Comparison of mass-transfer and isotopic dilution methods for estimating milk intake in Antarctic fur seal pups

Abstract

The efficacy of a new mass-transfer method for estimating milk intake was examined in Antarctic fur seals (Arctocephalus gazella) at Iles Kerguelen. Our method differed from previous mass-transfer approaches in that we estimated milk-mass transfer as the maternal mass lost (MML; kg) during an attendance bout, less the mass lost to metabolic maintenance (MML_E) over that time. MML was significantly related to pup mass-gain (PMG) and attendance bout duration (d days) as follows: MML=1.106PMG+1.002d (r²=0.998). Based on this and previous studies, we estimated that the MML_E was 0.0285 kg kg⁻¹ day⁻¹ for lactating females; and we developed the following milk-mass transfer equation: MML_M=1.106PMG+1.002d−0.0285MMd (where MM is maternal mass). Milk-mass intake was also estimated in an additional 21 pups, using the isotopic dilution method. These values were then compared with estimates based on the milk mass-transfer equation for the same individual pups. A pair-wise comparison indicated that milk-mass transfer estimated using tritium dilution methods were significantly lower than those based on mass-transfer (MML_M). Furthermore, the absolute PMG exceeded tritium dilution estimates of milk-mass transfer in 35% of cases. In contrast, all milk-mass transfer estimates using the mass transfer method were greater than PMG. Overestimation of metabolic water production (MWP), leading to a smaller proportion of the total water intake being attributed to milk ingestion, is believed to be the most likely cause for significant underestimation of milk-mass transfer using the tritium dilution method. Consumption of exogenous water by pups is the most likely reason for the overestimation of MWP, although errors in estimated milk water content may have also contributed to underestimates. We conclude that, in our study, the mass-transfer method provided a more reliable estimate of milk-mass transfer than the isotopic dilution method; and we argue that, under certain conditions, it provides a practical alternative method where the assumptions of isotopic dilution methodology (e.g., all exogenous water from maternal milk) and quantitative parameters (e.g., maternal milk water content) may either be violated or impractical to measure.