What I’m wondering is what happens physiologically to a dog eating chocolate that doesn’t happen to a human eating chocolate. Why is chocolate harmful to dogs, but not to people? What goes biochemically wrong within the canine system when chocolate is introduced?
Can any other mammals eat chocolate safely–simians, maybe? How about birds or reptiles? Can a pet iguana survive a chocoalte binge?
FWIW Whenever I hear the stories of chocolate being harmful/fatal to dogs, I’m reminded of an Easter morn long ago when our family mutt, shortly after puppyhood, ransacked our Easter baskets and ate all the chocolate bunnies. She lived a long and healthy life thereafter and never seemed to be adversely affected. Maybe she was lucky or maybe it was a type of chocolate lacking the essential “dog killing” ingredient. BTW she also loved ice cream and, not knowing we were doing any harm, used to give her a little scoop whenever we had some. Again, no adverse effects, just a happy dog.
IANAV, but I believe the problem with giving chocolate to dogs and other small animals is that the stimulants in the chocolate (caffeine and theobromine) may cause heart failure. It’s not a poison per se, but it’s fairly easy to over-stim the little mutts.
Of course, some dogs eat chocolate and don’t die, but the exception is not proof of safety. Many amphetamine addicts live long and happy lives, but too much crank will make you pop like a balloon.
Our little dog Benji used to routinely steal chocolate. We’d find the wrappers of those little mini-bars behind the couch.
The late great springer spaniel Miss Emily Kimberly died as an indirect result of eating chocolate. She got into a can of unsweetened cocoa powder one Friday night while we were away. The chocolate itself did not harm her – we don’t think she actually ate much, and she merely had a queasy tummy and rather fudgy-looking poops the next day – but she cut her gum on the metal rim of the can and got an infection that went to her heart. She seemed fine for a week after getting into the cocoa, but was ill the following Sunday and died that night. 
The vet said her symptoms were classic heart failure.
We have a new rule at our house: nobody canine gets even a whiff of chocolate – and the cocoa powder is now on a high, enclosed shelf instead of a low, open one. 
micco is right that it’s the caffeine and theobromine.
It looks like a 50-pound dog could eat 7 pounds of milk chocolate before it’s likely to be lethal, but less than one pound of unsweetened chocolate would likely kill it.
Amber, my mother’s cocker spaniel when I was a small child, ate an entire Whitman’s Sampler one Christmas and, IIRC, felt no ill effects. No, that didn’t kill her - my mother did by backing over her when Amber, who had grown deaf as a stone didn’t hear the car start.
Bibliophage, thanks for the great link! Good extra info on the general dangers of caffeine to pets.
From that link, the following detailed info:
Most domestic animals are more sensitive to the effects of theobromine in chocolate than people are. It is because they metabolize this compound more slowly, so it builds up. Dogs are especially sensitive. Theobromine is related to caffeine and theophylline and raises blood pressure, heart rate and respiration rate.
The following is from “The Handbook of Small Animal Practice” by Dr. Rhea Morgan.
"The toxic dose of theobromine is about 100 to 150mg/kg.
Milk chocolate contains 6mg of theobromine per ounce. Semi-sweet chocolate contains about 22mg/oz. and baking chocolate about 35 to 45mg/oz."
Hershey’s has a table on their website stating how much theobromine is in some of their products:
http://www.hersheys.com/consumer/nutrition/theobromine/index.html
A mention of reports of horses dying from theobromine poisoning are listed on this site:
http://www.apogeecomgrp.com/drkevin/chocolate.html
I’ve never heard of a cat being poisoned by chocolate, but cats seem to have more sense just in general not to eat things they shouldn’t (with the possible exception of plants).
As for reptiles and birds, I imagine it would be difficult to get them to eat chocolate too. I can tell you as a former bird owner, though, that they are extraordinarily sensitive to environmental toxins, particularly airborne ones. The fumes from cleaning products can kill a bird, while not outwardly harming you, your dog or your cat.
Its probably a good idea to add a link to Uncle Cecil’s column on this subject–Is chocolate toxic to dogs?
This column also appears in Triumph of the Straight Dope, along with an official illustration by Mr. Slug.
Thanks for the reminder, whitetho. I remembered Cecil’s article, but did not remember the amount of detail he went into. I remembered (incorrectly) that he had only glossed over it. I was interested in this question at a very detailed level – plus I was interested in the effects of chocolate on other animals.
Romansperson has pitched in on that last part. Thanks for the responses, all!
I hope this stuff never gets to Oz. We already have man-eating sharks, baby-eating dingoes and bird-eating spiders. The last thing we need is dog-eating chocolate.