A few years ago, a cousin of mine told me the story of the time she was at a public swimming pool (in Utah, I believe) when her infant son demanded to be fed. She did what came naturally: she began to breastfeed her son. This act didn't go over very well with at least one of the pool's patrons, who came over to my cousin and sneered "That's disgusting! I have young children and they don't need to see that!" My cousin mused on how curious it was that this lady wasn't bothered by her children seeing dozens of women and girls of various ages walking around the pool in scant string coverings, but a mother breastfeeding her infant was disgusting. (For the record, The Science Pundit is okay with both breastfeeding mothers and scantily string-clad bathers.)
lactation--is a natural process for all mammals. Many animals don't care for their young, but those that do have evolved a variety of strategies for feeding and protecting their progeny. Lactation is the strategy that mammals have evolved for that purpose. The "most primitive" mammals, the monotremes such as the duck billed platypus, give us a clue as to how lactation might have evolved. Suckling probably started as young proto-mammals liking their mother's skin to gain moisture and nutrients from the oils and perspiration exuded from the pores. Something similar to this (but more evolved) is seen today in the platypus. The most highly developed lactators (by which I mean the most developed mammary glands) are the placental mammals (which includes we humans). The milk we feed our youngest contains not only nutrients, but antibodies to help with immunity development. I wouldn't dream of calling this disgusting.
When I think disgusting, I think of something that promotes the gag reflex, in other words: puking. This brings me to the next strategy for feeding one's young. The simplest and most obvious strategy is for the parents to go get food and put it into their offspring's mouth. This strategy is also practiced by many species of mammal during a period known as weaning. This strategy does have some shortcomings, though.
So do we have a champion? Not by a long shot. Let's not forget that there is another way for the body to release nutrients which is intimately tied to the digestive system. I am of course speaking of excrement.
But is there an animal out there that actually craps out a partially digested meal to feed its young? Behold Ctenocephalides felis, the common cat flea!
The cat flea is the most common flea found biting pets and people. Even though there is a dog flea (Ctenocephalides canis) and human flea (Pulex irritans), the fleas you are most likely to find sucking away human or canine blood are cat fleas. So let's talk a little about our friend's life cycle.
EDIT: The flea in the picture is female; thanks Christine.), and the female cat flea is a blood sucking, egg laying, and pooping machine. Adult fleas are constantly shedding eggs and doodies. The flea dingleberries naturaly outnumber the eggs since (a) only females lay eggs, and (b) the nutrition required to produce a single egg results in several berries. It is also worth noting that female fleas will lay eggs even if they haven't mated, and mated females will lay non-viable as well as viable eggs.
While adult fleas have hooked barbs on their feet and legs to keep them attached to their host (not to mention mouth parts), eggs and scat don't avail of such adhesives. So what happens is that eggs and turds drop off the animal with the largest concentration (as statistically expected) falling off where the pet spends most of its time. The viable eggs then hatch into worm like larvae, a good proportion of which will find themselves surrounded by huevos and caca. This is a veritable smorgasbord for the larvae since that is what they eat.
It seems that the larval diet shifts from primarily feeding on stools in the first stage to actively seeking out eggs and sucking out their yolks in the third. I suppose that this is an improvement since being told to go suck an egg isn't quite as insulting as being told to eat shit. But this raises the interesting question: are fleas not only blood suckers and shit eater, but also cannibals? As long as you don't consider eating non-viable eggs as cannibalism, it seems the answer is yes, but they keep it to a minimum. There is some evidence that non-viable eggs are stickier than viable eggs and so easier for a larva to latch on to, but I think that the primary explanation is one of statistics. Viable eggs hang out for a limited period of just a few days, while non-viable eggs hang around until either they're eaten or sucked up by the pet owner's vacuum cleaner (I was taught that if you have fleas, you should put moth balls in your vacuum cleaner bag. That makes sense now.)
So which is the more important part of the flea larva diet: eggs or droppings? That's the question Drs. Hsu, Hsu, and Wu looked into back in 2002. The flea larvae were divided into five groups based on diet. The MF group was fed exclusively adult male feces. The FF group was fed exclusively adult female feces. The NE group was fed exclusively non-viable eggs. The FF+NE group was fed a mix of adult female feces and non-viable eggs. And the control group PBCP group was fed exclusively porcine blood curd product (The Science Pundit hereby renames the control group "bacon eaters"). There were thirty larvae in each group, which is large enough to see patterns, but The Science Pundit (that's me) would like to see a larger more comprehensive study. The FF+NE and bacon eaters both did fairly well with 90% & 83% survival to adulthood, respectively. The MF group did a bit more poorly with a 13% survival rate, but both the FF and NE groups were completely wiped out.
I guess the lesson here is that if you're a cat flea larva you should eat shit and suck eggs, but if you can only do one, then eat your daddy's poop.
Hsu, M., Hsu, Y., & Wu, W. (2002). Consumption of flea faeces and eggs by larvae of the cat flea, Ctenocephalides felis Medical and Veterinary Entomology, 16 (4), 445-447 DOI: 10.1046/j.1365-2915.2002.00388.x