Part 10
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.
_Parasites_
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.
Table 18. Seasonal Incidence of the Fluke Neorenifer lateriporus, in Mouths of Blue Racers on the Reservation and Rockefeller Tract
=============+=================+====================== | Number of | Percentage of sample | racers checked | having flukes -------------+-----------------+---------------------- Apr. 15-30 | 1 | 100 May 1-15 | 28 | 79 May 15-31 | 25 | 72 June 1-15 | 24 | 50 June 15-30 | 17 | 18 July | 30 | 0 Aug. | 5 | 0 Sept. | 9 | 0 Oct. | 2 | 0 -------------+-----------------+----------------------
Populations
_Composition_
Since there is a brief annual breeding season, any local population of racers consists of a series of discrete annual age groups. The population reaches its annual maximum in early September, after undergoing sudden increase by the addition of the annual crop of hatchlings. Throughout the remainder of the year numbers of racers undergo gradual reduction as a result of the many combined mortality factors that affect them. This mortality is distributed among all the age classes, but the heaviest losses, both percentage-wise and in actual numbers, are sustained by the first-year young. Being by far the most numerous group, these young suffer more mortality than all the other age classes combined. Presumably much of this mortality is concentrated in the early weeks of life, while the young are still near their minimum size; the rate of loss is gradually reduced as larger size is attained and some of the early hazards are outgrown. In the adult age classes also, the larger and older snakes live in greater security, and the rate of mortality is higher in the smaller and younger snakes. Even before hatching, the eggs are subject to heavy losses from predators, and probably from drying, flooding, and other unfavorable climatic factors. Unfortunately it was not possible to obtain definite figures on any of these losses since the eggs were never found under natural conditions and the hatchlings were seen only in relatively small numbers.
The records obtained from trapping racers in late spring and summer in fields provided a somewhat different picture of the population from the sample obtained along the ledges in autumn. In the former sample there were 400 males to 257 females, but in the latter sample there were 355 males to 379 females. I regard the summer sex ratio as a distorted one, brought about by the greater activity of the males in the breeding season. Racers are caught most easily in May, and the fact that two or more males often were trapped with the same female, while the reciprocal combinations did not occur, demonstrates the increased activity of the males in their search for mates at this season. In autumn there is no sexual activity; both sexes probably are equally active in seeking places to hibernate when they are trapped along the hilltop outcrops. The ratio of 51.6 per cent females in my sample of 734 may indicate that in the males greater activity at other seasons results in a somewhat higher mortality. This idea is borne out by the fact that for the supposed two-, three- and four-year-olds combined, females comprise 51.2 per cent, but they comprise 55.6 per cent of those more than four years old and 61.3 per cent of those more than five years old.
[Illustration: Fig. 20. Histograms comparing snout-vent length in random samples of _Coluber c. flaviventris_ (those captured on the Reservation and Rockefeller Tract in 1960 and 1961) and _C. c. mormon_ (specimens in the Museum of Vertebrate Zoology, University of California). In the museum sample, collected by conventional methods, the first-year young constitute a prominent and fairly distinct size group, whereas in the sample of racers from the Reservation and Rockefeller Tract, mostly caught in wire traps of quarter-inch mesh, relatively few young were secured. It is demonstrated that hatchlings are approximately the same length in both populations, but _flaviventris_ grows much longer, and that the differences in length between the sexes is approximately the same in each population.]
By assigning to each racer caught an arbitrary age, on the basis of size according to Table 17, I calculated the population (exclusive of those snakes in their first year of life) to have the composition shown in Table 19.
Table 19. Percentages of Adult Population of Blue Racers Comprised by Each Annual Age Group
=================+=============== | Percentage of Years of Age | population -----------------+--------------- 2 | 41.5 3 | 17.8 4 | 12.6 5 | 9.5 6 | 6.1 7 | 4.3 8 | 2.7 9 | 2.4 10 | 1.2 11 or more | 1.9 -----------------+---------------
Figures are completely lacking to show the relative numbers of juveniles, until, already approaching adult size, the young are about to enter their second hibernation. My combined fall samples include 303 of these latter young, as against 142 racers about to enter their third hibernation. Thus, after having nearly attained adult size, these adolescent snakes sustained a loss of 53 per cent in a year. Losses must occur at an even more rapid rate in the younger and smaller snakes. It may be speculated that of the approximately 300 eggs produced by a population of 100 adult racers, 150 are lost before or during the period of incubation which lasts nearly two months. Of the 150 hatchlings emerging in early September, at least one-third probably are eliminated by the following breeding season in late May, leaving 100. The 100 survivors at this stage are still small juveniles, but by autumn they have attained adolescent size. By this time, if they had undergone a further reduction by 53 per cent, only 47 would remain--approximately the number to be expected if the population were stable from year to year.
A notable difference between the fall sample and the spring sample that I obtained was the higher proportion of large and old racers in the former sample. This difference can be attributed to the year-to-year changes in the population during the 14 consecutive years spanned by my field work. The fall sample of 734 racers represented the combined catch of the years 1949 through 1962, rather evenly distributed, but the spring sample included few snakes from the years 1949 through 1957; most were from the years 1958 through 1961. In 1949 when the study was begun, the Reservation was being protected for the first time, and formerly overgrazed pastures or cultivated fields were acquiring a rank growth of grass and weedy vegetation, and thus becoming favorable habitat. The abundant new habitat promoted rapid increase in the population of racers until the newly available areas were filled to their "carrying capacity." Table 21 shows the changing trends of the different age groups. Although the separate annual samples are perhaps too small to show the composition of the population accurately, it is significant that in the fall of 1949 an unusually high proportion of the racers caught were one-year-olds, hatched in September, 1948.
Table 20 shows that in a typical group of 100 subadult and adult racers (second year and older) only a little more than one-fourth are productive females. The largest females, six years old and older, making up less than ten per cent of the adult population, contribute nearly half the total complement of eggs.
The calculated number of eggs pertains to a stage before oviposition, and subsequent losses through resorption under unfavorable conditions, through inviability of embryos and through deaths of some of the gravid females, are to be expected. In the weeks of incubation further losses are sustained. Although these losses cannot be measured, they must be severe as on numerous occasions scattered and torn eggshells representing entire clutches dug out and destroyed by predators, have been found. Probably other clutches are destroyed underground by such predators as moles and egg-eating snakes, and still others by insects. Tinkle (1959:195) wrote that in a clutch of 15 eggs found under a board, four were parasitized and had small perforations. Molds destroy a high percentage of all reptilian eggs that are incubated artificially and doubtless destroy many under natural conditions also. Excessive heat or moisture, or desiccation, resulting either from climatic extremes or from poor choice of a nest site by the female, would cause further loss. In four different years, Blair (1960:108) found that losses of eggs between laying and hatching in the Texas spiny lizard (_Sceloporus olivaceus_) ranged from 69 per cent to 86 per cent; no other comparable study of the extent of egg losses in a species of reptile is known to me. The racer is somewhat less prolific than the spiny lizard, and potentially longer lived; the racer's eggs are larger and thicker-shelled, and they are deposited in deeper burrows. It might be expected that losses during incubation would be somewhat less in the racer than in the spiny lizard.
Table 20. Calculated Productivity in a Hypothetical Group of 100 Subadult and Adult Blue Racers
===========+==============+============+============+============+========= | Percentage | Percentage | Percentage | Eggs | Age-Group | of | of fecund | productive | per | Number (years) | population | females in | females in | productive | of eggs | in age-group | age-group | age-group | female | produced -----------+--------------+------------+------------+------------+--------- 2 | 41.5 | 51.2 | 13 | 9.2 | 26 3 | 17.8 | 51.2 | 56 | 9.9 | 51 4 | 12.6 | 51.2 | 60 | 10.8 | 42 5 | 9.5 | 55.0 | 57 | 13.0 | 39 6 and over | 18.6 | 61.3 | 80 | 15.7 | 143 -----------+--------------+------------+------------+------------+---------
Table 21. Percentages of Racers in Each Annual Age Group (Exclusive of Hatchlings) in Autumnal Samples at Different Stages of the Field Study, Showing Shift Toward Older Age-Groups in the Later Years
==============+========================================================= | Year or combination of years represented by each sample +-----------+-----------+-----------+----------+---------- | 1949 | 1950 | 1953 | 1956 | 1959 Years of Age | | 1951 | 1954 | 1957 | 1960 | | 1952 | 1955 | 1958 | 1961 | | | | | 1962 --------------+-----------+-----------+-----------+----------+---------- 1 | 54 | 44 | 39 | 39 | 41 2 | 14 | 24 | 21 | 19 | 19 3 | 18 | 11 | 23 | 16 | 13 4 | 4 | 10 | 10 | 11 | 9 5 | 6 | 6 | 3 | 8 | 8 6 | 2 | 1 | 3 | 1 | 3 7 | 2 | 4 | 4 | 2 | 3 8 | ...... | ...... | ...... | .5 | 2 9 | ...... | 1 | .4 | .5 | 2 10 | ...... | ...... | .4 | 2 | 1 Older than 10 | ...... | ...... | ...... | .5 | ..... | | | | | Number in | | | | | sample | 49 | 126 | 117 | 194 | 242 --------------+-----------+-----------+-----------+----------+----------
Although figures for the youngest age groups--one-year-olds and hatchlings--are missing, approximations of them may be furnished by extrapolation, from the information available regarding the productivity of the population. Some factors involved in productivity are that the sex ratio deviates from parity, slightly in favor of the females in the adolescents but more markedly in favor of the females among the older age groups; that some adult females apparently fail to produce eggs in the breeding season, but the percentage decreases in the older snakes; and that number of eggs per clutch increases in proportion to the size and age of the female producing them. Too few figures are available concerning most of these factors to indicate more than the trends; nevertheless the available figures have been used in Table 20 in an attempt to estimate the productivity of a hypothetical population.
_Numbers_
Conant (1938:178, Pl. 7) published a photograph of 106 blue racers killed in February, 1932, by farmers near Bellville, Ohio, and Pope (1944:173) mentioned that scores of blue racers aggregated in October around an oak-covered dune near Chicago. In both these instances large hibernating aggregations were involved, and the areas represented by them are unknown; nothing has been recorded regarding population densities.
The records obtained through my fall trapping, along hilltop rock outcrops, yielded no information concerning population densities, but those obtained in fields in summer did provide significant information in this regard. Even after years of trapping on the same area, the catch still consisted largely of new individuals; the method was not sufficiently effective to catch all racers present at any one time, and the total catch for a season therefore provided only a crude index of the minimum number present.
The summer trapping was carried on in three separate areas. One of these was the area of bottomland pastures and formerly cultivated fields where the Reservation headquarters are located, a block of 39 acres bounded on three sides by woodland, and on the fourth by cultivated farm land. Effective trapping in this area was carried on through the years 1955 to 1961 inclusive. A second area, of 48 acres, was one of upland fields, mostly covered with re-established prairie grasses, in the northeastern part of the Reservation. A third area, of 137 acres, also upland, was that of the Rockefeller Tract, cultivated through 1956 and sown to prairie grasses the following year, and the adjacent northwestern hilltop portion of the Reservation. Effective trapping on these two latter areas was carried on in 1958 through 1962.
For the seven years of trapping in the House Field area, the catch was as follows: 30, 33, 38, 38, 34, 24, and 20. In four years of trapping, the northeast field area yielded 42, 28, 37, 59, and 19 blue racers, and the Rockefeller Tract yielded 52, 67, 67, 126, and 106. The actual catch was hence less than one per acre in nearly all instances, but the year-to-year differences in catch are believed to be caused chiefly by differences in numbers of traps used and in trapping effort, rather than by changes in the numbers of racers present.
Best index to the number of racers actually present is provided by the number of recaptures, and their ratio to first captures. The population of course, undergoes alteration from year to year, with many racers eliminated and replaced by others.
In 1955, 26 racers were caught in the May-June-July period, in the headquarters field area. In the August-September-October period of the same year five racers were caught of which only one was a member of the original 26. The five-to-one ratio indicates that the original 26 may have represented an actual population of 130, but of course the single recapture is much too small a sample to provide a reliable ratio. Some of the racers caught in May were recaptured in June, others in July, and still others not until late summer or early autumn. Somewhat different estimates can be obtained for the population depending on how the season's records are divided. For instance, in the headquarters field area in May 1955, eleven racers were caught; in the remainder of the season 20 were caught, of which two were members of the original group of eleven. The 20 to 2 ratio indicates that the 11 caught in May represented an actual population of 110. In the period May-June, 18 racers were caught, and in July-August-September-October, 15 were caught, including four of the original 18, hence indicating a population of 67. Although obviously the population underwent some change during the course of the season, the three sets of census figures apply essentially to the same population, and the divergence in them illustrates the wide range of error arising from insufficiently small samples.
Common sources of error in the censusing of natural populations of animals by the capture-recapture method ("Lincoln Index" or "Petersen Index") arise from the fact that the composition of a local population often changes between two sampling periods, or even within them. Some of the animals marked may move elsewhere, to be replaced by unmarked immigrants, or they may die and be replaced by unmarked maturing young. First-year racers that could pass through the quarter-inch mesh of the traps in spring and early summer became too large to escape in this way in the latter half of the summer, but these young were excluded from the census computations. There was doubtless some shifting of marked individuals away from the study areas and shifting of new individuals onto these areas in the periods of weeks between successive samplings. Jackson (1939) has explained a method of correcting census computations based on capture-recapture ratios when there is a consistent trend of diminishing recaptures with increase in elapsed time. However, in my records no such trend is discernible; furthermore it has been demonstrated that individual racers tend to stay within the same home range area throughout most of their season of activity. Therefore, I conclude that shifts of individuals away from the study areas or into them, in the intervals between samplings constitute only a minor source of error.