A well-established fact of infancy in mammals is that the mother is the primary adult with whom an infant will interact. This holds true across species, from the tiniest shrew to the most massive blue whale. However, infants of many species also interact with adults who are not their parents. This is called "allomaternal behavior" and it is commonly seen in social mammals that move in groups or herds. One of the allomaternal behavior is when young females without infants of their own handle and care for infants. Young females learn to raise infants, while the mother can forage for food more effectively.
Allomaternal behavior has been observed in many species of odontocetes—toothed whales, dolphins, and porpoises. However, zoologists have long believed that the narrow-ridged finless porpoise, Neophocaena asiaeorientalis, is a solitary species with no allomaternal behaviors. Finless porpoises rarely form pods, and their primary social structure is mother-infant pairs or adults cooperating to swim or feed.
A new study led by Associate Professor Mai Sakai of Kindai University has cast shadows of doubt on this long-held belief regarding finless porpoises. Dr. Sakai's team, which included Soeko Noro from the Marine Mammal Research Laboratory, Kindai University, and Assistant Professor Genfu Yagi from Mie University, observed porpoises in Ise Bay, Japan, and found examples of possible allomaternal behavior. Their findings were published on October 21, 2025, in the journal Mammal Study .
Earlier research has hinted at some kind of social interaction between finless porpoises. Dr. Sakai remarked that in a previous study, her team had observed synchronized diving by an adult and juvenile male pair. Dr. Tomoyoshi Terada, who is the member of the present study team, reported, "In Ise Bay, individuals within 15 m of one another engaged in social behaviors at a high frequency, suggesting that finless porpoises have a social structure where solitary and gregarious behaviors coexist ( Terada et al. 2024 )."
Using consumer-grade drones, Dr. Sakai's team observed finless porpoises in Ise Bay over a period of 34 days between February and July 2023. They looked for infant porpoises interacting with more than one adult during an observation flight, as that was definite proof of interaction with an adult other than their mother.
The team found interactions between four "infant + two adult" sets. In most cases, the infant would approach or separate from an adult and spend periods parallel swimming (PS). The team found that in all cases, infants were on the lateral side of adults during PS. This position might reduce water resistance and allow the infant to keep pace with the adult with reduced swimming effort. Infants may approach adults for protection and reduced swimming effort.
Adults approached infants in two instances. These may be examples of young females without offspring learning to interact with infants before giving birth to their own. However, the sex of the adults was not confirmed. In all cases but one, the time spent in PS with any one adult was less than 40%. This is significantly lower than the mother-infant PS time seen in other dolphins and porpoises. Dr. Sakai highlighted, "The findings suggest that neonatal finless porpoises swim alone for extended periods compared with neonates of other odontocetes. The behavioral tendencies observed in neonates of narrow-ridged finless porpoises may be indicative of a comparatively weaker mother-calf bond when compared with other delphinid species."
Put together, these observations reinforce the idea that N. asiaeorientalis may not be a mostly solitary species as was previously assumed. These findings are important for the conservation of finless porpoises. Orphaned or abandoned infants can be rehabilitated through allomaternal interactions with non-related adults. Lastly, this study proves the utility of relatively inexpensive and simple drones in wildlife monitoring studies.
"Allomaternal behaviors may be beneficial for neonates in the development of social relationships and/or as a supplement to the weaker mother-calf relationships that are typical of this vulnerable developmental stage," remarked Dr. Sakai, adding, "To understand allomaternal behavior in species with simple social structures, future studies need to evaluate the cost-benefit relationship for mothers, calves, and non-maternal adults."
