noun (plural of singular) of plural noun

Question

I am often confused whether to use singular or plural noun. Below I list some examples that I am not sure which one is correct...

EX1

• I measured the head width of lizards.
• I measured the head widths of lizards.

EX2

• The growth rate of birds is influenced by temperature.
• The growth rates of birds are influenced by temperature.

EX3

• The mean heights of this population and that population are different.
• The mean height of this population and that population is different.

EX4: In this example, because (like in other examples) each dog has a nose; dogs [as a group] have (multiple) noses. I want to use "Dogs have noses" but feel it is wrong.

• Dogs have a nose.
• Dogs have noses.

I hope these examples can give some idea what kind of confusions I am having... I want to know some rules that help me identify the correct form.

Answer 1

This is a very good question. In general, you can help yourself work through any confusion by trying to express the idea more precisely or in a slightly different way. I'll use each of your examples to explain this a bit further.

EX1: All lizards have a head width, but that isn't what you measured. You measured the different head widths of a group of lizards, or "the head widths of some lizards." I would at least add the "some", but perhaps it would be better to include the actual number.

EX2: You are talking about the growth rate of all birds, not, say, the growth rates of different species of birds.

EX3: Each of two populations has a mean height, therefore there are two mean heights. You are saying that the mean heights of these two populations are different.

EX4: Dogs have noses is correct, but this is a bit inconsistent. For example, dogs have a sixth sense that lets them know when someone is coming, and they have a nose for trouble. People have minds, but they have a combative streak. So, when you are talking about a shared characteristic of all dogs, you use the singular, but when you are talking about something that each dog has, you use the plural.