Fans of BBC’s Sherlock were give one tiny snap from the production of the show’s fourth season today; an image of Benedict Cumberbatch in his Holmes-costume hanging out with a dog. At a glance, this could mean nothing, but if we apply some Holmesian deduction, we can infer quite a bit. Sherlock has gone on the record condemning wild speculation (“DATA DATA DATA! I can’t make bricks without clay!”) but the picture does give loyal fans a fair amount of information.

There’s quite a lot of information about canines in Sherlock Holmes stories, giving us a some ideas as to which of Sir Arthur Conan Doyle’s original stories the modern-day BBC adaptation may be drawing inspiration from next.

First off, here’s Cumberbatch and the droopy-jowled hound in question in all their glory:

The dog game is afoot!

First off, we can rule out that dog is not “Redbeard,” Sherlock’s childhood dog briefly glimpsed in a flashback in the third season’s finale “His Last Vow.” In the flashback, Redbeard was a ginger Irish Setter, and the dog in the new production photo looks to be a Bloodhound.

Redbeard in "His Last Vow"

It’s highly unlikely that creator-writers Mark Gatiss and Steven Moffat forgot what kind of dog Redbeard was. Plus, Redbeard died when Sherlock was a kid. Additionally, Redbeard is also not derived from any original Conan Doyle story.

Holmes with Toby

While Redbeard may not be in the original canon, there are a good number of canines in the world of the original Holmes stories. The most obvious is the titular The Hound of the Baskervilles, but Sherlock already adapted that story in their second series. Still, the dog may have secrets to tell from Sir Arthur’s prolific source material.

Based on Sherlock’s cozy relationship with the bloodhound, the dog could be the show’s version of Toby, a Bassett Hound (close enough) from the the original novel The Sign of the Four. Toby helps Holmes and Watson in that particular adventure, and though elements of that book were loosely adapted in Sherlock’s episode “The Sign of the Three,” Toby did not appear. Though Toby is a minor character in the original canon, a version of him appears in Holmes stuff not written by Conan Doyle, notably in The Great Mouse Detective and in the Nicholas Meyer(director of Star Trek:II) Holmes novel, The Seven Per-Cent Solution.

The vicious Mastiff from "The Copper Beeches." Illustration by Sidney Paget. 

There’s also the possibility that the dog in the picture could be something a little less famous and perhaps a bit more sinister. The dog in the photo is not a Mastiff, but it is a fairly big dog. And a big Mastiff plays an interesting (and terrifying) role in the Holmes short story “The Adventure of the Copper Beeches.” In this one, Holmes and Watson visit the countryside investigating a case of switched or stolen identities, menaced by a vicious Mastiff throughout the mystery. “The Adventure of the Copper Beeches” isn’t one of Holmes’s better-known cases, which could fall in line with comments Moffat made recently in an interview with the Telegraph about the new season.

“There are stories that we are making use of in different ways,” Moffat told The Telegraph, “We’re using stories that people don’t know so much.”

The dogs from both stories — Toby and the vicious Mastiff — are equally obscure, at least in the way they’re used in the stories.

We'd be remiss if we didn't include this Basset Hound puppy running at you

Still, with this rumored to be the last season of Sherlock ever, we’ll probably have more to discuss than mysterious dogs very soon. Here’s hoping more clues appear about the fourth season of Sherlock and render our dog speculation totally, and completely, elementary.

Photos via BBC America