Speaking from experience irl, different dogs and cats demonstrate various personalities and degrees of intelligence. I've had little difficulty understanding what they want from me at any given time as their body language generally isn't too hard to read in most situations (generally speaking, cats tend to try and hide when they are hurt/ill, while dogs are big suck ups when they don't feel well), while they seem to also understand me to some extent.
As such, I'd propose the following explanation for what we observe in game:
1. We can communicate with animals by interpreting their body language and vocalizations as a form of language, while portraying our own words/intentions by the same means. This ability roughly corresponds to Summoning skill level, with the Magic of Summoning allowing this method of conversation to communicate more complex ideas than would normally be possible. Plants are also capable of some rudimentary communication between individuals (individuals near others that have been damaged by herbivores will ramp up their mechanisms for defense), so the cabbage speak amulet could just as well let us perceive and interpret these signals (with magic again allowing more complex communications via this method).
2. Some animals in RS possess a higher degree of intelligence than they do irl. This could be attributed to some special part of their environment (ex: animals on Zanaris), some natural part of their species in RS (ex: penguins), or unique instances for individual animals within a species.
3. Some cases where we observe intelligence could be us interpreting their actions and vocalizations as a false positive, or us just not being in our right minds at the time and hallucinating those actions entirely.
And finally:
4. Some instances are Jagex's humor and references.
25-Mar-2018 11:36:21
