While larger animals live longer they do things at a slower rate. So one kilogram of tissue in a cow uses less energy a day than one kilogram of tissue in a sheep all other things being equal. The cow eats more overall a day because it is bigger but less per kilogram of body weight. Detailed experiments made many years ago proved this and showed that energy requirements of animals scale (are proportional) to adult body weight (A) to the power of 0.73 (∝ A0.73).
The length of events scale to 0.27 (∝ A0.73) – therefore total energy used by one kilogram of tissue over the animal’s life scales to rate per day (A0.73) times length of life (A0.27). We do the algebra and find that A0.73 * A0.27 = A0.73+0.27 = A1.00. In other words, the total energy an animal uses in its life is directly proportional to its size. Or, one kilogram of tissue uses the same amount of energy over the lifespan of an animal whatever the size of the animal is. But since it lives longer a larger animal uses less energy per kilogram per day.
Associated with a lower rate of energy use in larger animals, events occur less frequently. A heart beat is an event so larger animals have slower heart rates. This is entirely consistent with using energy at a slower rate per kilogram of tissue. But since they have longer lives, larger animals are predicted to have the same number of heart beats in their lifetime as smaller animals. Some estimates have this as 1.5 billion heart beats per lifetime.
So how does the algebra shown cope with this? If animals have a constant number of events per lifetime not affected by body size, this scales to A0.00 (any number raised to the power of zero equals 1). Since lifespan scales to A0.27, rates of events is a function of number of events per lifetime (∝ A0.00) divided by lifespan (∝ A0.27) so event rates are proportional to A0.00 ÷ A0.27, which simplifies as A0.00-0.27 and then to A-0.27.
You can work out the algebra yourself to show that larger animals have a slower heart rate than smaller animals – see Table 1.For example, sheep are predicted to have 10.1 months between first giving birth and end of the associated lactation (20.4-10.3) while for cattle this is 15.8 months (35.3-17.7). We are fighting against biology trying to shoehorn cattle into a repeated 365 day cycle for pregnancy and lactation.
Cattle have a cycle inherently longer than the 365 day “Earth year” dictated by motion of the Earth around the sun, while sheep can easily fit within our “Earth year” provided we can feed them what they require.
In summary, size matters.
You can give B+LNZ Genetics or SIL your thoughts on this topic by emailing: silhelp@sil.co.nz or by leaving a message on 0800-silhelp (0800 745 435).
- Mark Young is senior geneticist with B+LNZ Genetics and SIL.
