Q.Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?

The kind of nitrogenous waste an animal makes depends largely on how much water is available to it.

• Ammonia is highly toxic and very soluble, so it can only be got rid of safely if it is diluted in and washed out with large amounts of water.
• Land animals cannot afford to spend so much water, since they must conserve it.
• So instead of excreting ammonia they convert it into urea (ureotelism) or uric acid (uricotelism), both of which are far less toxic and need much less water to be removed. …

The form in which an animal disposes of its nitrogen is closely tied to its habitat, and especially to how freely water is available. Three main nitrogenous wastes exist — ammonia, urea and uric acid — and they differ sharply in toxicity and in how much water is needed to remove them.

Ammonia is the most toxic of the three and is highly soluble in water. An animal that excretes ammonia therefore has to keep it very dilute and flush it out continuously with plenty of water. This is workable for animals that live in or around water, but it is a serious problem on land.

Terrestrial animals face a constant pressure to conserve water. Spending large volumes of water simply to carry away ammonia would quickly lead to dehydration. Their solution is to change the chemical form of the waste before excreting it:

• Ureotelism — ammonia is converted into urea, which is much less toxic and can be excreted with a moderate amount of water. …

Method: Answering a "Why" Comparative-Adaptation Question

Questions that ask WHY an organism uses one strategy instead of another (here, why land animals avoid ammonotelism) are answered by comparing the same two variables across every option, then applying the organism's specific constraint to pick the winner — never by describing each option in isolation.

Steps

Step 1: Identify the two competing costs behind every option

Before comparing the specific strategies, name the two properties that differ between them — here, the toxicity of the waste, and the amount of water needed to excrete it safely. Almost every excretion-strategy "why" question in this chapter reduces to this same toxicity-versus-water trade-off.

Step 2: Rank all the options on both variables

Lay out ammonia, urea and uric acid against both variables: ammonia is the most toxic and needs the most water; urea is intermediate on both; uric acid is the least toxic in its stored form and needs the least water.

Step 3: Identify the organism's specific constraint

State the one environmental fact that decides which trade-off the organism can afford — here, that terrestrial life means water is scarce and must be conserved, unlike an aquatic habitat where water is freely available for diluting a toxic waste. …