Isotope recycling in lactating dogs (Canis familiaris)

Abstract

Isotope-based techniques for the measurement of water turnover, energy expenditure, and milk intake often assume that there is no recycling of isotopes once they have left the labeled animal. In experiments involving lactating females or their suckling offspring, there are several possible routes of isotope recycling. These include the consumption of labeled milk by offspring, the ingestion of labeled excreta, and the rebreathing of exhaled labeled CO(2) or water vapor by both mother and offspring. Isotope recycling might be especially important during lactation because the offspring are in close contact with each other and their mother for prolonged periods. We show here in 24- to 30-day-old domestic dog Canis familiaris puppies that there was no detectable transfer of (18)O or (2)H from labeled to unlabeled pups in two litters (16 pups, 8 labeled, 8 unlabeled) that were weaned early and independent of their mother. However, there was a significant transfer of both isotopes from labeled to unlabeled pups and from labeled pups to their mothers in nine equivalent nursing litters of the same age (27 labeled, 26 unlabeled pups). The increases in enrichment of isotopes in unlabeled offspring were greater than the increases in enrichment of the mothers. This indicates that maternal ingestion of offspring excreta and subsequent transfer of isotope in milk is not the sole pathway of recycling. Additional routes must also be important, such as exchange of isotope between pups on saliva-coated nipples and perhaps direct ingestion of excreta by unweaned young. Recycling is unlikely to be an important factor when determining maternal metabolic rate during peak lactation in domestic dogs. However, experiments that are designed to assess the energy demands of pups and isotope-based estimates of water turnover in offspring may need to take into account any effects of isotope recycling. In a theoretical example, removing the effects of recycling increased the measured energy expenditure in pups by up to 7% and increased the calculated elimination rates of both isotopes by up to 11.1% in (18)oxygen and 10.9% in (2)hydrogen.

Limits to sustained energy intake. XXII. Reproductive performance of two selected mouse lines with different thermal conductance

“…Measurements were made across 2 days to minimise the potential day-today variability in DEE (Speakman et al, 1994;Berteaux et al, 1996). Recycling of isotopes between the mother and her pups was considered negligible (Scantlebury et al, 2000). On day 15 of lactation, mice were weighed (±0.01 g) and injected intraperitoneally with ~0.25 g of water enriched with 18 O (27.8 atom%) and 2 H (15.9 atom%).…”

Limits to sustained energy intake XIX: A test of the heat dissipation limitation hypothesis in Mongolian gerbils (Meriones unguiculatus)

“…Day-to-day variability in estimated energy metabolism suggests that measurements spanning multiple 24h periods may give a superior representation of energy metabolism (Berteaux et al, 1996;Speakman et al, 1994). Studies of lactating mammals suggest that recycling of isotopes between a mother and her offspring is negligible (Scantlebury et al, 2000).…”

Limits to sustained energy intake. XI. A test of the heat dissipation limitation hypothesis in lactating Brandt's voles (Lasiopodomys brandtii)

“…Day-to-day variability in estimated energy metabolism suggests that measurements spanning multiple 24h periods may give a better estimate of average DEE (Speakman et al, 1994;Berteaux et al, 1996). Studies of lactating mammals suggest that the recycling of isotopes between a mother and her offspring is negligible (Scantlebury et al, 2000).…”