DNA studies suggest that native American dogs entered North America from Siberia roughly 10,000 years ago, and were then isolated for some 9,000 years until the arrival of the first Europeans; these pre-contact dogs exhibited a unique genetic signature that is now almost gone.[6] A study based on sequencing of ancient dog genomes, published in 2020, suggests that this pre-colonial ancestry survives in two Mexican breeds, to the extent of about 4% in the Chihuahua (and some 3% in the Xoloitzcuintli).[7]
Colonial records refer to small, nearly hairless dogs at the beginning of the nineteenth century; one claims that sixteenth-century conquistadores found them plentiful in the region later known as Chihuahua.[8] In a letter written in 1520, Hernan Cortés wrote that the Aztecs raised and sold little dogs as food.[9]
A shorthair apple-head Chihuahua, showing pronounced stop
Chihuahua breed standards specify an "apple-head" or "apple-dome" skull conformation.[12] Chihuahuas occur in virtually any color combination, from solid to marked or splashed.[13] Apple-dome Chihuahuas have large, round eyes and large, erect ears, set in a high, dramatically rounded skull.[14] The stop is well defined, forming a near-90-degree angle where the muzzle meets the skull.[3] Dogs of the older "deer" type, with a flat-topped head, more widely set eyes, larger ears, and longer, more slender legs, may still be registered, but the deer head is not considered a separate type in competition and a deer-head dog's digression from the breed standard is considered a fault.[12][3]
Breed standards for this dog do not generally specify a height; only a weight and a description of their overall proportions. Generally, the height ranges between 15 and 23 cm (6 and 9 in);[14] some dogs grow to 30 to 38 cm (12 to 15 in).[citation needed]
The Fédération Cynologique Internationale standard calls for dogs ideally between 1.5 and 2.5 kg (3.3 and 5.5 lb); those outside the range 1.0–3.0 kg (2.2–6.6 lb) are disqualified from exhibition.[1] The American breed standard sets a maximum weight of 2.7 kg (5.9 lb) for showing.[14] In the British standard a weight of 1.8–2.7 kg (4–6 lb) is preferred – the clause "if two dogs are equally good in type, the more diminutive one is preferred" was removed in 2009.[15]
Pet Chihuahuas (those bred or purchased as companions rather than as show dogs) often range above these weights, even above 4.5 kg (10 lb), if they have large bone structures or are allowed to become overweight.[14] This does not mean that they are not purebred Chihuahuas; they just do not meet the requirements to enter a conformation show. Chihuahuas do not breed true for size, and puppies from the same litter can mature in drastically different sizes from one another. Also, larger breeding females are less likely to experience dystocia (obstructed labor). Many breeders try to breed Chihuahuas to be as small as possible, because those marketed as "teacup" or "tiny teacup" demand higher prices.[16]
The international standard disallows the merle coat pattern, which appears mottled.[1] In 2007 The Kennel Club of Great Britain amended its breed standard to disqualify merle dogs because of the health risks associated with the underlying gene.[17]
Like many other small dogs, the Chihuahua may display above-average aggression toward people and other dogs.[18][19]
A 2018 study in Japan of pet cemetery data found the Chihuahua to have an average life expectancy of 11.8 years compared to 15.1 for crossbreeds and 13.7 overall.[26] A 2022 UK study on life expectancy of dog breeds based on veterinary data showed the average life expectancy to be 7.91 for the breed compared to 11.82 years for crossbreeds.[27][b] A 2024 UK study found a life expectancy of 11.8 years for the breed compared to an average of 12.7 for purebreeds and 12 for crossbreeds.[28] A 2024 Italian study found a life expectancy of 8.5 years for the breed compared to 10 years overall.[29]
A UK study found the Chihuahua bitch to be more than 10.4 times more likely to experience dystocia.[30]
^The Japanese study reviewed cemetery data which is unlikely to have any records of still-births and altricial deaths whilst a veterinary clinic likely would have data on these.
^"CHIHUAHUEÑO"(PDF). fci.be. The Fédération Cynologique Internationale. October 4, 2019. p. 5. Retrieved June 19, 2024. Color: All colors in all possible shades and combinations are admitted, except merle color which is exotic color variation.
^Pedro Baptista Pino y Juan Lopez Cancelada, Exposición sucinta y sencilla de la Provincia del Nuevo México y otros escritos. Ed. Jesus Paniagua Perez. Valladolid: Junta de Castilla / León: Universidad de León, 2007, p. 244: "Even in the desert the tiny dogs could be found, hunting rats, mice, and lizards." The following footnote alludes to starving Conquistadores reportedly hunting and stewing the dogs (Universidad Veracruzana, Arquivo Viejo, XXVI.2711).
^Coile, C. (2013). Chihuahuas: Everything about purchase, care, nutrition, behavior, and training. Hauppauge, NY: Barron's Educational Series, p. 7: "Only in 1904 did the American Kennel Club (AKC) register its first Chihuahua; a total of five were registered that year."
^Curtis W. Dewey, Ronaldo C. Da Costa (2016). Signalment and History: The First Considerations. In: Curtis W. Dewey, Ronaldo C. Da Costa (2016). Practical Guide to Canine and Feline Neurology, third edition. Ames, Iowa; Chichester, West Sussex; Oxford: Wiley Blackwell. ISBN9781119946113, pages 1–8.
^Chihuahua (long and smooth coat): Hydrocephalus. Genetic Welfare Problems of Companion Animals. Wheathampstead, Hertfordshire: The International Animal Welfare Science Society/Universities Federation for Animal Welfare. Accessed February 2021.
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