This is only a small point of reference, but my experience with rats has shown that females benefit from being spayed as it greatly reduces their chances of developing mammary tumours in later life. Of my 13 rescue rats (only 5 of whom are still with me) I've had a total of 11 operations, all to remove tumours or reoccurring lumps that have formed. Of those 11, 1 has died from lumps that were too attached to be removed, and I have a second that is unfortunately in the same position and has been given days to live.
I had one of my girls spayed a week ago because she was passing blood from down below. That operation actually saved her life. The vet showed me what he took out - one of my girl's ovaries was full of pus and there was a tumour the size of a golf ball attached to her uterus. My girl should hopefully have another year of life to live, and it's incredible how she is no longer this sick little girl who sleeps all day and looks fed up. She has so much energy now and it's obvious her life has improved.
So even if my experience of the rats is all I can go by, this is enough for me to recognise the benefits of spaying.
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