**2.6 Ontogenetic changes in δ13C and δ15N values in bowhead whales**

Large-scale ontogenetic studies on baleen whales, focusing on lactation using δ13C and δ15N signatures, have only been conducted in BW samples from native subsistence hunts [20, 72, 73]. The BWs are large baleen whales inhabiting the icy Arctic waters, and they are born at 4–5 m BL, weaned at 10 m BL, with sexual maturity at over 13.5 m BL [72, 74]. BWs feed on pelagic zooplankton and benthic amphipods, where the δ13C and Cd levels of benthic amphipods are lower and higher than those of zooplankton, respectively, and Hg is in trace concentrations [20].

**Figure 3** shows the ontogenetic changes in δ15N and δ13C signatures in the muscle samples of BWs by Lee et al. [72]. They analyzed the δ15N and δ13C values of nonlipid–extracted muscle samples. Line smoothing shows the δ15N-enriched peak at 8 m BL and a <sup>δ</sup>15N-depleted peak at 10 m BL. A large study on BWs conducted thereafter by Horstmann-Dehn et al. [73] showed prominent δ15N-enriched peaks in muscle (n = 133) and epidermis (n = 130) samples of BWs at 8 m BL. The increase in <sup>δ</sup> 15N values to the enriched peak (8 m BL) could represent nursing, and this peak is likely to be the onset of weaning (shift from milk to solid foods). Furthermore, the BL *Typical Changes in Carbon and Nitrogen Stable Isotope Ratios and Mercury Concentration… DOI: http://dx.doi.org/10.5772/intechopen.103067*

**Figure 3.**

*Ontogenetic changes of δ<sup>15</sup> and δ13C signatures in muscle of bowhead whales. These figures were drawn based on the data from Table 5 (n = 47) reported by Lee et al. [72] with permission. Two outliers were not drawn.*

at the δ15N-depleted peak may represent BL at complete weaning in BWs, which coincides with the weaned BL at 10 m reported previously [72, 74]. After weaning, the δ15N values gradually increased and reached a plateau. The ontogenetic change in the δ13C profile was similar but more prominent than that of δ15N change; no correlation was found between the δ13C and δ15N values. The prominent increase in post-weaning δ13C values may be related to the feeding on zooplankton and benthic amphipods, although the cofounding effect of tissue lipid content on δ13C values should be considered: large differences are found between the lipid-extracted and non-extracted muscle samples when δ13C values are low [73]. Dehn et al. [20] reported trace levels of Hg in the muscle (0.02 0.01 μg/wet g) and a moderate level of Cd in the kidney (15.08 14.94 μg/g) of BWs, and these levels of Hg and Cd could be consistent with their habit of feeding on pelagic zooplankton and benthic amphipods. Despite a large number of BW samples, Lee et al. [72] and Horstmann-Dehn et al. [73] did not investigate sex-related differences in the δ13C and δ15N profiles of BW samples.

Dehn et al. [20] reported a marked depletion in <sup>δ</sup>13C values (between <sup>24</sup>‰ and 25‰) in two fetus of BWs at 1 m and 4 m BL as compared to those in adults (approximately <sup>20</sup>‰), and these values are in the range of the <sup>δ</sup>13C values in milk found widely in marine mammals (between 24‰ and 26‰) [3, 9, 43, 64]. Dehn et al. [20] did not mention the δ15N-enrichment in the muscle of BW fetuses, but Horstmann-Dehn et al. [73] reported the enriched δ15N values in the epidermis of BW fetuses. Borrell et al. [3] reported higher δ13C and δ15N values in FW fetuses (17.45 0.53‰ and 11.46 0.38‰, n = 10) than in lactating females (18.06 0.22‰, and 9.46 0.55‰, n = 13), although they did not mention the BL of the analyzed fetuses. More research on Δ13Cfetus-mother values in addition to Δ15Nfetus-mother values is needed to clarify the transport of nutrition from mother to fetus. The BW and FW are mysticetes and are classified as capital breeders.
