Main comments:
In this manuscript, Danaher-Garcia et al. documented a long-term partial fusion of two spotted dolphin groups that were originally ranging 160 km apart based on repeated peaceful associations between subgroups of immigrant and resident dolphins. Using comprehensive data collected from sightings and encounters in 2014-2018, the authors showed that the resident BIM dolphins and the immigrated WSR dolphins did not associate as separate communities. Also, some of the mixed-group associations were as strong as those within the resident group, with immigrant and resident individuals initiating affiliative contacts with each other.
While the authors showed that there is no clear cluster of groups based on the data collected during mixed-group sightings, only half of the immigrated dolphins associated with the resident dolphins (L263). I wonder if the immigrant dolphins still on average have much stronger associations with each other than with any of the residents, suggesting that WSR and BIM may still be two separate groups, or at best in a transition phase, undergoing a long-term group fusion. The authors also referred to some of the associations between immigrant and resident dolphins as stable “social bonds,” without really demonstrating that dyadic relationships remained consistently strong over the years. Perhaps the authors can plot the sociograms separately for the different years to better illustrate the strength and stability of specific mixed-group relationships.
The authors argue that the partial group fusion was driven by mating benefits. Have the authors observed any mating events during dolphin encounters? Given that the strongest mixed-group associations were among males rather than between males and females, and that most contacts occurred in same-sex dyads (L359), I wonder if other factors such as interspecific competition with sympatric bottlenose dolphins may be a stronger driver of the integration of WSR and BIM. Also, do the authors have information about kin relationships within and between the study groups? This will be relevant information when discussing the formation of affiliative relationships between groups.

Minor comments:
Title: Authors may want to avoid using the term “norm” in the title or elsewhere in the manuscript as it refers to something else in cultural evolution. Based on the findings presented in the manuscript, I suggest changing the title to something like “Potential integration of immigrant and resident dolphin groups”
L94-96: Please provide information about the dispersal patterns of spotted dolphins here. Also, is there information about the average home ranges of WSR and BIM prior the immigration event?
L116-124: The reference to primates seems a bit out of place here as you are introducing your study species. Please consider moving it to the discussion or elsewhere in the introduction.
L135-136: Replace this sentence with hypotheses and predictions.
L234-235: Why don’t the authors add an autocorrelation term in the model to account for same-day observations and include all contacts when assessing dyadic relationships between groups? Contacts after the first contact on a given day are still meaningful to assess relationship quality.
L380, 385: What does “fused only” mean?
L449-450: Any observations of mating between WSR males and BIM females?
L475: Most of the time only subgroups of bonobos (can be large in size) come together rather than the whole community.

The authors have done a great job revising the manuscript by providing more background information about the social organisation of their study species, especially with regard to the bisexual philopatry, dispersal patterns, immigration events and the range of the two groups. These details really helped make a good case that this unique observation represent a partial merge of two previously distinct social groups. I am also happy to see the new figure depicting the proportion of different dyadic associations across the years. I only have a few minor comments and I am excited to see this exciting report published in the Royal Society Open Science! Congratulations!

Minor comments:

line 44, line 135: Social bond is a very specific term to describe social relationships that are differentiated from others, while being stable and equitable within the dyad. But it seems like the authors are using this term to simply describe new social relationships formed between groups. I recommend the authors consider replacing this term with “social relationships” instead. Otherwise, a clear definition of social bonds used in this manuscript is required.

line 94: Fig 1 instead of Fig S1.

line 272: Shouldn’t this be “Forty-six”?

line 530-532: Consider revising this sentence as “… indicate that the adaptations for aggressive responses to non-group members that are so common in social terrestrial mammals are diminished in spotted dolphins, just as in bonobos.” Currently, it reads like aggressive response to non-group members are common in bonobos, which I guess is not what the authors intend to convey.