ALLOMETRIC GROWTH

Allometry is the study of relative growth, of changes in proportion with increase in size.

Allometric growth ratios for muscle groups of beef.

Albrecht Durer (1471-1528), famous German painter and engraver, used to draw the shapes of animals onto home-made graph paper to analyse their relative proportions. Nowadays, the cartesian coordinates may be transformed mathematically and an anatomical plan stretched to its new shape, thus facilitating the perception of ontogenetic and phylogenetic changes in 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.
Sir John Hammond, one of pioneers in the study of farm animal growth, 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 HAM 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 technique showed the animal shape that would be most likely to yield the greatest mass of edible meat.
Unfortunately, shape then became a more important criterion than yield, despite the fact that many generations of blocky meat animals failed to yield their promised return of lean meat on the butcher's block. The problem was casued by subcutaneous and intermuscular fat.
To estimate meat yield in live animals it is necessary to 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. Unfortunately, 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.
Hammond's concept of the ontogeny of shape in sheep was:
- that a wave of growth starts from the head and passes back along the vertebral column.
- that secondary waves in the limbs pass from distal to proximal,
- that 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,
- tha the difference in shape between the fetus and adult resembles the shape change that occurred in the transformation of wild to domestic farm mammals.

The major changes in both individual development (ONTOGENY) and in evolutionary change (PHYLOGENY) are a reduction in the head relative to the body, and a reduction in the limbs relative to the trunk of the body. In pigs, however, the anterior to posterior wave may or may not be detectable. <

Allometric growth ratio

Sir Julian Huxley in 1932 described a simple mathematical method for the detection and measurement of the allometric growth. In order 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,

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.

This equation has become popular because it appears to work well: scattered data points are pulled into a neat line with a high coefficient of correlation. However, these features do not automatically prove that a logarithmic transformation is necessarily the best or the only way in which to transform a set of data.

Allometric growth is normally regarded in a positive light. Thus, a unit of the body with a positive allometric growth ratio (k > 1) is thought of as exhibiting rapid relative growth. When we attempt to fit the phenomenon of allometric growth into the general framework of our knowledge of sigmoid growth curves, the concept of allometric growth as a positive phenomenon is inappropriate. During the period of growth after birth (and even somewhat before birth) the velocity of growth tends to decline. Thus, those parts of the body with high postnatal allometric growth ratios also may be regarded as retarded parts, relative to those parts that have already decelerated their growth.

Allometric growth of carcass muscles

Allometric growth ratios for muscle groups of the pig.

Allometric growth ratio may be used to categorize muscles into one of three monophasic categories (high, average and low impetus) depending on whether their allometric growth ratio is greater than, equal to, or less than a value of 1.

Many of the postnatal allometric growth ratios of muscles change during development so that a diphasic or dual categorization was needed.

The dominant diphasic types are:

high followed by average,
average followed by high,
and low followed by average.

The growth patterns of certain muscles in the beef carcass also may show an abrupt change in growth rate at birth.

Proximal hindlimb and spinal muscle groups (group 5) are nearly all either monophasic low, monophasic average or biphasic high-average impetus muscles.

The only group 5 muscles that reach a monophasic high level of growth are the tensor fascia lata and the obliquus abdominis internus. A possible explanation is that the abdominal support provided by these two muscles might continue to expand in response to continued visceral growth.

Distal hindlimb muscles (mostly group 3) and distal forelimb muscles (mostly group 2) have either a monophasic low or a biphasic low-average impetus. The requirement for locomotion immediately after birth may be responsible for an early prenatal acceleration of growth in these muscles, so that subsequent postnatal growth is slower than in the remaining more proximal muscles. Since many of the distal limb muscles have a complex pennate structure, constraints resulting from muscle structure might also be involved. Pennate muscles are restricted in their potential for growth, whereas overlying muscles that are composed of relays of intrafascicularly terminating fibers are able to grow radially and longitudinally for a longer period.

The proximal forelimb muscles (mostly group 4 muscles) are mostly either monophasic low or average impetus muscles. Monophasic high and diphasic average-high impetus muscles are in the thorax-forelimb and neck-forelimb groups. This might be a relic of the heavy head and neck muscles that wild bulls once used for fighting. Rams also show considerable growth of their neck muscles. Since many of the ventral neck muscles act as antagonists to the ligamentum nuchae, it is possible that this is an ontogenetic, rather than a phylogenetic feature. In Zebu cattle (Bos indicus) the shoulder hump is composed primarily of the rhomboideus cervicis muscle. The growth of the skull and the ligamentum nuchae are probably interrelated so that the enlargement of ventral neck muscles is required to counterbalance a stronger ligamentum nuchae. The distribution of the carcass muscle mass is changed when steers lose weight on a low plane of nutrition.

Pigs

Pietrain pigs may have greater muscularity in the loin and hindlimb, while Large Whites have relatively greater development of neck, shoulder and forelimb muscles. These two breeds do not differ in bone distribution. In pigs, sex and level of feeding have only a small effect on the proportion and distribution of muscle and bone. Differences in muscle weight distribution under hormonal control might be caused by differences in skeletal growth.

Sheep

Breed differences in muscle weight distribution also may occur in sheep. Ewes may have a lower percentage of shoulder, shank and neck than wethers. However, in sheep, sex differences in muscle weight distribution may be absent in situations where breed differences do exist.

Cattle

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 and the effect is centered on the splenius muscle at the cervical-thoracic junction.

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, growth gradients for intramuscular fat in different muscle groups are similar to those for the muscles.

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.

Poultry

Allometric growth is quite conspicuous in the muscles and bones 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.

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 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.

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, another 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 precisely because they are at the low end of an allometric growth gradient.

Allometry and domestication

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 auroch (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.

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 gray matter, and the parts of the brain that were involved with the senses of smell, sight and hearing showed the greatest reduction, perhaps in relation to a safer environment and controlled feeding. Pigs that have 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.

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 that may have occurred 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.

In wild and domesticated ducks, appropriate differences occur in the pituitary-adrenal system, but the nature of the relationship between domestication and changes in behavior and adrenocortical activity is still open to question.

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.

Another factor that may be involved in the interaction of animal breeding with growth is the general trends towards an inverse relationship between functional maturity and growth rate in muscle. Thus, muscles that must be able to function effectively for locomotion at birth may not be able to grow rapidly, and vice versa. This has been demonstrated in different species of birds and could also be operative in domestic animals. In splayleg piglets, for example, perhaps selection for rapid muscle growth has reduced functional maturity of the muscles at birth.