Final answer:
The injections the rats received likely made them more intelligent and longer-lived, which would pose various challenges for Mrs. Frisby and her husband, such as resource competition and potential harm from complex rat societies.
Step-by-step explanation:
Based on the excerpts provided, it appears that the injections the rats received caused an increase in their intelligence and longevity, which could have posed challenges for Mrs. Frisby and her husband. The rats, being smarter and living longer, could outcompete Mrs. Frisby's family for resources, or their enhanced abilities could lead to them inadvertently harming the Frisby family's efforts to survive.
In addition, if these rats were like the ones that received enhanced intelligence in the novel 'Mrs. Frisby and the Rats of NIMH', they might also have developed complex social structures and technology that could clash with the natural lives of the Frisbys.
The concept of enhancing animals through injections is a significant plot element in the novel 'Mrs. Frisby and the Rats of NIMH', where the eponymous rats gain human-like intelligence and longevity after being experimented on by scientists. This makes coexistence between the normal wild animals, like Mrs. Frisby, and the enhanced rats difficult. Their increased capabilities could lead to unintended consequences for the local ecosystem and Mrs. Frisby's family.