The Elements of Evolution (51-1-1) Allometry


Body size plays a crucial role affecting generation times, energy demands, and population sizes. ~ American biologists Lauren Sallan & Andrew Galimberti

Allometry is an umbrella term for the relations between body size to shape, anatomy, physiology, and behavior. The term is also used to refer to the study of such relationships.

In insects, an incremental increase in body size can result in disproportionate enlargement of appendages: legs, antennae, and, in some beetles, horns.

Allometric relationships are often expressed as a power law. Allometric scaling deviates from the isometric, which is a 1-to-1 relationship.

Allometric power laws are often strongly conserved across evolutionary time, invariant across taxa, and have long been hypothesized to reflect developmental constraints. ~ Norwegian evolutionary biologist Geir Bolstad et al

In flies, small wings are typically rounder than large wings. Researchers bred fruit flies to violate that allometry. Once they stopped, the natural allometric relationship of wing roundness rapidly returned (in 15 generations).

Allometric scaling laws often express dynamics related to ontogeny (developmental growth). In an organism that grows as it matures, size increases but bodily shape remains similar; still there can be allometric changes during development. Lizards often exhibit such changes.

Based upon study of a variety of species, Swiss agricultural biologist Max Kleiber concluded in 1932 that animal metabolic rate is a 0.75 power of body weight. This allometric law is a consequence of the physics and geometry of animal circulatory systems. Allometric relations suggest that Nature follows laws for biology, just as in physics.

In the same species, smaller youngsters respire more per weight unit than larger oldsters because growth carries an overhead. By contrast, small adults of a species breath more per unit of weight than larger ones of another species because a larger proportion of their body weight is of structure rather than reserve. Structural mass has maintenance costs, whereas reserve mass does not.

To nourish itself, an animal needs energy. Employing that energy generates heat. An animal must rid itself of excess body heat. The obvious way is surface cooling.

As vertebrates get larger, they have relatively less surface area to dissipate heat. So, to be able to rid excess heat, metabolism must increase at a slower rate than body volume enlarges. According to Kleiber’s law, the relation between mass and metabolic rate is fixed. Something seems amiss. As it turns out, the missing element leads to another power law.

As vertebrates get bigger, the speed at which nutrients are carried through the body and heat is carried away increases to ensure heat disposal. The velocity of blood flow to an animal’s mass is a 1/12th (0.0833…) power.

Animals need to adjust the flow of nutrients and heat as their mass changes to maintain the greatest possible energy efficiency. That is why animals need a pump – a heart – and trees do not. ~ Italian hydrologist Andrea Rinaldo

Allometric scaling applies from the genome on out. In vertebrates, there is a correlation between the sizes of genomes and cells, especially red blood cells, and metabolic rate.

Having a petite genome affords smaller cells. Smaller cells support a larger surface-to-volume ratio, which makes for more efficient gas exchange.

Flying is energetically expensive. Birds and bats that take to the air must be as metabolically efficient as possible. Among birds, the strongest fliers have the smallest genomes, while flightless birds possess some of the largest.

Hummingbirds approach the theoretical limit for aerobic metabolism. Their genome weighs an average of 1.03 picograms (pg).

An average bird genome weighs 1.42 pg, reptiles 2.24 pg, and humans 3.5 pg. Some salamanders lug around 100 picogram genomes. Slimming down the avian genome began with the theropod dinosaurs from which birds descended.

Adaptive or environmental demands cause body sizes to diminish or enlarge. Body size is a resultant trait, not a driver. Size is determined by way of functional or developmental association, which ultimately relates to risk-based efficiency; with risk expressed as an extent of adaptability to environmental fluctuation. Depending upon ecological interaction, risk-based efficiency favors a certain size.

Allometry illustrates that life forms exist within physics-based mathematical constraints, via intricate component relationships.