Why do some animals like swans mate for life but others don’t? Do they form attachments like humans?

Question: Why do some animals like swans mate for life but others don't? Do they form attachments like humans?
- Keywords:
  attachment,
  mate,
  monogamy,
  swan
Asked by alexrive to Charlotte, Jo, Kevin, Louise, Valeria on 18 Jun 2012.
- Joanna Cruden answered on 18 Jun 2012:
  
  They just form a very deep bond, but I don’t think we really know why some species do and some don’t, I could not find any information on why this is so. Male swans will find another mate if the female does not produce viable eggs though!

Question: Why do some animals like swans mate for life but others don't? Do they form attachments like humans?