This article is from the Nov./Dec. 1993 AFRMA Rat & Mouse Tales news-magazine.

By Sara B. Conrow, 1915, Anat. Rec., vol. 9, no. 10, p. 777
From the Wistar Institute of Anatomy and Biology

ED. NOTE: While going through my files of Genetec/Medical literature, I came across this article sent to me in 1986 from a member. I have taken excerpts from it to share with you. I found it to be very interesting. I would be interested to know if anyone has read any more recent research on the Tailless rats regarding breeding, heredity of the gene, or any updates on the study of the actual vertebrae. Even though we have recently recognized Tailless rats, we do have a strict standard to disqualify any evidence of physical abnormality due to being tailless, e.g. difficulty walking or climbing, skeletal problems, etc. The Tailless rats we have here don’t seem to have the “spina bifida” and other serious problems found in Tailless rats elsewhere.

There seems to be a general opinion that when a rat appears without a tail it means the loss of the tail by accident early in the animal’s life, and it is usually suggested that it was bitten off by another rat at the time of birth or soon after. The objects of this paper are to describe skeletal conditions in the region posterior to the thoracic vertebrae of several tailless rats, and to correct the existing impression that a tailless rat occurs through the accidental loss after birth of a once existing tail. As to the word tailless; by tailless we mean here “with no caudal vertebrae.” There are cases, of course, where, from disease or from some accident after birth, the tail has become simply a stub, but in these cases some caudal vertebrae remain.

Little data seems to have been recorded concerning the tailless condition of the higher animals. Short tails have been noted among cats, fowls, and dogs, while the number of caudal vertebrae has been found to vary in some other animals, but only among dogs have cases been recorded where the caudal vertebrae of a mammal were completely absent.

Hind (1889), Anthony (1899), Kennel (1901), and Davenport (1905) all give accounts of mating Manx cats or short-tailed cats with the long-tailed variety and having the short tail appear in many of the offspring.

Concerning fowls, Godron (1865) in a footnote says that complete lack of coccyx has been observed in a large number of fowls and that the character is very readily transmitted.

These conditions referred to are congenital and not the result of accident.

The material for this study was obtained from The Wistar Institute rat colony and consisted of rats of the species Mus norvegicus albinus and Mus norvegicus (pied). Specimens for the work were not abundant, since, during the past nine years, only five tailless rats have appeared in the colony, although forty thousand rats have been observed. Of these five, one was eaten by an older rat soon after birth, one male is at present mated in the colony, and the remaining three rats were killed and examined.

Of the three tailless rats whose vertebrae were examined, one was a female and two were males. The data concerning them are presented in table 1. The normal rat of this species (Mus norvegicus) has six lumbar vertebrae, four sacral, and twenty-nine to thirty-one caudal.

Rat No. 1, the female, was of the species Mus norvegicus (pied). Its parents were some pied pet rats of the colony. It was two years old when killed, its body weight was 171.3 grams, and its body length was 193 mm. The arrangement of the most posterior vertebrae of this rat and their relation to the pelvic girdle may be seen in figure 1. Supposing all the lumbar vertebrae to be present, we have here, posterior to the lumbar vertebrae, only three modified sacral vertebrae. Thus one sacral vertebra and all of the caudal vertebrae are missing. The striking feature here then is that the vertebral column ends posteriorly about midway of the long axis of the pelvic girdle, far in from the posterior end of the body.

Table 1
Data on tailless rats

Rat No. 2, a male, was a half-inbred albino (Mus norvegicus albinus) of the eleventh generation. Its mother was a strict inbred of the tenth generation and its father a stock male. When killed it was 388 days old, its body weight was 190.9 grams (one month earlier it had weighed 210 grams), and its body length was 194 mm. The arrangement of the most posterior vertebrae and their relation to the pelvic girdle may be seen in figure 2. Supposing all the lumbar vertebrae to be present, we have here, posterior to the lumbar vertebrae, only two modified sacral vertebrae. Thus two sacral vertebrae and all the caudal vertebrae are missing. Here again the vertebral column ends posteriorly far up the long axis of the pelvic girdle, even anterior to the middle of its axis.

Rat No. 3, a male, was an albino rat (Mus norvegicus albinus) of unknown parentage. It was thirty days old when killed, its body weight was 21.7 grams, and its body length was 89 mm. This rat was small and in rather poor condition. The arrangement of the most posterior vertebrae and their relation to the pelvic girdle may be seen in figure 3. We have here no sacral vertebrae, but two good lumbar vertebrae and two or three modified lumbar vertebrae. The last (sixth and perhaps fifth also) lumbar vertebra, all of the sacral vertebrae, and all of the caudal vertebrae are missing. In this case then the vertebral column is more modified than in the other two rats, for here the vertebrae reach only to the anterior part of the pelvic girdle, and the girdle is attached to the column merely by a small surface near its anterior end. This mode of attachment allows the posterior end of the girdle to hang very low down, almost at right angles to the column. In the living rat, the sagging of the girdle was very noticeable, as it allowed the head of the femur to drop far down and thus gave an odd appearance to the posterior part of the rat’s body.

Fig. 1 — Rat No. 1, ventral aspect; 1, pelvic girdle; 2, sixth lumbar vertebra; 3, three modified sacral vertebrae. Fig. 2 — Rat No. 2, ventral aspect; 1. pelvic girdle, 2, fifth lumbar vertebra; 3, sixth lumbar vertebra; 4, two modified sacral vertebrae. Fig. 3 — Rat No. 3, right lateral aspect; 1, thirteenth thoracic vertebra; 2, two first lumbar vertebrae; 3, two or three modifed lumbar vertebrae; 4, pelvic girdle

This completes the description of the three tailless rats whose bones have been examined. As to the tailless male albino rat which has been mentioned as at present mated in the colony, we are confident from inspection that in this animal also the vertebral column ends far forward along the long axis of the pelvic girdle, for this condition may be felt distinctly by pressing the finger on the rat’s back in the pelvic girdle region.

The vertebral structure of all the tailless rats which we have examined seems to show that the deformity is not due to an accident after birth, since in each case the column ends in the pelvic region far from the posterior end of the body. We conclude, therefore, that the rats were born tailless, and even more than tailless, since they lack more than the caudal vertebrae.

An examination of the vertebrae of three tailless rats showed that all of them lacked all of the caudal vertebrae; and besides, the first lacked one sacral vertebra; the second, two sacral vertebrae, and the third lacked one (and perhaps two) lumbar vertebra and all four of the sacral vertebrae.

In each case the vertebral column terminated in the pelvic region far anterior to the posterior end of the body, showing that the tailless condition was due not to accident after birth but to a congenital deformity of the vertebral column.

• Anthony, R., 1899, Heredity in Manx cat. Bull. Soc. Anthr., p. 303.
• Davenport, C. B., 1905, Details in regard to cats. Report on the work of the Station for Exp. Evol., Cold Spring Harbor; Fourth year-book, Carnegie Institute. 1906 Inheritance in poultry. Publ. Carnegie Inst., No. 52.
• Godron, D. A., 1865, De la suppression congéniale de l’ appendice caudal. Observée sur une famille de chiens. Mem. Ac. Stanislas.
• Hind, W., 1889, Taillessness in the Manx cat. Ann. Rep. N. Staffs. Field Club, p. 81.
• Kennel, J., 1901, Ueber eine stummelschwänzige Hauskatze und ihre Nachkommenschaft. Zoöl. Jahrb., Syst., vol. 15, p. 219.

Sara B. Conrow, 1917

This paper has three objects: first, to describe skeletal conditions in the region posterior to the lumbar vertebrae of two tailless rats which have appeared in the colony since the writing of the preceding paper (Conrow 1915, Anat. Rec., vol. 9, no. 10, p. 777); second to describe skeletal conditions in the same region of four rats whose tails were removed by operations soon after birth; and third, to emphasize, by a comparison of the skeletal conditions in these two sets of rats, the conclusion of the previous paper, namely, that taillessness in the rat is due not to accident after birth but to a congenital defect in the vertebral column.

There seems to be a general opinion that when a rat appears without a tail it means the loss of the tail by accident early in the animal’s life

Since the writing of the previous paper (Conrow ’15) four tailless rats have appeared among the standard rats (Mus norvegicus albinus) of the Wistar Institute Colony, making nine tailless specimens observed during the past ten years in a total of 71,500 rats. Of these four, two are at present mated in the colony, and two (a male and a female) have been examined and will be described here. The data concerning these latter are presented in table 2.

Table 2
Data on tailless rats

In rat No. 4, a male, the vertebrae and pelvic girdle were studied. All vertebrae caudal to the second sacral were lacking, as was a large part of the second sacral. The first sacral was somewhat deformed and had what remained of the second fused with it. Its points of attachment to the pelvic girdle were a little more caudal than usual. (Two of the cervical vertebrae also were partly fused.) The striking feature in this rat was that the vertebral column terminated far from the posterior end of the body, even anterior to the middle of the long axis of the pelvic girdle.

In rat No. 5, a female, the four sacral vertebrae were present, but they were more crowded together and fused than is usually the case. There was also one deformed caudal vertebra; the rest were lacking. This rat might not be called strictly tailless, since tailless, as previously defined, means “with no caudal vertebrae,” but the vertebral column ended several millimeters anterior to the posterior ends of the pelvic girdle. There was an odd, external structure found here just dorsal to the anus, in the position which normally would have been held by the tail. It was a fleshy papilla 11 mm long and 2.5 mm in diameter, with the characteristic scaly tail covering.

The conditions found in these two tailless rats are then (with the exception of the fleshy papilla just mentioned) similar to those found in the tailless rats previously studied.

The experiment of removing by operation the tails of several rats was conducted as follows. Two litters of standard albino newborn rats were chosen. In one litter the tails were cut off as close as possible to the origin. In the other litter a slender silk thread was tightly tied about the tail of each rat at the same place. The tails of the first litter healed quickly; the tails of the second litter dropped off in a few days with apparently no inconvenience to the animals. Both litters grew normally and their eyes opened at the usual time–at the end of fifteen days. All were allowed to live, and when nearly a year old, four animals (a male and a female from each litter) were killed and examined. The data concerning them are presented in table 3.

Table 3
Data on rats tailless by operation

Only changes in the caudal vertebrae need be mentioned here since all the sacral vertebrae of the four rats were present and normal.

Of the two rats with tails cut, rat No. 1, a male, had the first four caudal vertebrae and about half of the fifth remaining. Rat No. 2, a female, had the first three caudal vertebrae and about half of the fourth remaining. The appearance of the most posterior vertebrae of this rat and their relation to the pelvic girdle may be seen in figure 2.

Of the two rats with tails tied, rat No. 1, a male, had the first four and about half of the fifth caudal vertebra; while rat No. 2, a female, also had the first four and a part of the fifth caudal vertebra. The appearance of the most posterior vertebrae of this last rat and their relation to the pelvic girdle may be seen in figure 3.

Fig. 2 — Tail cut rat No. 2, dorsal aspect; 1, sixth lumbar vertebra; 2, four sacral vertebrae; 3, first four caudal vertebrae; 5, pelvic girdle. Fig. 3 — Tail tied rat No. 2, dorsal aspect; 1, sixth lumbar vertebra; 2, four sacral vertebrae; 3, first four caudal vertebrae; 4, fifth caudal vertebra; 5, pelvic girdle.

In each of these four rats the vertebral column extended to the posterior end of the body and all of the caudal vertebrae present were normal, except the fraction of the vertebra actually cut or tied and this showed only slight deformity.

A comparison of the vertebral conditions in the congenitally tailless rats and in those rendered tailless by operations gave a marked contrast. In the former the vertebral column ended in the pelvic region, far anterior to the posterior end of the body, and also the terminal vertebrae were more or less deformed and fused; in the latter the vertebral column extended to the posterior end of the body and the vertebrae retained their normal size and shape, except those cut or tied, and even they showed only slight modification.

Since in the experiment the tails were severed as close to the body as possible, and since the vertebrae anterior to the severance not only did not disappear but kept their normal form, it is apparent that the removal of a rat’s tail by operation soon after birth does not cause the disappearance of any of the remaining vertebrae, nor does it give rise to deformity or fusion among them. In other words, it does not produce such a condition of the vertebrae as is found among rats born tailless, therefore, taillessness in the rat as defined is due not to accident but to a congenital defect of the vertebral column.

While this paper was passing through the press, one of the two rats spoken of (p. 11) as mated in the Wistar Institute colony was killed and examined, and also two new tailless rats appeared in the colony, making a total of eleven tailless rats observed to date. The rat examined was killed on December 14, 1916. It was a female of standard strain (Mus norvegicus albinus); its age was 273 days, body weight 168.5 grams, and body length 193 mm. The four sacral vertebrae were present, the third and fourth being slightly deformed. There was also one deformed caudal vertebra and the rudiment of a second. The vertebral column ended far anterior to the posterior ends of the pelvic girdle. The other mated rat (a male) that was not examined is of the inbred albino strain, the 22nd generation. The two new tailless rats (a female and a male) appeared in Dr. King’s strain of small-eyed rats and the left eye of the female is small. The vertebral columns of these three rats all end anterior to the posterior ends of the pelvic girdle, for this condition may be felt distinctly by pressing the finger on the rats’ backs in the pelvic girdle region.

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