06 December 2010

Essential requirements to be alive

A recent paper* of mine opened with this sentence: "During their daily survival, terrestrial gastropods have two primary requirements they need to fulfill: the intake of adequate food and the maintenance of proper hydration."

In the context of a paper on natural selection that I was reading today, I started thinking about this again. Then I came up with following list of the essential requirements in the life of an animal. They are in no particular order.

1. Survival of biological dangers (predators, parasites, bacteria, viruses, etc.).
2. Survival of physical dangers (temperature extremes, dehydration, loss of habitat, etc.).
3. Procurement of enough food to supply energy and raw materials for growth and maintenance and of oxygen to maintain an adequate level of metabolism.
4. Reproduction.

Have I forgotten anything?

Of course, none of these processes are independent of each other. For example, reproduction also requires energy, while the act of searching for and consuming food exposes an animal to biological and physical dangers. Even the food consumed could be, and often is, a source of parasites. This interdepency was something I emphasized briefly in my paper: "Food intake requires that the animals be active, but activity results in water loss as evaporation from the body surfaces and as slime left behind. The surface a gastropod is on must be sufficiently wet, the air humidity adequately high, and its food of a high enough water content so that daily activity doesn’t result in a negative water balance."

The requirement to obtain enough oxygen doesn't seem to be a major concern for terrestrial animals. However, it is indeed a critical requirement for aquatic animals.

Because I work with animals, I normally think in terms of animals. But these requirements are also essential and sufficient for all other forms of life. The requirement #3 must, however, be modified to suit the idiosyncrasies of particular organisms. For example, photosynthetic plants synthesize their own "foods" from sunlight.

Natural selection operates during the fulfillment of each and every one of the 4 requirements.

*Örstan, A. 2010. Activities of four species of land snails at low temperatures. Journal of Conchology 40:245-246. See this post for more on this paper.


Coyote said...

What about the excretion of metabolic wastes? All life forms take in nutrients, and excreting the resulting wastes is necessary to maintain health.


Excretion is part of metabolism.

coturnix said...

Physiologists of old teach this as the Three Fs:
- 1.Feeding (which includes all the ins and outs of energy and metabolism)
- 2.Fighting (which includes everything regarding survival)
- 3.F... Reproduction (obvious)