Which animal, including humans, has the longest memory span?

Question: Which animal, including humans, has the longest memory span?
- Keywords:
  human,
  memory,
  neuroscience
Asked by to Edward, Ian, Mathew, Naomi, sakshisharda on 16 Jun 2014. This question was also asked by .
- Naomi Osborne answered on 16 Jun 2014:
  
  Dolphins! They can remember fellow dolphins after 20 years apart by remembering their whistles. Elephants and chimpanzees have similar abilities but haven’t yet been tested.

Question: Which animal, including humans, has the longest memory span?