And we thought we understood the nonapeptides...
The protein sequence and structure of oxytocin is widely thought to be conserved among Eutherian mammals. Curiously, these researchers found a nonsynonymous mutation in a handful of platyrrhines and tree shrews, changing a leucine to proline at position 8 in the signal peptide. Of the noncatarrhines sampled, owl monkeys, capuchins, marmosets, squirrel monkeys and tree shrews had the mutation, while titi monkeys didn't.
I took a quick look at OXT across a handful of mammals after reading this study and found that in that 9 protein sequence, mouse lemurs and tarsiers shared the conserved sequence, suggesting that this was indeed convergent in the platyrrhines and tree shrews. Unfortunately, without sequencing this region in atelines and pitheciines, we don't know if titi's lost the P8 mutation or if it never evolved in that clade.
In a nonprimate note, it seems like elephants have protein changes at positions 3 and 4.
This is an exciting find and it will be very interesting to see what researchers are able to find out about the functional and evolutionary significance of these mutations.