It’s unknown whether Kipenzi rejected Kaius due to the stress of her illness following his delivery; or because of her uncertainty around maternal care. In the case of the latter, it’d be advantageous to discover that now rather than cease breeding for a decade and have another Kaius raised in the absence of peers down the line.
I've rather presumed her rejection was due to the shock/stress of her illness, though we don't know for sure. I think in the early days he showed some video of Kaius 'meeting' the other gorillas and there is one of them up close to the grille looking interested in him- I presume that was his mother Kipenzi but not sure.... I think a zoo more experienced with fostering might have attempted it then. The problem now is that Kiaus will be unfamiliar to the group and Kisane might reject him- or worse- if they tried it now.
I forgot to mention that some years ago Artis(Amsterdam) introduced a rejected female baby from Hanover- one of Buzandi's daughters- in the hope a female would adopt her. None did but despite that Yanga grew up in the troop and is still living in it now. The main difference there was it was a group situation with at least one similar-aged baby she did seem to bond and have physical contact with. I guess Yanga will stay in their group permanently now. The Kaius/Ganne situation is of course a 1.1. situation without that option available.