Milk Energy Output

Although a general relationship exists between peak milk yield (or milk energy output) and maternal metabolic body size (body mass075),[5,6] there are large differences in peak production among species independent of metabolic body size. In terrestrial mammals, species with large litters, such as rats, dogs, and pigs, have peak milk energy outputs per maternal metabolic size that are two to three times those of ungulates with one young (Table 1). The peak energy outputs of most primates are even lower. The highest daily outputs of milk energy are found among seals. The hooded seal appears to be the mammalian champion: Its daily milk energy output is four times that of a pig, 11 times that of a horse, and 27 times that of a human, relative to metabolic size.

Among terrestrial mammals, milk energy outputs are higher for species with large litters, reflecting the greater energy demands of additional offspring. This effect can be accounted for by expressing milk energy output relative to litter metabolic mass (litter size x offspring mass0 83).[6] Among terrestrial mammals, milk energy output expressed in this way varies from about 0.71 to 1.11 (Table 1). However, much higher values have been observed in seals with very short lactations.

Mammals with long lactations tend to have higher total energy outputs over the lactation period, presumably because they supply the maintenance requirements of suckling young for a longer time. Some species, such as seals and dogs, opt for a short, intensive lactation with high peak energetic costs, while others, such as horses and

Table 1 Milk and energy outputs at midlactation in terrestrial mammals

Energy

EO per

EO per

Maternal

Milk

output,

maternal

litter

Litter

Lactation

mass

Peak

yield

EO

MBSa

MBSa

Species

size

length

(kg)

(d after birth)

N

(kg/d)

(MJ/d)

(MJ/kg075/d)

(MJ/kg083/d)

Human

1

6 36 mo

57

60 120

5

1.05

3.03

0.15

0.72

Baboon

1

12 18 mo

16.7

120

48

0.40

1.34

0.16

0.85

Reindeer

1

5 mo

107

21 28

3

1.59

11.05

0.33

1.19

Horse

1

11 12 mo

515

39

5

17.6

37.21

0.35

0.88

Dorcas gazelle

1

3 mo

20.6

30 60

4

0.56

3.59

0.37

0.88

Black tailed deer

2

3 4 mo

49.8

28

6

1.26

9.28

0.49

0.98

Sheep

2

4 6 mo

52.6

17

5

2.47

11.47

0.59

1.05

Mink

5

8 10 wk

0.96

20

6

0.12

0.59

0.60

0.89

Striped skunk

5

8 16 wk

2.22

31

5

0.15

1.24

0.69

1.05

Greater spear nosed

1

11 wk

0.077

30 45

13

0.015

0.11

0.76

1.00

batb

Rabbit

7

4 wk

4.40

21

5

0.27

2.32

0.77

1.01

Dog

5

6 8 wk

12.7

26

5

1.05

6.45

0.96

1.11

Brown rat

8

4 wk

0.20

14

?

0.041

0.24

1.01

0.89

Pig

9

3 4 mo

120

24

44

7.16

37.34

1.03

0.88

aMBS metabolic body size (mass ' in adults; mass0 83 in suckling young). (From: Refs. 6 and 7.) bSpecies binomial: Phyllostomus hastatus.

aMBS metabolic body size (mass ' in adults; mass0 83 in suckling young). (From: Refs. 6 and 7.) bSpecies binomial: Phyllostomus hastatus.

humans, have evolved long, extensive lactations with lower peaks but large overall energy costs.

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