The scientists noted that rats suffering from the rare degenerative di

In the first example below, the ‘they’ is clearly ambiguous because it can refer to either the control group mice or those who receive Xylanol.

In the second sentence, the ‘they’ is apparently ambiguous. However were we to say that the ‘they’ could refer to mice, we are saying that it is possible that mice could dress up in laboratory gear. This is clearly absurd because outside of Disney mice do not wear clothes. Thus a pronoun is not ambiguous if can reasonably only refer to one thing/group.


Clearly Ambiguous

Compared to mice in a control group, those mice receiving Xylanol were more likely to develop tumors, even if they had tumors before the study.


Not Ambiguous but Absurd

Scientists who test potentially carcinogenic substances on mice may be at a slight risk for cancer, even if they wear special protective clothing.


Original Sentence

The scientists noted that rats suffering from the rare degenerative disease had begun to die six months earlier, even though they had shown no signs of the disease then.


As for the original sentence, the potential ambiguity is not absurd the way it is in the second example above. After all, scientists could potentially show signs of a degenerative disease (vs. mice donning lab gear). Nonetheless, it is clear from the sentence that the scientists are studying the rats for a disease. Therefore, when the second half of the sentence mention ‘they’ in reference to the disease, it is clear who the ‘they’ is.

Now let’s contrast the original sentence to the first one above. The ‘they’ could refer to either of the two groups, i.e. either the control group of the Xylanol group could have consisted of mice that had tumors before the study began. However, in the original sentence the scientists were the ones studying a degenerative disease they were not the ones suffering from it.