There is no definitive answer to this question as it depends on a number of factors, including the individual's body size and the definition of "smallest brain." However, based on available research, some of the candidates for smallest brain might include certain species of fish, reptiles, amphibians, and invertebrates.
One way to determine which animals have the smallest brains is to look at their brain-to-body size ratio. This metric can be misleading, however, as it does not take into account the different proportions of white matter and grey matter in different brains. For example, a human brain has a much higher proportion of white matter than a mouse brain (which is mostly grey matter), but we would not say that humans have smaller brains than mice.
Another way to compare brain sizes is by looking at absolute values or weight. Again using humans as an example, our average adult male has a brain that weighs about 1.4 kg (just over 3 pounds), while an average adult female has abrain that weighs about 1.2 kg (about 2.6 pounds). But there are many other animals with much smaller brains than either sex of human; for example:
A common goldfish has a brain weighing only 0.01 grams â€“ less than 0% the weight of a humanâ€™s
A hummingbirdâ€™s tinybrain weighs just 0.02 grams â€“ less than 2% the weightof ours
The largest recorded specimenof African elephant hada massivebrain weighing in at 9 kilogramsâ€“ still only about 650%the sizeof ours