Perhaps certain mammalian predators are even more important natural enemies than are raptorial birds, but records of predation on racers by mammals are few. Substantial samples of scats of opossums and coyotes from the Reservation have contained no remains of racers. The Fish and Wildlife Service files include a record of an opossum from Adrian, Michigan, on April 26, 1934, that had racer remains in its stomach. No racers were specifically recorded from a collection of 820 scats and 22 stomachs of raccoons (Procyon lotor) from Douglas County, Kansas, reported upon by Stains (1956:43), but occurrences of unidentified snake may have pertained in part to the racer. Skunks are probably more important natural enemies, but food habits data from the Reservation are lacking for the two kinds of skunks occurring there. Crabb (1941:356) in a study of the food habits of the spotted skunk (Spilogale putorius) in southeastern Iowa, did not report any reptiles among the many kinds of prey found in scats. Several times in many years of residence near Medford, Jackson County, Oregon, I saw remains of racers which appeared to be victims of striped skunks (Mephitis mephitis). Tracks and other sign of the skunks were often noticed along a little-used road on a hillside, passing between a pasture and a hay field, with an oak grove, high weeds, and brush, bordering the road. The racers sometimes found as victims along this road were small- or medium-sized individuals. In every instance the predator had begun eating on the tail end of the snake, and later had abandoned the remains leaving the head and part of the forebody still intact. Predation had occurred at night. Skunks foraging mostly in twilight or darkness, probably find racers inactive beneath flat rocks or in shallow burrows. The skunks would be too slow and clumsy to catch the snakes in the open when they were fully active.
The funnel traps used for catching the racers also caught many other kinds of animals. Often a racer and another kind of snake were caught together, but, ordinarily, in these instances no damage to either resulted, although racers, copperheads, king snakes, and garter snakes are all known to eat each other's young. At times, large predatory ground beetles (Calosoma scrutator and Pasimachus sp.) were so abundant that a dozen or more were caught together in a trap, and several times such groups attacked and killed and partly ate young racers caught in the same traps. White-footed mice (Peromyscus leucopus), deer mice (P. maniculatus), harvest mice (Reithrodontomys megalotis), short-tailed shrews (Blarina brevicauda), and least shrews (Cryptotis parva) all were caught rather frequently in the traps, and each, on one or more occasions, gnawed and killed or severely injured a racer trapped with it. The attacks probably were motivated by hunger in all instances, the snake in some instances failing to defend itself or escape because of low temperature. Occasionally such encounters might occur even under natural conditions, the mouse or shrew finding and attacking the snake while the latter was torpid and helpless in its hibernaculum or in a more superficial temporary shelter.
Relatively few of the racers examined showed signs of disease. The most common type of affliction was an infection of the skin causing cankerlike sores, chiefly on the ventral plates, sometimes on the head or tail. These were seen most often in snakes recently emerged from hibernation, but persisted later in the season in years of exceptionally wet weather. Some of the racers most severely afflicted appeared to be in debilitated condition. The causal organism was not determined, nor was it definitely determined whether this type of disease causes mortality.
The ectoparasites of the racer are chiefly chiggers, the parasitic larvae of mites. Loomis (1956) in his study of the chigger mites of Kansas examined many of the racers captured in the early years of my field work. He checked a total of 130 racers, mostly from the Reservation, and found four different species of chiggers, all of the genus Trombicula: T. alfreddugèsi, T. lipovskyana, T. kansensis and T. sylvilagi. The common pest chigger, T. alfreddugèsi, parasitizes most of the species of reptiles, birds, and mammals occurring in Kansas, and it was by far the most common kind on the racers. The numbers per racer in different months were as follows: June, 81; July, 285; August, 432; September, 123; October, 15. Many of the racers were collected in the relatively cool and moist summers of 1950 and 1951. In a year of typical weather, heaviest infestations occur in early summer, June or the first half of July, and the numbers taper off rapidly in the hot but often dry weather of late July, August, September, and October. In hot, humid weather of early summer a racer may have several hundred chiggers attached to it, filling most of the areas of exposed skin between the scales. The chiggers are conspicuous because of their bright orange color. Like T. alfreddugèsi, T. lipovskyana was also found in large numbers on racers and is found on many kinds of hosts. Loomis (op. cit.:1281) recorded it from one kind of frog, one kind of toad, one kind of turtle, two of lizards, six of snakes, 19 of birds, and nine of mammals from eastern Kansas. Five larvae of T. sylvilagi were recorded from a racer captured in October. That kind of chigger is primarily a parasite of small mammals, and perhaps cannot develop successfully when it attaches to a snake. Unlike most other kinds of chiggers, this species is most in evidence in autumn and winter. A single larva of T. kansensis was found on a racer in October. This relatively rare kind of chigger has been found on several kinds of snakes and small mammals (including pocket gophers) and is known from hot and dry rocky places. Even the racers that were heavily loaded with chiggers showed no obvious ill effects, but the chiggers are potentially vectors of various diseases.
Of the many endoparasites found in racers, the lung fluke, Neorenifer lateriporus was the only one identified and frequently observed in my study (Stewart, 1959). This is a digenetic trematode of the subfamily Reniferinae. The racer is its specific host. The life history is still unknown, but in other members of the subfamily, all of which parasitize snakes, an aquatic snail and a frog are required as hosts at different stages of the life cycle. Presumably N. lateriporus has similar requirements. The two common local water snails, Heliosoma trivolvis and Physa anatina, are both potential hosts. By far the most probable frog host is the leopard frog. In wet weather of July the recently metamorphosed leopard frogs leave the water and disperse to all habitats, probably carrying with them the parasites acquired in the tadpole stage. The racers in turn probably acquire their flukes by eating the young frogs in summer. In any case, the adult racers are nearly all parasitized, but the flukes have not been found in those racers that were less than one year old. During their first few months, the racers are too small to swallow leopard frogs, even the young. The flukes have been seen in the live racers mostly in May, when most adults are infested with the flukes. Seemingly at this season the flukes migrate forward into the mouth of the host. Probably this is the time when the flukes breed and lay eggs; if so, the eggs would pass through the digestive tract of the snake and escape with its feces. The latter are usually left in terrestrial situations unfavorable for the development of an aquatic stage, but perhaps some of the eggs are washed into ponds by heavy summer rains. In late summer and fall the flukes are not to be found in the mouths of the live snakes.
Most complete records of the flukes present in racers were kept in 1959. The following table shows the numbers of racers examined and the percentage having flukes in that year.
Ortenburger (1928:182) recorded lung flukes (Renifer ellipticus) from blue racers. In Maryland, McCauley (1945:76) also recorded numerous lung flukes (Pseudorenifer sp.) in an immature racer 490 millimeters in total length. Parker (1941:34) recorded Neorenifer septicus from racers collected at Reelfoot Lake, Tennessee, Greensboro, Georgia, and Kissimmee, Florida; also he recorded N. georgianum from racers collected at Reelfoot Lake. N. septicus was recorded by the same author from the water moccasin (Agkistrodon piscivorus) and N. georgianum was also recorded from the king snake (Lampropeltis getulus).
Cloacal smears from the racers examined usually showed an abundance of ciliate protozoans, either parasites or commensals, and occasionally nematode worms. Harwood (1933:66) examined two racers from the vicinity of Houston, Texas, and found four kinds of helminths: Kalicephalus agkistrodontis, K. rectiphilus, Ophidascaris sp., and Polydelphis sp. Each parasite was found in only one of the two snakes. McCauley (loc. cit.) recorded nematodes (Physaloptera obtusissima) from black racers in Maryland, and Ortenburger (loc. cit.) recorded Physaloptera sp. from the blue racer.