"Humans -- who enslave, castrate, experiment on, and fillet other animals -- have had an understandable penchant for pretending animals do not feel pain. A sharp distinction between humans and 'animals' is essential if we are to bend them to our will, make them work for us, wear them, eat them -- without any disquieting tinges of guilt or regret. It is unseemly of us, who often behave so unfeelingly toward other animals, to contend that only humans can suffer. The behavior of other animals renders such pretensions specious. They are just too much like us."
-- Dr. Carl Sagan & Dr. Ann Druyan Shadows of Forgotten Ancestors, 1992
Here are some casualty comparisons I found on Vegan Outreach's site, from The State of Animals Today portion of "Vegan Spam":

* The number of animals killed in shelters is approximately equal to the human population of New Jersey.
* The number of animals killed for fur is approximately equal to the human population of Illinois.
* The number of animals killed in experimentation is approximately equal to the human population of Texas.
* The increase in the number of land animals farmed and slaughtered is greater than the total human population of the United States.
* The total number of mammals and birds farmed and slaughtered is approximately equal to one and two-thirds times the entire human population of Earth.
(This is just an overview of yearly casualties in the U.S., not worldwide. And the point is, that as heartbreaking as it is to think about how many unwanted pets are destroyed, or how horrendous the treatment is of animals on fur farms or in laboratories, they're numbers are just a drop in the bucket, compared to the number of animals we kill for food every year. And that is something each person has the option to change, and to make a real difference, if they'd really want to. The most pitiful thing is that they don't.)