Domestic Dog Skull Variation and Genetics

Domestic dogs exhibit a wide range of skull variations due to their diverse genetic makeup. This genetic diversity is a result of selective breeding and natural selection over thousands of years.

The skull shape and size of domestic dogs can vary significantly from one breed to another. For example, the Pug has a flat skull, while the Greyhound has a long, narrow skull.

A study of 12 breeds found that the skull shape is influenced by multiple genes, with some breeds having a more complex skull shape due to the interaction of multiple genetic factors.

For more insights, see: Domestic Dog Breeds

Variation in dog skulls is a result of domestication, with studies suggesting that it began as early as 135,000 years ago.

Dogs have undergone significant changes in their skull shape, with some ancient dogs being massive with shortened snouts and widened palates, while others were smaller than wolves.

The overall size of dogs' brains relative to that of wolves has decreased by nearly 30%, with a particular decrease in the limbic system, which is integral to fight or flight responses.

This reduction in brain size may have reduced areas of the wolf brain that enabled tolerance to human contact, leading to changes in skull shape in response to changes in brain morphology.

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Domestication of dogs is believed to have started between 15,000 and 36,000 years ago, with some estimates suggesting it may have occurred as far back as 135,000 years ago.

The earliest archaeological records show that dogs were much larger than wolves, with shortened snouts and widened palates. In contrast, ancient dogs were often smaller than wolves.

Modern dogs have a larger brain-to-body-size ratio, a trait known as encephalization, but their brains are actually smaller than those of wolves, with a decrease of nearly 30% in some areas.

Domestication may have reduced areas of the wolf brain that enabled tolerance to human contact, and skull shape likely changed in response to these changes in brain morphology.

In an experiment with silver foxes, changes in cranial dimensions and other morphologic features were noted as a correlate to tameness over a period of 40 years.

Some researchers liken dogs to wolf pedomorphs, proposing that they are juvenilized wolves that are developmentally restrained in behavior and physical maturation.

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Domestic dogs have been tamed by humans and kept to cater to various human needs.

Domestic dogs are descendants from common wolves and have been kept in homes since the ancient era.

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Genetic variants play a crucial role in determining the shape of a domestic dog's skull. Early efforts to identify these variants focused on candidates identified in humans.

Human brachycephaly, which is associated with morbidity and is diagnostic of many syndromes, occurs as a result of growth zone defects within the developing skull.

Mutations that affect fibroblast growth factor (Fgf) signaling feature prominently in brachycephalic-type craniosynostosis.

Craniosynostosis, which is a condition where the bones of the skull fuse prematurely, implicate genes such as MSX2 and twist homolog 1 (TWIST1) transcription factors.

Other genetic defects leading to craniosynostosis implicate ephrin-B1 (EFNB1), the ras-related protein RAB23, and transforming growth factor, beta receptor 1 (TGFBR1).

In dogs, coronal synostosis is normally absent, but the cranial base of brachycephalic breeds is disproportionate to overall body size.

Regulation of growth at the synchondroses plays a role in the genesis of brachycephalic and dolichocephalic skull conformations.

The TCOF1 gene, which is associated with Treacher Collins-Franschetti syndrome, has been studied in dogs, but the results are disputed.

Variation at FGFR3 has been examined for its role in canine chondrodysplasia, but no variants were found in the Boxer breed.

The domestic dog skull has undergone significant changes over time, with fossil records showing that the first dogs appeared around 15,000 years ago.

Studies have identified several key characteristics that distinguish the domestic dog skull from its wolf ancestors, including a shorter, more compact skull shape.

Domestic dogs have a wider range of skull shapes and sizes compared to wolves, which is likely due to the process of artificial selection by humans.

The incredible diversity of canine craniofacial shape is on full display in Figure 1. A montage of canine craniofacial shape showcases the remarkable differences between breeds.

The lateral views of various breeds are articulated so that the skull base is approximately parallel, allowing for a side-by-side comparison of their craniofacial features.

Palate shape, indicated by white dots, is a prominent difference across breeds. The angle of the palate relative to the cranial base is also noteworthy.

The neurocranium shape, enclosed by blue dots, varies significantly between breeds. The cranial base length, marked by a red line, is another key difference.

The wolf's skull base is used as a reference point to align the lateral views of the different breeds. This allows researchers to compare the craniofacial features of various breeds in a more standardized way.

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Canine skull length is a complex trait, and it's fascinating to see the different variations across breeds.

The surface scans in Figure 2 illustrate the differences between brachycephalic, ancestral, and dolichocephalic skull states of canids. Brachycephalic dog breeds have a shortened rostrum, wide zygomatic arches, and a rounded neurocranium.

The GWAS of skull length in Figure 2 demonstrates that multiple QTL are highly associated with face length. Each dot represents a single marker, and the y-axis represents the strength of the association.

At the other end of the continuum is the dolichocephalic appearance of breeds such as the Saluki, Borzoi, and collie, which tend to have narrow, sometimes elongated, snouts and orbitals that are less forward set.

These dogs were originally bred for coursing small prey, and their dolichocephalic morphology is exactly what one would predict based on the relationship between morphology and ecology/hunting behavior of wild canids.

Many dolichocephalic breeds, such as Afghans and Salukis, were bred for coursing small prey, which requires horizon scanning to spot prey.

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Figure 3 showcases the diverse craniofacial shapes of dog breeds.

The image highlights the palate and brainstem in each skull example, demonstrating the varying degrees of airorhynchic and klinorhynchic features.

A continuum of breeds is arranged in order of severity, from Pekingese to Borzoi, illustrating the range of morphological variations.

Bull terrier skulls are arranged chronologically, from the oldest to the most modern, showcasing the evolution of breed characteristics.

Many small dog breeds, such as Chihuahuas, have brain cases reminiscent of human hydrocephalus, characterized by an "apple dome" skull shape.

The neuroanatomy of brachycephalic dogs is unusual, with the brain rotated with respect to its mediolateral axis.

This raises questions about the cause and effect of this rotation, and how it influences cranial vault shape.

Geometric morphometric analysis has found evidence of modularity between the rostrum and neurocranium of dogs, allowing for independent regulation of development.

This modularity may have been achieved through selection of genetic variants that alter the development of bone-forming tissues.