227
Chapter ten: Post-partum
pinnipeds have some of the highest circulating levels of fatty acids found in nature.
By the end of lactation, northern elephant seals have plasma fatty acid levels that average
over 3 mM, a concentration that would be considered harmful (dyslipidemia) in humans
(Crocker et al. 2014a). These high levels of fatty acids not only support maternal maintenance energy needs, but also facilitate rapid uptake and use by the mammary gland. The
mechanisms that allow such rapid mobilization of stored fats have begun to be elucidated
in seals and appear to vary widely, even between species with similar life-history patterns.
This suggests that lipid mobilization during fasting is under strong evolutionary selection
pressure in seals.
The primary endocrine features associated with lactation in seals are a profound reduction in the hormone insulin and elevation in the hormone cortisol. While insulin’s primary
role is the regulation of carbohydrate metabolism, it also exerts strong anti- lipolytic effects.
This means that insulin promotes the production of new fats and inhibits the mobilization of stored fats. Low insulin levels appear critical to fat mobilization in seals, resulting
in a decline in insulin release over lactation when compared to other life-history stages
(Fowler et al. 2008). Correlative analysis suggests that low insulin levels are the primary
determinant of fatty acid mobilization during lactation in some species. The hormone cortisol helps mobilize fat and protein in most species of animals and usually also leads to
increased use of mobilized proteins for energy metabolism and for making sugars, features that might lead to vital organ damage during extended fasting. Several species of
phocids and otariids elevate plasma cortisol concentrations over their lactation periods.
This elevation is strongly associated with both circulating fatty acid levels and milk fat
Table 10.2 Lactation variables for selected pinnipeds
Lactation
duration
Maternal
foraging
Milk composition
Pup
growth
Species
Days
% Time at sea
% Fat
% protein
kg/day
Bearded seal
24
84
47
10
3.3
Crabeater seal
17
51
11
4.2
Hooded seal
4
0
59
5
7.0
Harbor seal
25
55
50
9
0.6
Harp seal
12
71
50
8
2.2
Gray seal
16
0
54
7
2.5
N. elephant seal
26
0
55
11
4.0
Ringed seal
39
69
38
10
0.4
S. elephant seal
23
0
40
10
3.4
Weddell seal
50
40
48
8
2.0
Australian sea lion
532
60
31
10
0.11
Ca. sea lion
332
73
36
9
0.12
S.A. sea lion
548
59
32
10
0.21
Stellar sea lion
332
76
22
9
0.35
Antarctic fur seal
117
71
41
10
0.08
Galápagos fur seal
540
50
29
12
0.07
Subantarctic fur seal
300
80
43
12
0.04
N. fur seal
118
73
42
14
0.07
Walrus
730
na
26
8
0.45
Source: Data from Schulz and Bowen (2004).
Chapter ten: Post-partum
pinnipeds have some of the highest circulating levels of fatty acids found in nature.
By the end of lactation, northern elephant seals have plasma fatty acid levels that average
over 3 mM, a concentration that would be considered harmful (dyslipidemia) in humans
(Crocker et al. 2014a). These high levels of fatty acids not only support maternal maintenance energy needs, but also facilitate rapid uptake and use by the mammary gland. The
mechanisms that allow such rapid mobilization of stored fats have begun to be elucidated
in seals and appear to vary widely, even between species with similar life-history patterns.
This suggests that lipid mobilization during fasting is under strong evolutionary selection
pressure in seals.
The primary endocrine features associated with lactation in seals are a profound reduction in the hormone insulin and elevation in the hormone cortisol. While insulin’s primary
role is the regulation of carbohydrate metabolism, it also exerts strong anti- lipolytic effects.
This means that insulin promotes the production of new fats and inhibits the mobilization of stored fats. Low insulin levels appear critical to fat mobilization in seals, resulting
in a decline in insulin release over lactation when compared to other life-history stages
(Fowler et al. 2008). Correlative analysis suggests that low insulin levels are the primary
determinant of fatty acid mobilization during lactation in some species. The hormone cortisol helps mobilize fat and protein in most species of animals and usually also leads to
increased use of mobilized proteins for energy metabolism and for making sugars, features that might lead to vital organ damage during extended fasting. Several species of
phocids and otariids elevate plasma cortisol concentrations over their lactation periods.
This elevation is strongly associated with both circulating fatty acid levels and milk fat
Table 10.2 Lactation variables for selected pinnipeds
Lactation
duration
Maternal
foraging
Milk composition
Pup
growth
Species
Days
% Time at sea
% Fat
% protein
kg/day
Bearded seal
24
84
47
10
3.3
Crabeater seal
17
51
11
4.2
Hooded seal
4
0
59
5
7.0
Harbor seal
25
55
50
9
0.6
Harp seal
12
71
50
8
2.2
Gray seal
16
0
54
7
2.5
N. elephant seal
26
0
55
11
4.0
Ringed seal
39
69
38
10
0.4
S. elephant seal
23
0
40
10
3.4
Weddell seal
50
40
48
8
2.0
Australian sea lion
532
60
31
10
0.11
Ca. sea lion
332
73
36
9
0.12
S.A. sea lion
548
59
32
10
0.21
Stellar sea lion
332
76
22
9
0.35
Antarctic fur seal
117
71
41
10
0.08
Galápagos fur seal
540
50
29
12
0.07
Subantarctic fur seal
300
80
43
12
0.04
N. fur seal
118
73
42
14
0.07
Walrus
730
na
26
8
0.45
Source: Data from Schulz and Bowen (2004).