Generally, nitrogen isotope values are correlated with an organism's
position in the food chain and typically increase by 2
e
3
per trophic level (
Tykot, 2006
).
For example, in a study sampling species from numerous trophic positions,
Katzenberg
and Kelly (1991)
demonstrated distinct differences in
15
N levels between humans and
lower-position animals from the Sierra Blanca region of New Mexico. Trophic level distinc-
tions were also apparent in a similar study from Lake Baikal, Siberia (
Katzenberg and
Weber, 1999
), as freshwater seals (animals close to the top position of the food web) had
the highest
d
15
N values of all species sampled at approximately 14
&
.
In addition to questions surrounding trophic levels, researchers utilize nitrogen isotopes
to differentiate between plant and animal protein sources, as well as to separate terrestrial
from marine protein. In the case of differentiating between terrestrial and marine protein
sources (and human consumers), it is well known that marine plants are more enriched
for
d
15
N than their terrestrial counterparts. As a result,
d
15
N values increase with each subse-
quent trophic level; therefore, human consumers of marine protein sources present signifi-
cantly enriched
&
d
15
N values when compared to their terrestrial resource consuming
counterparts.
Finally, several researchers have indicated that
d
15
N is variable in hot, dry climates (
Hea-
ton et al., 1986; Ambrose, 1991
). This phenomenon is in part due to the way in which nitrogen
is excreted by mammals through urination. In essence, in parts of the world where water is
(or at times becomes) scarce, mammals excrete nitrogen in their urine at higher levels. At the
same time, however, this higher level of excretion must be balanced by the retention of
d
15
N
in the organism's tissues. As a result,
Ambrose (1991)
demonstrated that water-stressed
2
Dietary spacing is represented by the notation
D
13
C
CO
-
AP
. The
D
symbol indicates a change (i.e.,
difference) between
d
13
C
CO
and
d
13
C
AP
(see
Hedges, 2003
for a review).
