Use of stable-carbon and -nitrogen isotopes to assess weaning and fasting in female polar bears and their cubs

Abstract

In some species, stable-isotope techniques can provide insights into dietary regimens where there are temporal shifts in trophic level or feeding frequency. We determined stable carbon (δ13C) and nitrogen (δ15N) isotope values for plasma and milk proteins and δ13C values for milk lipids from female polar bears (Ursus maritimus) and cubs to (i) ascertain whether cubs are at a higher trophic level than their mothers as a result of nursing and whether we can determine when weaning occurs, and (ii) determine the impact of seasonal fasting on δ13C and δ15N values. The plasma δ13C values for mothers and cubs were similar to milk-protein δ13C values and were significantly enriched in 13C compared with those for milk lipid. Plasma from cubs of the year (COYs) in spring, when milk was their only diet, was isotopically enriched in 15N by 1.0‰ over that of their mothers (δ15N = 21.5 ± 0.8‰ (mean ± SD) for cubs and 20.5 ± 0.5‰ for mothers) and depleted in 13C by 0.8 ‰ (δ13C = –19.6 ± 0.5‰ for cubs and –18.8 ± 0.8‰ for mothers). For bears who fasted between summer and fall (3–4 months), plasma became depleted in 13C by 0.5‰ and in 15N by 1‰. Plasma from females, who had fasted from summer to spring (7–8 months) and given birth to cubs, became enriched in 13C by 0.7‰ and in 15N by 2‰. By using stable-isotope analyses we were able to show that (i) young cubs were at a higher trophic level than their mother when milk was their only food source, and (ii) seasonal fasting influenced δ13C and δ15N values. However, we were not able to use stable-isotope analyses to determine the exact time of weaning.

Trophic niche partitioning among sympatric baleen whale species following the collapse of groundfish stocks in the Northwest Atlantic

“…While calves were not isotopically distinct from juveniles and adults, they occupied higher trophic positions when compared to their own mothers. This is consistent with the mother-offspring trophic enrichment re ported for several other mammals (Jenkins et al 2001, Polischuk et al 2001, and most likely related to milk consumption. Lastly, reproductive state of adult female humpbacks appears to have no effect on isotopic niche.…”

Stable isotope turnover and variability in tail hairs of captive and free-ranging African elephants (Loxodonta africana) reveal dietary niche differences within populations

“…Based on our tail hair data, elephant ␦ 15 N niches were also broader in the south than in the north and some of the variation in the south was linked with age-related decreases. Reduced ␦ 15 N values from calf to juvenile stages are consistent with expected trophic shifts between suckling and nonsuckling individuals (see e.g., Balasse et al 2001;Polischuk et al 2001). …”

Nitrogen and carbon isotope fractionation between mothers, neonates, and nursing offspring

“…The mean δ 15 N difference between the plasma of grizzly bear mothers and their cubs when milk is the only nourishment for the cub (1.2±0.5‰) is similar to that in polar bears (1.0‰) (Polischuk et al, 2001) and the overall interspecific average (0.9±0.8‰), but well below that suggested for cave bears (3.1-5.1‰) (Nelson et al 1998). Similarly, the above δ 15 N enrichments between mother and offspring plasma (~1‰) or milk and offspring plasma (1.9±0.7‰) are small compared to the mean, interspecific δ 15 N enrichment between diet and maternal plasma in the current study (4.1±0.7‰).…”

“…3 The C isotope signature (a) and N isotope signature (b) of plasma from adult female moose, caribou, grizzly bears, black-tailed deer, coyotes, domestic rabbits, sheep, cows, pigs, rats, and cats and their offspring at 12-14 days after birth (current study). Isotope signatures of plasma from free-ranging, adult, female polar bears and their cubs (Polischuk et al, 2001) were also included. All offspring had consumed only milk 1.7±0.5 for RBCs on day 14 (P<0.01).…”