The Microbial Origins of Affection: Rethinking Social Behaviors in Dogs and Humans

Abstract

Many animal behaviors traditionally classified as social—such as licking, grooming, and kissing—may have originated not purely as mechanisms of bonding, but as survival strategies rooted in microbial exchange. This perspective challenges the dominant narrative that such behaviors are primarily symbolic, proposing instead that their origins lie in the biological necessity of maintaining healthy gut and immune microbiomes. Over time, the survival function weakened, yet the behaviors persisted as vestigial instincts now interpreted as social bonding. This paper explores the hypothesis that microbial transfer may have been a primary driver in the evolution of these behaviors, with social cohesion as a powerful byproduct.

Introduction

Dogs licking each other, wolves regurgitating food for pups, humans kissing, and primates grooming—these behaviors span species, environments, and evolutionary branches. Traditionally, they are interpreted as acts of bonding or hierarchy maintenance. But what if their original evolutionary utility lay elsewhere? Specifically: microbial and nutrient exchange.

Gut microbiota are critical to digestion, immune regulation, and survival. In environments where pathogenic threats were abundant and access to diverse microbial sources was limited, behaviors that facilitated the sharing of beneficial microbes may have conferred strong evolutionary advantages. This paper argues that such pressures could have hardwired microbial exchange into instinctual behaviors that we now interpret primarily as social or affectionate.

Evolutionary Logic of Microbial Exchange

1. Survival in Hostile Microbial Landscapes

Early carnivores and primates lived in pathogen-rich environments where a disrupted gut microbiome could lead to death. Accessing, maintaining, and restoring a healthy microbial community was vital. By licking, grooming, or ingesting fecal matter, individuals may have increased their chances of reseeding their microbiome with resilient strains.

2. The Incidental Emergence of Social Bonding

While the initial driver may have been microbial transfer, the repeated positive outcomes of these behaviors (better digestion, stronger immunity, reduced infant mortality) would have naturally reinforced their prevalence. Over evolutionary time, neurological systems may have adapted to associate these acts with pleasure, comfort, and safety, embedding them as bonding behaviors.

3. Parallel Examples Across Species

• Wolves & Canines: Puppies lick adult muzzles to induce regurgitation—nutritional transfer directly tied to saliva exchange.

• Primates: Grooming spreads skin and oral microbes while reinforcing alliances.

• Humans: Kissing transfers oral microbiota and historically may have enabled mate assessment and microbial compatibility.

• Insects (e.g., ants, bees): Trophallaxis (mouth-to-mouth food sharing) ensures colony-wide microbial uniformity.

These cases illustrate a convergent evolutionary pattern: microbial sharing tightly coupled with behaviors that later assumed primarily social meanings.

Vestigial Persistence in Modern Contexts

Today, domesticated dogs lick, humans kiss, and primates groom in environments where microbial exchange is no longer life-or-death. Modern sanitation, antibiotics, and abundant food supplies have reduced the necessity of microbial seeding through direct contact. Yet the behaviors persist—much like the human tailbone—as vestigial remnants of once-critical survival strategies.

This persistence suggests that the social and emotional associations of these behaviors have become self-sustaining, even after the original microbial utility diminished.

Reframing the Narrative

The dominant narrative frames these behaviors as social first, biological second. This perspective inverts that order:

1. Original Function: Microbial/nutrient sharing for survival.

2. Reinforced By: Positive health outcomes leading to greater reproductive success.

3. Evolved Into: Hardwired instincts carrying strong emotional and social valence.

Thus, what we interpret today as affection may have begun as microbial pragmatism.

Implications

1. For Evolutionary Biology: Opens inquiry into the extent to which microbial pressures shaped social behaviors across species.

2. For Microbiome Science: Suggests that ancient “natural probiotics” (via saliva, feces, grooming) influenced survival before modern diets and medicine.

3. For Human Psychology: Highlights how seemingly abstract social rituals (hugs, gifts, kisses) often rest on forgotten biological foundations.

Conclusion

Licking, grooming, and kissing may not be mere social niceties. They may be echoes of ancient microbial survival strategies. What was once a pragmatic way to seed and strengthen the gut microbiome has become one of the most powerful mechanisms of social bonding across species. Recognizing these behaviors as evolutionary byproducts of microbial necessity reframes affection itself as an artifact of survival.