Why do large dogs tend to live shorter lives than their smaller counterparts? According to a study of nearly 900 dogs, the answer may be written into their epigenome. The findings reveal that larger and male dogs undergo accelerated molecular aging, with pronounced DNA methylation (DNAm) on the X chromosome and at transposable elements (TEs).

Aging is a universal process, but its pace and biological mechanisms substantially differ between individuals and species. Domestic dogs offer a particularly useful model for understanding the biology of aging because their lifespans vary dramatically with body size.

For example, smaller breeds can live nearly twice as long as larger breeds. Moreover, companion dogs also live in human environments, consume commercial diets, and receive routine medical care, making them a relevant translational model for studying how genetic background and environmental exposures shape aging.

However, how intrinsic factors such as size and biological sex influence variation in dog aging and lifespan remains unclear. Previous research has demonstrated that DNAm is a useful indicator of biological aging, with “epigenetic clocks” offering a way to understand the factors that accelerate or decelerate biological aging.

Blaise Mariner and colleagues generated 1,640 methylomes from a cohort of 894 dogs in the Dog Aging Project. They combined these molecular data with detailed genetic and demographic data.

Mariner et al. found that molecular aging occurs most rapidly early in a dog’s life. What’s more, larger and male dogs – both of which have shorter lifespans than their respective counterparts – age more quickly at the molecular level.

The authors identified different epigenetic patterns underlying these effects. According to the findings, sex-related changes were concentrated on the X chromosome, while body-size-related changes were especially prominent in TEs, stretches of DNA that can influence genome stability and gene regulation.

Scientists—and dog lovers—have long known that dogs are remarkably good at interpreting our laughs, frowns, and tears. But the biology behind this ability has remained largely unexplored until now. 

In a study published in the Cell Press journal iScience, researchers peered inside the brains of dogs as they viewed images of various human expressions. The brain scans revealed that dogs process smiles in a unique brain region and, for the first time, provided evidence that dogs can distinguish between negative facial expressions, including anger, sadness, and fear. 

“Humans are really good at picking up small details on the face, and so are dogs,” says first author Raúl Hernández-Pérez from the University of Vienna. “This makes dogs very interesting. By studying their brains, we can understand what adaptations have developed to support their remarkable social and emotional understanding of humans.” 

The researchers first focused on happiness, an expression that previous studies suggest holds particular significance for dogs. They scanned eight dogs while showing them images of strangers with either happy or neutral expressions. The happy faces lit up the temporal cortex and caudate nucleus in the dogs’ brains—regions associated with higher cognitive and reward processing, respectively. 

“The activity in the caudate nucleus is particularly interesting,” says Laura Cuaya, also of the University of Vienna, who began the research at the National Autonomous University of Mexico and later expanded it through a collaboration at Eötvös Loránd University in Hungary. “It suggests that happy human faces may have emotional significance for dogs.” 

The team then asked whether the same brain regions respond to other emotions. They showed images of people displaying happiness, anger, fear, and sadness to 12 dogs and analyzed their brain activity using machine learning. They found that the dogs did not react in the same to these negative emotions; the temporal cortex-caudate nucleus network responded uniquely to happiness, allowing researchers to easily distinguish brain reactions to smiles from all three negative expressions.  

But the canine brain was not merely sorting the world into good and bad, the team found. A whole-brain analysis revealed distinct patterns of activity that separated fear from anger and sadness, providing the first evidence that dogs can distinguish between specific negative facial expressions.  

The researchers say that this ability does not mean that dogs experience emotions exactly as humans do. Also, a still photo of a human face doesn’t tell the whole story; in real life, dogs watch human body language, listen to nuances in our voices, and detect information from our scent. 

“When we compare humans with primates, we are looking at abilities that may have come from a common ancestor,” says senior author Laura Cuaya of the University of Vienna. “With dogs, the story is totally different. They followed a very different evolutionary path, yet through domestication, they developed the social abilities needed to understand and cooperate with us.” 

In this study, the canine participants were all pets from loving families, notes the team. Free-ranging dogs, such as those that live on the street, navigate a very different social world and may process human cues differently. 

Next, the researchers plan to adopt a more dog-centric perspective to understand how dogs integrate these brain signals to make sense of the humans around them. Their future work will also compare how human and dog brains process the same emotional signals. 

“I would like people to see dogs as emotional beings,” says Cuaya. “They have evolved with us to understand our expressions and cooperate with us, and recognizing that can change how we communicate with and care for them.” 

Children who have been in contact with dogs in early childhood are on average healthier and require fewer antibiotic courses than children without such contact. The protective association is explained, at least in part, by the spread of dog-associated microbes in the home.

In the recent study, the presence of specific dog-associated microbes in the home were linked to fewer respiratory tract infections, fewer antibiotic courses and more healthy weeks during the child’s first year of life. The richness and diversity are higher in homes with a dog than in homes without a dog, but these general characteristics of the microbiota were not associated with the incidence of respiratory tract infections or morbidity in the child.

The study shows that certain dog-associated microbes or their combinations can explain up to one quarter of the protective effect of dog ownership on children’s respiratory tract infections, antibiotic use and the number of healthy weeks during the first year of life.

The protective effect of a dog ownership remained also after accounting for other factors known to influence childhood infection risk, including family size, living environment and exposure to tobacco smoke. In addition to the dog’s microbiota, a more outdoor-oriented lifestyle in dog-owning families, shared responsibilities related to caring for the dog and partly similar dietary habits shared among dog owners may influence the microbiota in the home and among its inhabitants, and thereby the development of the child’s resistance to pathogens. These factors were not examined in this study.

“The results strengthen our understanding that the microbial environment during early childhood plays an important role in the development of child’s immune system. It has been suggested that in the future it may be possible to modify the home microbiota in ways that promote health. Our study shows that dog ownership already does this today,” says paediatrician Jenni Mäki.

“Respiratory tract infections in early childhood are a significant risk factor for asthma. When some of children’s respiratory tract infections can be prevented with dog-associated microbes, this may also reduce the child’s risk of developing asthma later in life,” says chief researcher Anne Karvonen.

The LUKAS study included nearly 400 families from Finland. About one third of the families lived on farms, half in rural areas and the rest in towns.

The study was carried out by the Finnish Institute for Health and Welfare, Kuopio University Hospital and the University of Eastern Finland.