Morphometry and skeletopy of kidneys and renal vessels in "ring-tailed coati" (

Nasua nasua is a carnivore belonging to the family Procyonidae and is widely distributed throughout South America. The details of its anatomy are fundamental to the application of antomy and understanding of its natural history. This study aimed to measure the average length, width, thickness, and volume of the kidneys; the average length of the renal artery and vein, and to describe the renal and vessel skeletopy in Nasua nasu a. For this purpose, 32 kidneys and renal vessels of 16 cadaveric specimens (eight male and eight female) were dissected and measured using a digital caliper. Pearson's correlation coefficients were calculated between the rostrum-sacral length and the renal and vessel variables. The kidneys had a "bean-shaped" aspect with smooth surfaces and were unipapilate. On average, the N. nasua kidneys measured 30 × 16 × 13 mm, with no significant difference between the sexes. The average right renal artery (1.74 ± 0.67 cm) was longer than the left (1.26 ± 0.43 cm), and the right renal vein (1.22 ± 0.34 cm) was shorter than the left renal vein (1.82 ± 0.46 cm) (p < 0.05). One male animal (6.8%) presented with a double right renal vein as an anatomical variation. Both the right and left kidney cranial poles prevailed at the level of the L2 vertebra, assuming a practically symmetrical position. There was a positive and moderate to high correlation between rostrum-sacral length and renal dimensions and renal vessel lengths. The present data may be useful for interpreting the diagnosis of nephropathies that affect renal dimensions in this species and may contribute to the comparative anatomy of carnivorans.


Introduction
Morphological studies regarding Nasua nasua ("ring-tailed coati") are exciting tasks since they contribute to comprehending ecological traits from a species distributed over practically the entire South American continent. Furthermore, anatomy, one of the oldest biomedical sciences, is a crucial tool for resolving issues regarding the biology and evolution of species, especially concerning interactions involving the habitat environment, feeding, and reproductive adaptations. In addition, the morphology of anatomical structures reveals the natural history of a species (1) .
Nasua nasua (Linnaeus, 1766) is found widely throughout South America and predominantly inhabits Brazil's Atlantic Forest, Amazon Rainforest, and Pantanal regions (2) . They belong to the order Carnivora and family Procyonidae. This family is subdivided into two subfamilies: Procioninae and Ailurinae. The representatives of the subfamily Procioninae include six genera; in South America, there are only the genera Nasua, Procyon, Potos, and Bassaricyon (3) .
In domestic dogs, the kidneys are bean-shaped and retroperitoneally positioned against the sublumbar muscles. The usual position of the right kidney is ventral to the first three lumbar vertebrae; the left kidney is ventral to the second to fourth lumbar vertebrae, although they may be observed more caudally. The right kidney is more settled because it is deeply recessed within the liver and medially related to the right adrenal gland and caudal vena cava, laterally to the last rib and abdominal wall, and ventrally to the liver and pancreas. The left kidney is cranially related to the spleen (or stomach when enlarged), medially to the left adrenal gland and aorta, laterally to the abdominal wall, and ventrally to the descending colon (4) .
The kidneys of domestic cats are relatively large and have a distinctive appearance with capsular veins converging over the surface toward the hilus. They are more mobile than the kidneys of dogs, and the left kidney can be displaced cranially or caudally from their usual position below the second to fifth lumbar vertebrae. Both kidneys are palpable in cats (4) .
Each kidney has a cranial and caudal pole, medial and lateral borders, and dorsal and ventral surfaces. The convex lateral border joins the cranial and caudal poles. The medial border has an indentation (hilus) that defines the renal sinus. The sinus contains the ureter, renal artery, renal vein, lymph vessels, and nerves. Of these structures, the renal artery is the most dorsal and the renal vein is the most ventral. Typically, the renal vein is paired on one or both sides, and sometimes the renal artery may also be paired (5) . Usually, in most species, the right renal artery is more cranial than the left, per the relatively more cranial position of the right kidney (6) .
Knowledge of average renal measures may help diagnose different kidney diseases. Variations in these measures can occur in nephropathies owing to hypertrophic processes and/or atrophy (7) . Therefore, it is imperative to establish a pattern of typical renal measures for each species. It has been postulated from human necropsy studies that variations in renal dimensions and weight are related to gender, being higher in males. The left kidney is known to be larger than the right kidney regardless of sex (8) . However, this information is scarce for wild animals.
Despite an increase in basic and applied research involving wild animals, anatomical studies describing renal anatomy and vascularization in these species are scarce. However, concerning morphology, a small number of studies concerning these species have shown that it is crucial to obtain anatomical information both for future work and for preserving the species in captivity or in natural habitats. Therefore, this study determined the morphometric variables of the kidneys and renal vessels in N. nasua and compared the data, with an emphasis on the order Carnivora.

Materials and methods
This study was approved by the Research Ethics Committee of Federal Rural University of Rio de Janeiro (018/2017). Anatomical dissections were performed on 16 adult ring-tailed coatis cadavers without visible signs of renal pathology: 8 males and 8 females with a rostrumsacral length of 35.38 ± 9.620 cm and 35.56 ± 5.500 cm, respectively. Specimens were obtained from the Laboratory of Teaching and Research in Morphology of Domestic and Wild Animals collection of the Department of Animal and Human Anatomy, Federal Rural University of Rio de Janeiro.
For fixation of the specimens, the thorax was first opened and dissected, and urethral catheter number 6 was introduced into the thoracic aorta. The arterial system was then "washed" with 0.9% NaCl solution and the cadaver was fixed with 10% formalin solution. Next, an aqueous solution (1:1 dilution) of Petrolátex S-65 (Refinery Duque de Caxias-REDUC-Petrobras, Duque de Caxias-RJ) was injected together with a dye (Xadrez ® ). Finally, the specimens were immersed in a 500 L low density polyethylene box containing 10% formaldehyde solution to complete the latex fixation and polymerization processes. At least seven days after latex injection, the cadavers were washed in running water, and their peritoneal cavities were opened and dissected to demonstrate the kidneys, their respective vessels, and skeletopy. Renal measurements (length, width, and thickness) were obtained using a digital caliper (0-150 mm, 0.01 mm resolution, accuracy ± 0.02 mm).
The ellipsoid volume of each kidney was estimated according to Sampaio (9) (1995). The length and origin of the renal arteries and main branches, and the length of the renal veins were also determined. The mean and standard deviation of kidney and renal vessel measurements were calculated and compared between the sexes using unpaired Student's t-test. A Chi-square (χ2) test was performed to test the null hypothesis for skeletopy of the right and left kidneys. Pearson's correlation coefficient (-1< r < 1) estimated the relationship between the renal measures and the rostrumsacral length. Differences were considered statistically significant at P < 0.05. Data were analyzed using GraphPad Prism 5 software.

Gross anatomy
Both kidneys were symmetrical in shape and presented a format that resembles a "bean seed." They were brownish, pale, smooth on their surface, and devoid of lobations. The kidneys consisted of an outer renal cortex and an inner medulla, with the pelvis and renal crest in the longitudinal and transverse sections. They were simple and unipapillary (Figures 1 and 2).
The average measurements of the left and right kidneys of the coatis did not differ, despite significant antimeric differences in the lengths of the renal arteries and veins (Table 1).   Average measurements of kidneys and renal vessels did not differ between sexes ( Table 2).
The skeleton of the right kidney cranial pole prevailed at the L2 or L3 level in 68.8% of samples. In addition, the left kidney cranial pole predominated at L2 or caudally in 56.3% of the animals (   The average rostrum-sacral length was 35.47 ± 7.57 cm (n = 16), being 35.38 ± 9.62 cm in males (n = 8) and 35.56 ± 5.49 cm in females (n = 8), without statistically significant differences (P = 0.96). When considering the whole sample (n = 16), the rostrumsacral length was positively and significantly correlated with right and left renal lengths, renal volumes, renal artery lengths, and renal vein lengths (Table 4), except for left renal artery length. On the other hand, the coefficients calculated separately by sex still demonstrated a positive correlation between these measurements but were not always statistically significant (Tables 5 and 6).  Table 4. Pearson's correlation coefficient ("r") between the rostrum-sacral length (RSL) and renal length and volume, and vessel lengths in N. nasua (n = 16) Table 5. Pearson's correlation coefficient ("r") between the rostrum-sacral length (RSL) and renal length and volume, and vessels lengths in male N. nasua (n = 8) Table 6. Pearson's correlation coefficient ("r") between the rostrum-sacral length (RSL) and renal length and volume, and vessels lengths in female N. nasua (n = 8)

Renal vessels
The average length of the right renal artery (1.74 ± 0.67 cm) was significantly greater than that of the left renal artery (1.26 ± 0.43 cm). In all males (n = 08) and females (n = 8), the renal arteries were dissected and emerged directly from the abdominal aorta as a single vessel (Figure 3). The right renal artery emerged at the level of L1 in 12.5% of males, between L1 and L2 in 12.5%, L2 in 62.5%, and L3 in 12.5%. The left renal artery emerged at the level of L1 in 12.5% of males, L2 in 50%, between L2 and L3 in 12.5%, and L3 in 25%.

Veins
The average length of the right renal vein (1.22 ± 0.34 cm) was significantly less than that of the left renal vein (1.82 ± 0.46 cm). The renal vein was a single vein in 15 animals (93.8%); however, one male specimen had a duplicated right renal vein (6.2%).
In males, the right renal vein drained into the caudal vena cava at the level of L1 and L2 in 12.5%, L2 in 62.5%, and L3 in 25%. The left renal vein addressed the caudal vena cava at the level of L1 and L2 in 12.5%, L2 in 50%, L2 and L3 in 12.5%, and L3 in 25% of male coatis.
In females, the right renal vein drained into the caudal vena cava at the level of L1 in 12.5%, L2 in 25%, between L2 and L3 in 25%, and L3 in 37.5%. The left renal vein emerged at the level of L2 and L3 in 62.5% and L3 in 37.5% of female specimens.
These differences in renal measurements between carnivorans may correlate with body size of the species. In fact, all renal measurements were positively and significantly correlated with rostrum-sacral length in N. nasua (r > 0.5, and P < 0,05). The same has been reported for the ellipsoid volume in domestic cats (12) .
Renal measurements are relevant to clinical decisions and serve as surrogates for renal functional reserve (14) . In addition, morphometric values provide indirect clues regarding renal disease progression and stability. Unfortunately, there is a lack of information regarding the kidney sizes of species commonly cared for in wild animal medicine. Among the common kidney diseases in the wild, dioctophymiasis can affect N. nasua (15) .
The set of renal dimensions reported in this study serves as a preliminary parameter for interpreting imaging findings and necropsy in N. nasua. However, these data must be cautiously applied because the body size of one particular specimen might result in a normal kidney with higher or lower dimensions than those proposed in our study. For example, in C. thous individuals, the kidneys of the southern population were significantly larger, probably because of different body sizes and diets (11) . Furthermore, the present sample of coatis provided data from free-living young adults, whereas captive animals certainly have a different diet and are more long-lived. Furthermore, anthropogenic interactions may include coatis prone to obesity (16) . An abdominal ultrasound study in seven N. nasua (17) revealed kidney dimensions (3.0 × 1.8 × 1.6 cm) very close to our data, confirming the reliability of the morphometric base. These aspects must be considered when interpreting our findings.
Despite the strong correlation between renal dimensions and body size in N. nasua, there were no differences in kidney measures between the sexes. One explanation is that rostrum-sacral lengths were similar between males and females (P > 0.05) in our sample. However, biometric reports indicate that male coatis are generally larger (16,18) . L. gymnocercus (10) and C. thous (11) are other examples of carnivorans in which renal dimensions do not differ between sexes. Nonetheless, a number of imaging studies have suggested that the kidneys of male dogs are significantly larger (19)(20)(21)(22) .
The "bean-shaped" or ovoid kidneys observed in N. nasua represent a trait observed in other carnivorans such as domestic dogs (5) and cats (12) , L. gymnocercus (10) , C. thous (11) and M. p. furo (13) . However, bean-shaped kidneys are not synapomorphic in Carnivora species. For example, Lontra has an elliptical multilobular kidney that presents lobes at the organ surface (23) . In addition, an abdominal ultrasound study revealed an oval shape of N. nasua kidneys (17) .
The skeleton of the cranial pole of the right kidney prevailed caudally or at the L2 level (68.5%), whereas the cranial pole of the left kidney predominated caudally or at the L2 level (56.3%). With the cranial pole at the level of L2 or caudally, both kidneys may be, at least partially, identified on abdominal palpation during clinical examination. Based on skeletal data, the right and left kidneys are considered practically at the same vertebral level, although an abdominal ultrasound study suggested that the right kidney is slightly cranial in N. nasua (17) . This symmetric positioning at the same vertebral level differs from previous reports in other carnivorans. C. thous (11) , domestic dogs (5) and cats (12) , and M. p. furo (13) have the right kidney usually cranial to the left, similar to other wild mammals such as Didelphis albiventris (24) . A cranially positioned right kidney is also prevalent among domestic ungulates, other than pigs (25) .

Renal vessels
In N. nasua the right renal artery is longer than the left artery, and the left renal vein is longer than the right vein. This confirmed our expectations because the abdominal aorta is positioned slightly to the left and the caudal vena cava to the right of the median plane. However, the right renal artery is not longer in C. thous (11) , L. gymnocercus (10) , and domestic felines (12) . As in coatis, the left renal vein is longer in domestic dogs (5) , L. gymnocercus (10) , C. thous (11) , and domestic cats (12) .
One male specimen had a duplicated right renal vein. Observation of the double renal vein has been reported in domestic dogs as being as rare as 1%, but also restricted to the right vein (26,27) . Double and triple right renal veins have been described in domestic (28,29) and wild carnivorans, such as Leopardus pardalis (30) , Leopardus tigrinus (31) , and Puma concolor (32) . Therefore, variation in the number of renal vessels, especially the double right renal veins, must be considered in planning kidney surgery for carnivorans.

Conclusion
The kidneys of N. nasua have a shape similar to that of most other carnivoran species; however, the right and left kidneys are symmetrical inside the cavity. Furthermore, N. nasua kidneys did not differ between the sexes. Our data may serve as a basis to recognize an enlarged or reduced kidney during wild animal medical procedures; however, the positive and strong correlation of the dimensions of the kidneys with the individual body size must also be considered.