21 Allometry

21.1 What is allometry?

Allometry is the study and measurement of relative growth. Why is relative growth important in animal agriculture?
  1. Different breeds may have different shapes, and animal shapes are determined by differences in relative growth.
  2. Relative growth of different body tissues (say, muscle, fat and bone) determines the economic yield of a carcass.
  3. Relative growth helps explain the evolution and selective breeding of our meat animals.

21.2 Graphical analysis

In the graphical analysis of animal shape, pioneered by D'Arcy Thompson in 1917, rectangular coordinates were used to analyze and manipulate anatomical plan views. The coordinates were transformed mathematically and the anatomical plan was stretched to its new shape, thus facilitating the perception of ontogenetic and phylogenetic shape. Before the advent of computer graphics, this technique was too difficult for the routine analysis of body shape and, without a computer, the technique is largely subjective, since in most cases it is easier to fit a transformed cartesian grid to a drawing than vice versa. Thus, there is a risk of merely using the cartesian transformation to illustrate a preconceived idea of a shape change, as in the example below.

At the top is a wild boar who used to live in the London Zoo, and underneath is a pig who used to live in Guelph. The deformation of the grid shows how one picture is related to the other. For example, the eye is roughly four squares across and two down. I wonder which pig has more pork chops and ham?

In the 1950s, Sir John Hammond at the University of Cambridge (one of the greatest of all animal scientists) simplified the application of this method of analysis by making photographic prints of animals at magnifications giving a constant skull length. Lateral views of animals at constant skull length were superimposed onto graph paper grids to demonstrate the major changes in animal shape resulting from the domestication and selective breeding of farm animals. Thus, when skull length was held constant photographically, the loins and hams of meat‑type pigs appeared to balloon‑out phlyogenetically from the diminutive rump of the wild boar. The same technique revealed a comparable inflation of the rear end as pigs grew from birth to slaughter weight.

21.3 Growth waves

21.4 Huxley's allometric growth equation

Way back in 1932,  Huxley described a simple mathematical method for the detection and measurement of the allometric growth patterns revealed by D'Arcy Thompson's graphical methods. To compare the relative growth of two components (one of which may be the whole body), they are plotted logarithmically on X and Y axes,

            y = bxk

so that

           log y = log b + k log x


Here are some allometric growth ratios from a study by Berg and Butterfield (1976). Do not bother to learn the numbers - just the pattern of high and low numbers.

The allometric growth ratios in pigs (above, from a study by Davies, 1974) are more favourable. We have made more progress in improving meat yield in pigs than we have with cattle. The ratios are high for the loin and ham, and low for the less valuable cuts.  The high ratio in the belly is good news for bacon producers.


   In summary, the genetic inflexibility of the allometric growth patterns of beef muscles discourages any attempt to change the distribution of muscles in the carcass. In cattle, the opposite tactic is more attractive, namely, selection against fatness throughout the carcass. In poultry, large allometric growth gradients extend along the breast‑leg axis, but we are not yet sure how they will respond to genetic selection. Perhaps leg muscle weights have a lower heritability than breast muscle weights because they are at the low end of an allometric growth gradient?

21.5 Allometry and domestication

Further information

Structure and Development of Meat Animals and Poultry. Pages 455-463.