21 Allometry

21.1 What is allometry?

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.

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.

• At birth, meat animals tend to have large heads and long slender limbs.
  
• Subsequent growth is marked by an increase in body length and depth until, finally, the hams fill out convexly.
  
• Hammond's method showed the animal shape  most likely to yield the greatest mass of edible meat.
  
• BUT, unfortunately, shape then became a more important criterion than yield, despite the fact  many generations of blocky meat animals failed to yield their promised return of lean meat on the butcher's block.
  
• The problem was subcutaneous and intermuscular fat. To estimate meat yield in live animals we must visualize the muscle mass beneath the animal's outward shape. Body regions where subcutaneous fat is scarce, and where muscle mass may be judged by the stance between the limbs are, therefore, particularly important in the judgement of live meat animals.
  
• As cattle grow fatter, fat deposition tends to shift from relatively inexpensive cuts in the ventral part of the carcass to the more valuable cuts of the dorsal part of the carcass. This makes it progressively more difficult to judge the amount of muscle in the high-priced cuts.
  
•  Thus, animal scientists now look closely at the YIELD of meat we can sell from an animal - more or less regardless of its shape. Shape is still very important in traditional animal judging - but this is more of a beauty contest than a scientific method for improving animals. But, there's nothing wrong with beauty contests.  Who wants to look out the window at ugly animals?

21.3 Growth waves

• Hammond's concept of the ontogeny of shape in sheep was a wave of growth starts from the head and passes back along the vertebral column (anterior to posterior wave).
  
• Secondary waves in the limbs pass from distal to proximal.
  
• The union of the anterior to posterior wave in the vertebral column, with the distal to proximal wave in the hindlimb results in the late, but extensive growth of the pelvic region.
  
• The major changes in both ontogeny and phylogeny are a reduction in the head relative to the body, and a reduction in the limbs relative to the trunk of the body.

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 = bx^k

so that

           log y = log b + k log x

• The slope of the resulting regression is called the allometric growth ratio, often designated as k.
  
• With k = 1, both components are growing at the same rate.
  
• With k < 1, the component represented on the Y axis is growing more slowly than the component on the X axis.
  
• With k > 1, the Y axis component is growing faster than the X axis component.

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 bad news is all those valuable steaks along the animal's back only grow a little faster (k = 1.02) than the rest of the carcass.
• Throat muscles are growing much faster than the carcass, probably because they act against the ligamentum nuchae which is supporting the weight of the head. Remember, beef animals once used their horns on the skull as an offensive weapon!
• More bad news, the abdominal muscles are also growing very fast because they support the weight of the viscera plus the FAT they are developing.
• Low allometric growth ratios in the distal limbs.  This is good news, because this is low grade meat for stewing.
  

• In cattle, muscle distribution is influenced more by sex than by breed. 
  
• Proximal hindlimb and abdominal muscles are heavier in heifers than in steers, and heavier in steers than in bulls. The order is reversed for muscles of the neck and thorax.
  
• In cattle, castration causes a marked decrease (up to 55%) in the growth of shoulder muscles.
• Many carcass muscles may acquire appreciable amounts of intramuscular fat in older animals. This cannot be removed by dissection and is, therefore, included in the muscle weight. Fortunately for carcass dissectors, growth gradients for intramuscular fat in different muscle groups are similar to those for the muscle.
  
• When expressed on the same basis (weight of muscle plus bone), heifers are generally fatter than steers, and steers are fatter than bulls. These differences are related to the time of onset of fat deposition.
  
• Since the anatomical distribution of muscle mass in different breeds of cattle is fairly constant, the genetic reduction of fat content probably provides the best means of selecting for an increased proportion of lean meat.

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.

• Different species of poultry show quite marked differences in their overall growth: ducks and geese grow rapidly at first, but only geese keep up the pace, whereas turkeys and chickens tend to have a slow start followed by a long period of a sustained growth.
  
• Thus, allometric growth is quite conspicuous in the muscles and bones of different species of poultry, although not along the vertebral column as in mammals.
•  In ducks, the leg muscles are well developed at an early age, while the growth of the breast muscles is quite late.
  
• The same overall pattern of growth occurs in chickens  and in turkeys , but to a less extreme degree.
  
• This pattern of muscle development is an obvious advantage to an animal that, in the wild state, would depend on its legs for early locomotion and only later would learn to fly.
  
• Poultry exhibit considerable variability in their body proportions between breeds.  Broad Breasted Bronze turkeys, for example, may have shorter legs than other smaller strains.
  
• At present, it is doubtful whether differences in conformation have any relation to the relative distribution of the muscle mass and differences in nutrition do not appear to affect the distribution of the muscle mass in poultry.
<>In chickens and turkeys, the early deposition of subcutaneous fat is a desireable trait, but in ducks and geese the reverse is true.
Genetic selection for live weight gain in poultry is difficult to uncouple from increased fatness .
 The heritabilities of the weight and the proportion of the breast muscles are relatively high in ducks. But, because of the marked allometry of muscle growth in ducks, selection for gain in the breast muscles may not improve the meat yield of the leg.

21.5 Allometry and domestication

•  CATTLE. The domestication of farm animals several thousand years ago was accompanied by allometric changes in body shape and by an overall decrease in body size, but the archeological record of animal domestication is difficult to interpret because traces of different species may overlap. In cattle, for example, the bones of the ancestral wild aurochs (Bos primigenius) may give way to those of the extinct shorthorn of the Iron Age (Bos longifrons) and finally to the ancestors of our present species (Bos taurus). The bones of B. longifrons, however, might simply have been those of B. primigenius cows. The frequency of occurrence of bones may reflect the balance of hunting, containment and progressive breeding at any point in time. The identification of individual bones is confounded by the non‑random nature of specimens left by hunters, and by technological practices such as castration and the development of herds of cows. Selection may have encouraged the development of paedomorphic forms with persistent juvenile characteristics, such as pliable behavior, as well as neotenic forms with precocious reproductive development. Superimposed on these genetic changes there may have been changes in body structure associated with the new environment created by animal husbandry.
• BRAIN. Domestication was associated with considerable reductions in the weight of the brain; a 24% reduction in sheep , a 33% reduction in pigs and a 16% reduction in horses. The diminution of brain size relative to body weight was accompanied by allometric changes between parts of the brain. In sheep, the cortical reduction of white matter exceeded that of the grey matter, and the parts of the brain involved with the senses of smell, sight and hearing showed the greatest reduction, perhaps in relation to a safer environment and controlled feeding. Pigs reverted to a wild state do not restore their original brain size, although allometric changes may occur: feral pigs have a smaller cerebellum and a larger medulla oblongata.
• ENDOCRINE GLANDS. Domestication is accompanied by allometric changes in the endocrine glands. The relatively recent derivation of laboratory rats from wild Norway rats (Mus norvegicus) provides an interesting model of events during the domestication of farm animals. The dominant changes in rats were a reduction in the adrenal cortex and an increase in the activity of the gonads.
  
• DUCKS. In wild and domesticated ducks, changes occur in the pituitary‑adrenal system.,
• SHEEP provide a striking example of phenotypic changes that may be produced by what Charles Darwin called unconscious selection. These changes include: (1) the ratio of wool to hair, (2) an increase in tail length or adiposity, (3) development of the lop‑eared condition, (4) increased convexity of the nose due to a decrease in jaw length, and (5) a reduction in the number and complexity of horns.

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