MCDONALD, B.I.; CROCKER, D.E.; Sonoma State University, California; Sonoma State University, California: Lactation efficiency in northern elephant seals

The efficiency with which mothers convert acquired energy into milk is a key determinant of the magnitude of parental investment in mammals; however, the mechanisms underlying lactation efficiency are poorly understood. Previous investigations on northern elephant seals, Mirounga angustirostris, have shown that lactation efficiency, measured as the proportion of total energy expenditure that goes to the pup as milk, increases with age. Lactation efficiency can be increased through decreased activity or through increased milk production efficiency. The behavioral and physiological determinants of lactation efficiency were investigated in 8 young and 7 prime female elephant seals by conducting behavioral observations throughout lactation and collecting milk, blood, and tissue samples on day 3 and 22 of lactation. Milk composition, blubber and mammary lipoprotein lipase (LPL) activity, and circulating free fatty acid (FFA) and triglyceride concentrations were determined. Prime females spent significantly more time resting (95.3�0.8%) than young females (92.1�1.8%). Prime females had significantly greater percent milk fat on day 3 of lactation (25.0�3.3%) than young females (14.7�6.2%), but this difference disappeared by day 22. Prime females had significantly greater circulating FFA on day 3 (prime 1.35�0.35 mmol/L, young 0.81�0.29 mmol/L), but this difference also disappeared by day 22. LPL activity in the mammary gland (2.93�2.33 U) and blubber (0.63�0.26 U) did not differ between age classes and showed no relationship to milk fat content. The ability of prime females to produce higher energy-rich milk early in lactation may allow them to increase lactation efficiency by reducing lactation duration. A combination of reduced activity and more rapid energy delivery likely explains the increase in lactation efficiency with age.