Crossing a single street in a busy city, a guide dog might encounter a dense crowd, a pulsing jackhammer, and an impatient driver. To safely lead his human partner through the commotion, the dog must be calm, alert, and responsive without becoming panicky or overwhelmed.

Breeders select for these traits by seeking healthy dogs with desirable qualities to produce the next generation. But a new study from the School of Arts & Sciences, the School of Veterinary Medicine, and Perelman School of Medicine suggests that parenting preferences of the mothers themselves also affect puppies’ success as guide dogs. The researchers found that pups that faced small challenges in the first few weeks of life were more likely to complete their training and become working guide dogs.

Emily Bray, who graduated from Penn in May with a Ph.D. in psychology and is now a postdoctoral researcher at the University of Arizona, led the study at a facility in New Jersey called The Seeing Eye, which breeds and trains dogs to help visually impaired individuals live independent lives. Despite The Seeing Eye investing significant time and money in each animal, not every puppy completes the training program. Learning more about which factors influence a dog’s success in a specific training regimen could help the organization and others like it improve the breeding and training of future service dogs of all kinds.

At The Seeing Eye, Bray witnessed the puppies’ births and tracked their progress over several years. She noted key “mothering” behaviors, such as whether a mom nursed her puppies standing up or lying down, and measured the stress hormone cortisol in each mother’s saliva to assess her anxiety level. The Seeing Eye program itself provided the final measure of success: whether a dog completed training.

Constant supervision ensured that all the puppies received adequate motherly attention, but puppies whose mothers were especially doting did not fare as well in guide dog training. Those attentive mothers were also more anxious, as indicated by salivary cortisol.

“The findings suggest that the mothers who are more casual and not so inclined to coddle their pups produce more successful dogs,” says James Serpell, the Marie A. Moore Professor of Ethics & Animal Welfare at Penn Vet and one of Bray’s advisers. Further studies could investigate the mechanisms by which these changes occur.

Mothering style is not the only determinant of a puppy ultimately becoming a guide dog, but this study is the first to demonstrate its effect.

“It’s just one piece of the puzzle,” Bray says, “but it is an important piece.”

In addition to Serpell, Dorothy Cheney of the Department of Biology and Robert Seyfarth of the Department of Psychology served as Bray’s co-advisers. Mary Sammel, a Penn Medicine professor of biostatistics, was also involved in the study, which appeared in Proceedings of the National Academy of Sciences.