Utilitarian philosophers Jeremy Bentham and Peter Singer have argued the suffering of sentient animals to be extremely relevant to ethical discussion, and many modern vegeterians and vegans believe the same. Some ethical vegans argue that exploitation of animals is a form of "speciesism" that will one day be looked down upon as racism and sexism are today. Conditions for animals raised on intensive factory farms (or "concentrated feed operations," as they are sometimes called) are often downright gruesome: they are kept in cramped, unsanitary cages or stalls, and subjected to numerous inhumane practices.
Since all U.S. laws govern the welfare of nonhuman animals specifically exempt those raised for food, the well being of the animals themselves is usually of little concern to the companies running the operation as long as productivity is maintained. The notion that an efficient animal is a healthy animal is pure fallacy; drugs, hormones and other chemicals are used to artificially boost the "productivity" of animals and mask adverse conditions such as severe stress, disease, and degenerative disorders. Ewes, cows, and sows are forcibly inseminated on what the industry refers to ironically as the "rape rack." Breeding sows spend most of their lives in gestation crates and farrowing stalls which are too small for them to even turn around, and their piglets will face a similar fate as they are fattened in their own pens.
Babies of all species are separated from their mothers almost immediately following birth, upsetting natural motherly instincts and causing untold psychological damage to both mother and offspring. Males not selected for breeding are castrated, usually without anesthesia. Even "cage-free" animals don't always fare much better than their factory farm counterparts. Cage-free in some instances may simply mean that they are "free" to roam about a dark, cramped shed, and the animals may never see the light of day. Veal calves are confined in tiny crates and deliberately made anemic to keep their flesh pale for aesthetic appeal. Chickens and turkeys are debeaked with hot irons, pigs' tails are cut off, cattle are dehorned and branded --- all without anesthesia. Animals of all varieties are often beaten by uncaring farm or slaughterhouse workers.
Laying hens may be crammed four to six to a cage in a space barely larger than a folded newspaper; they may only last one year in these conditions before they are considered "spent" and shipped off to the slaughterhouse. Other times they are starved for weeks and kept in absolute darkness to induce "forced molting," and a form of shock which produces a few more eggs before the birds are sent off to be turned into chicken pot pie, bouillon, pet food, and other cheap chicken products. Male chicks born to laying hens are killed within minutes of being born, sometimes by suffocation, other times by being thrown -- alive -- into rendering machines, because they cannot lay eggs and growing them for meat is deemed "inefficient."
Dairy cows can naturally live up to twenty-five years, but under the conditions in modern dairies, they are typically "spent" (to use industry terminology) in three or four years before they are sent to the slaughterhouse to make cheap hamburgers. These cows have a nine-month gestation period and are forced to calve every year, which, along with being forced by hormones (including genetically engineered forms of bovine growth hormone) to manufacture ten times more milk than they would produce naturally. All of this takes an enormous toll on the cows' bodies, particularly their bones. There is also the disturbing connection between the veal and dairy industries; veal calves are most often the male offspring of dairy cows, who, because they cannot produce milk and are not profitable to grow to adulthood before slaughter, are only good for their starved flesh and are killed before their eighteenth month.
The colloquialism that "fish don't have feelings" is pure myth; a fish or other aquatic animal being pulled out of the water is like a person being held under water, and many scientific studies have clearly shown that these animals do indeed feel pain. Farm-raised fish face cramped conditions not unlike their terrestrial counterparts, crowded into confined spaces and made susceptible to suffocation and disease, especially parasites such as sea lice. There are no humane regulations whatsoever regarding the slaughter of aquatic creatures; they are often sliced while fully conscious and left to bleed to death, or they may simply be left to suffocate.
The nonhuman animals to be utilized or consumed directly aren't the only creatures who suffer. The U.S. government's Animal Damage Control Bureau kills an estimated 250,000 wild animals every year -- black bears, bobcats, coyotes, foxes, mountain lions, prairie dogs, timber wolves -- ostensibly to protect ranchers' livestock. Wolves and coyotes in particular are systematically gunned down from the ground or from helicopters and often poisoned. When rainforest is burned or temperate forests clearcut to make room for livestock grazing areas, the habitat of countless species is lost in the process.
Commercial fishers often inadvertently net whales, dolphins, porpoises, seals, turtles, sharks, and other "unwanted" sea life along with their intended catches in practices such as trawling. "Purse seining," a common method of catching tuna, is particularly notorious for netting dolphins. Diseases which run rampant among farmed fish may also spread to their wild counterparts.
Reverence for life
Great thinkers from Pythagorus to Da Vinci to Thoreau to Einstein have abstained from meat for ethical or moral reasons. Ethical vegetarians often equate meat with murder. (While some smart-alec meat eaters point out that vegetables are alive, too, if you consider that animals have to be fed tons of grain, grasses, and legumes in order to be prepared for slaughter, meat eaters actually "kill" far more plants than do vegetarians.) Ethical vegans and vegetarians argue that there is no such thing as "humane slaughter," and may view eating a nonhuman animal just as abhorrent as cannibalism. It could be argued that acculturation is the only reason we may believe that it is acceptable to eat the flesh of a cow but not the flesh of a dog. From a moral standpoint, it may argued that it is not ethical to kill animals for food when plenty of delicious and healthy options are available that don't require the killing of a sentient being.
The few "humane slaughter" laws on the books for terrestrial livestock exclude animals slaughtered for kosher or halal meat (see notes on ritual slaughter, and most of these same laws do not apply to birds (kosher or non-kosher), either. Of course, there is also the question of whether or not there can be such a thing as truly "humane slaughter." Some scholars -- both religious and secular -- believe that gazing into the animal's eyes while killing it is the only way to truly slaughter consciously. Author Rynn Berry remarks at this, "There would be damn few meat-eaters if they had to do that every time they ate a cheeseburger." (Maybe that's the point.) Reverence for life may have its roots in spiritual or religious beliefs as well.
Ethical vegetarians or vegans may also avoid use non-edible animal products such as leather or silk because they were produced by the death of a living creature; creating less demand for these items may theoretically create less demand for the slaughter of the animals. Many ethical vegetarians later become ethical vegans once they learn that dairy cows and laying hens are sent to the slaughterhouse as soon as they are no longer efficient as egg or milk "machines." Sheep are killed for mutton when they aren't capable of efficiently producing wool anymore. Bees may be killed in the harvesting of honey, and if a hive has become less productive, some beekeepers kill the bees and start a new hive rather than wait out the period of non-productivity with their current hive.
Other people do see meat-eating as evil per se, but believe that modern-day factory farms and slaughterhouses are a far cry from the plains and savannahs our ancestors hunted on years ago, and as such, refuse to support the systems by purchasing conventional meat from their local supermarkets.
Links and resources:
http://www.TheMeatrix.com and http://www.themeatrix2.com
Adams, Carol J. -- Neither Man Nor Beast
Coats, C. David -- Old MacDonald's Factory Farm
Davis, Karen -- Prisoned Chickens, Poisoned Eggs
Eisnitz, Gail -- Slaughterhouse: The Shocking Story of Greed, Neglect, and Inhumane Treatment inside the U.S. Meat Industry
Fox, Michael W. -- Eating With Conscience: The Bioethics of Food
Hill, John Lawrence -- The Case for Vegetarianism: Philosophy For a Small Planet
Jensen, Derrick -- A Language Older Than Words
Lyman, Howard -- Mad Cowboy: Plain Truth from the Cattle Rancher Who Won't Eat Meat
Marcus, Erik -- Vegan: The New Ethics of Eating
Reinhardt, Mark Warren -- The Perfectly Contented Meat Eater's Guide to Vegetarianism
Rifkin, Jeremy -- Beyond Beef: The Rise and Fall of Cattle Culture
Robbins, John -- The Food Revolution
Schlosser, Eric -- Fast Food Nation: The Dark Side of the All American Meal
Singer, Peter -- Animal Liberation
Spencer, Colin -- The Heretic's Feast: A History of Vegetarianism
Stepaniak, Joanne -- The Vegan Sourcebook