1. Introduction
The large group of marine mammals has developed numerous anatomical adaptations in response to their aquatic environment over time. Among them, cetaceans and sirenians stand out for presenting the highest degree of anatomical modifications, because they have achieved a complete adaptation to aquatic life. These two groups are distinguished, among other characteristics, by their feeding. Thus, while sirenians are the only herbivorous marine mammals, cetaceans are considered mostly carnivorous.
Cetaceans are divided into two main groups: those with teeth (Odontocetes) and those without teeth, or baleen cetaceans (Mysticetes). The Pontoporia dolphin (
Pontoporia blainvillei), also known as the Franciscana dolphin, is classified in the suborder Odontocetes, which comprises six families and 71 species. Between them, the best known are the dolphins, but the odontocetes, or cetaceans with teeth, also include the killer whale, the sperm whale, the beaked whales, the beluga whale, the porpoises, and the narwhal. Many of them have undergone remarkable adaptations to their corresponding habitats and/or feedings, such as the loss of teeth in the upper arch (sperm whales), a small number of teeth (beaked whales), long beaks with small dorsal fins (river dolphins), or a rounded head and the absence of a beak and dorsal fin (narwhals and beluga whales). Many of these adaptations are related to their digestive system [
1,
2].
The Franciscana dolphin is endemic to the Atlantic coast of South America, and it represents one of the most emblematic species of the coastal waters of Argentina and Brazil [
3]. Within its distribution range, up to five genetically distinct populations have been identified [
4]. Like other cetaceans with coastal habits, the Franciscana is particularly vulnerable to the impact of human activities [
5,
6,
7]. Habitat degradation, pollution, and, above all, bycatch have led to a high mortality rate of the species in recent years [
8,
9,
10,
11,
12]. As a consequence, it has become the most endangered small cetacean in the southwest Atlantic Ocean [
13,
14], and has been classified as “Vulnerable” by the International Union for Conservation of Nature (IUCN) [
15]. According to the Scientific Committee of the International Whaling Commission [
16], an incidental population mortality of 1% per year is of concern in small cetacean populations, and a mortality of 2% may not be sustainable. For the Franciscana dolphin, annual mortality reaches between approximately 2% and 5% [
8,
11,
17,
18].
In Samborombón Bay (Argentina), where the species has been studied in greater detail, two distinct habitats have been observed: a brackish water estuary and an adjacent marine system. Studies have confirmed differences in the predominant prey species, but fish, especially juveniles less than 8 cm, as well as small loliginid squids constitute the preferred food in both territories [
19]. In addition, evidence of predation by Franciscanas has been observed from a very early age, at approximately 2.5–3 months, when they initiate a transitional diet combining mother’s milk with solid food. Weaning in Pontoporia is gradual, with early predation on crustaceans and fish, and the lactation period is estimated to last 6–7 months [
19,
20].
In light of the above, the authors wonders what is meant by “diet”. In fact, what is diet? What does this term mean? What is meant when one talks about diet? The word diet comes from the Latin “diaeta”, and this in turn derives from the Greek “dayta”, which means regime of life. Diet, in nutrition, refers to the regular consumption of food and beverages. Therefore, a balanced diet is one that contains different types of foods in certain amounts and proportions so that requirements of calories, proteins, minerals, vitamins, and alternative nutrients are adequately met. But diet can also be defined as the total amount of food consumed by an animal or a group of animals. Moreover, diet can be considered the sum of the meals that an individual eats. This first statement includes certain parameters, such as food preferences, the prey’s size, the degree of feed hardness, and daily feeding frequency, among others.
These are the parameters that were considered in carrying out this work. Thus, the main goal of the present study was to describe in detail the digestive system of the Franciscana dolphin in trying to determine whether these anatomical characteristics could be conditioned by several factors in relation to its diet or if the digestive system of this species is modified by phylogeny.
2. Materials and Methods
This anatomical study was carried out on 19 cadavers of the species
Pontoporia blainvillei, made up of 5 adults, 12 juveniles, and 2 neonates. The individuals were classified into three groups according to age and body development according to data published by Arruda Ramos et al. [
21]. Neonates have a body length of approximately 71 cm and no teeth. The vibrissae and umbilical cord can also be observed. Juveniles include individuals with a body length up to 130 cm and the presence of developing teeth (
Figure 1). Adults have a body length greater than 130 cm and the presence of developed teeth.
In all cases, except one, they were found stranded dead on the beach. The dolphin cadavers, in all cases in good conditions, were transferred to the necropsy room of the rescue and rehabilitation center Fundación Mundo Marino, where anatomical dissections of each animal were carried out. Only one neonate was found stranded alive on the beach, because quite often, sick animals go to the shore to die. It later died during the rehabilitation process. All of them belonged to the group Area FMA sub-B (1), a population that inhabits the coast of the province of Buenos Aires. In no case were the animals frozen. Dissections were carried out over a period of less than 12 h from the time the cadavers were found. When it was necessary, the cadavers was kept under refrigeration (for less than 12 h). Practically in all cases, dissections were performed according to the “Documento Técnico sobre Protocolo Nacional de Actuación para Cetáceos Varamientos de Cetáceos” published by BEVACET [
22], with the animals placed in right lateral recumbency. A few cases were placed in ventral recumbency and supine recumbency in order to have a complete visualization of the digestive system’s topography. All dissections were carried out by veterinarians.
To keep the esophagus intact, the superficial musculature of the neck area was removed, exposing the trachea and esophagus. Then, a triangular incision along the inner sides of the jaws was made. The hyoid apparatus was cut, and the tongue was extracted through the intermandibular space. Then, the cardiorespiratory system was progressively separated.
To make the esophagus independent, its connections with the respiratory system (esophagus–trachea) were removed, and it was sectioned at the beginning, exactly at the level of the pharynx. To maintain the esophagus–stomach union, an eyelet-shaped cut was made around the esophageal hiatus. Finally, the entire digestive system was externalized by sectioning the most caudal part of the rectum, including the liver and the pancreas.
As mentioned before, the present study included different specimens—5 adults, 12 juveniles, and 2 neonates—each with obviously different degrees of development. Because of this, the corresponding arithmetic means of the data obtained were calculated separately to avoid deviations and erroneous data.
4. Discussion
Among the described organs of the digestive system of the Franciscana dolphin, there are some that share the typical characteristics of the Delfinidae, and these will not be discussed in this paper. However, the focus will be on those that present particular characteristics of this species related to its diet.
First of all, the low skull asymmetry seen in these animals is directly related to its small prey size. The existence of a relationship between prey size and skull asymmetry has been previously described: animals with more asymmetrical skulls are able to capture larger prey than similarly-sized, more symmetrical animals [
23,
24]. Secondly, because what ultimately determines prey size is the diameter of the maw isthmus and the pharyngeal passage [
25,
26], the width of the maw isthmus in this study was consistent with the size of the prey consumed: small-sized or juvenile fishes, small marine cephalopods (67% = 11–13 cm mean length), and slightly larger squids [
27,
28,
29,
30,
31,
32].
One might think that the elongated and thin snout of the Pontoporia is a common feature in river species and that it could be related to freshwater life. However, this hypothesis does not hold true for Sotalia, which is also a river dolphin. The authors hypothesize that the Pontoporia, the Amazon dolphin, and the Ganges and Yangtze dolphins probably share a common ancestor, contrary to the Sotalia dolphin, which adapted to life in freshwater much later than the others. In summary, this snout morphology would have more justification from an ontological than an adaptive point of view.
These animals have conical teeth of different lengths, as do other estuarine dolphins, such as the Ganges dolphin (
Platanea platinids), which are ideal for catching fish and shrimp at an early age [
33]. The number of teeth in the upper arch of the Franciscana is similar to that of the lower arch, with 110 pieces on average vs. 108 in juveniles and 113 vs. 110.5 in adults, which differs from those species whose diet is based exclusively on squid and that lack teeth in the upper arch, as explained by Cozzi et al. and Martin Diaz [
26,
34].
Unlike the tongue of most dolphins [
35,
36,
37], the tongue tip of the Franciscana is barely mobile [
38] and very short, even differing from other freshwater dolphins [
38]. This is probably compensated by the presence of the anterolateral papillae (anterolateral fimbriae) consisting of compact outgrows placed along a single row developed mainly in infants [
35,
36,
38]. According to Ferrando et al. [
39], these latter papillae may help to create a tight seal between the tongue and the roof of the oral cavity and therefore help with suction feeding, as the period of development of these papillae is coincident with the lactation period [
40]. Even as we have described in the Franciscana, these papillae tend to be reduced or disappear with age, although they may persist in some individuals [
26]. From the age of two months, this species begins to consume solid food and its feeding becomes mixed, and then these papillae assist in holding the prey against the palate while water flows through the papillae [
33,
34,
36,
39,
41] until they begin to involute due to disuse and completely lose functionality in adulthood.
In the present work, the average number of marginal papillae was 3 in adults, 30 in juveniles, and 50 in neonates. These data are consistent with those of Guimaraes et al. [
41], who obtained an average number of marginal papillae of 53 in immature and 36.7 in mature dolphins and only 3.75 on average in adults (being nonexistent in some specimens), but they differ from the 13 to 15 projections counted in juveniles by Yamasaki et al. [
38], which was probably measured in older juveniles.
A multiple-chambered stomach is unusual in carnivores. However, all cetaceans are carnivores and have the presence of a multichambered stomach. The Cetartiodactyla shows a tendency towards plurilocular stomachs, which can be found and characterized by very different architectures in Suidae, Tayassuidae, Camelidae, Tragulidae, Pecora, Hippopotamidae, and Cetacea [
26,
28]. After all, whales are closely related to artiodactyls, which also have multichambered stomachs. While their multiple chambers may relate to the mechanical and enzymatic breakdown of an herbivorous diet (e.g., separation of food to be regurgitated and re-chewed as cud), it is unclear what functions multiple chambers play in the carnivorous cetaceans [
24]. Perhaps, in a sense, they compensate for the absence of chewing by grinding and compressing in the first chamber before absorption takes place in the subsequent compartments [
26].
While all dolphins share the multicameral anatomy of their stomachs, Mead [
42] describes three morphological appearances of the stomach, which he calls the ziphiid stomach (one main stomach, one pyloric stomach), the derived stomach type I (two main stomachs, one pyloric stomach), and the derived stomach type II (two main stomachs, two pyloric stomachs). The Franciscana shows macroscopic features similar to those of a zyphid-type dolphin, with a main stomach, a communicating channel, and a pyloric stomach, as also described by Yamasaki et al. [
43].
In the Franciscana, there is no pre-stomach present, as in all known zyphids and contrary to most eutherian mammals (
Figure 25). This feature is also thought to be shared with the probably extinct Chinese river dolphin (
Lipotes vexillifer), but not so with the Amazon river dolphin (
Inia geoffrensis). Marigo and Groch [
44] reconfirm that some odontocete species lack an anterior or pre-stomach chamber and, instead, the main chamber is divided into two or three compartments separated by septa [
34]. Yet, this is not the case in Franciscana. Storage and mechanical degradation functions have been attributed to the anterior or pre-stomach chamber [
25,
44,
45], which encourages us to think that in the face of the continuous consumption of small prey, this possibility of storage and this reinforcement in digestion for the chemical and physical degradation of the prey are not necessary. With this, the authors say that the storage function is probably being replaced by the increase in feeding frequency (continuous feeder), and, on the other hand, the mechanical action can be performed in the muscular chamber of the main stomach. This structure of the stomach with one less compartment could agree with its antiquity and primitiveness with respect to other cetacean species more evolved or new in the phylogenetic tree. It may also be compensated by the greater length of its small intestine, where enzymatic degradation processes occur.
The stomach values recorded are consistent with the values published by Kamiya and Yamasaki [
46]. After the animal has reached a body weight of 20 kg, the relative weight of the stomach at adulthood contributes only approximately 1.5% of the body weight and does not show considerable changes during body growth. While the relative size of the stomach to the live weight of the Franciscana is consistent with values for a strict carnivore of that size [
47], the same is not true for the relative size of the intestines.
The small intestine of the Franciscana is similar to that of other carnivores. Yet, the total length of the gut in the Franciscana is remarkably high. The reason for this is the extraordinary length of the jejunum, corroborating what Yamasaki et al. described [
20]. According to Crespo (2002), this species feeds mostly near the bottom on fish belonging to several families; it is listed as a predominantly teutophagous odontocete [
48]. The reason for the long intestine in Franciscana and the influence of the types of food are not known. Büker, however, gives an interpretation of the lengthening of the intestinal tract in aquatic mammals: during diving, blood supply to the intestines is reduced, and a longer gut could compensate for the necessary assimilation of nutrients [
49]. The authors would like to add the possibility of the development of a relatively longer small intestine that allows more time for the food to be in contact with the enzymes that will degrade it, as well as a greater absorption surface in response to a lower-quality diet compared to that of other dolphins, comprising bottom-dwelling juvenile teleosts, squid, and shrimp [
31,
50,
51,
52,
53,
54].
Among the toothed whales, there is considerable variation in the ratio between the length of the small intestine and the body length, ranging from 5/1 in bottlenose whales to 14/1 in some species of dolphin [
55]. Baleen whales tend to show lower ratios (fin whales, 4/1; little piked and humpback whales, 5.5/1). These ratios do not necessarily correlate with either the diet or size of the species [
56], with the pontoporie ratio being 24–37.7/1.
The small intestine of Franciscana occupies the greater part of the lower abdominal cavity. The length of the small intestine examined by Yamasaki et al. [
20] is similar to the values found in this work. However, for this same author, the ratio varied considerably from specimen to specimen, and it seems to show no correlation to body length. Yet, it might have a close relationship with the lapse of time after death rather than individual variation. Also, the length varies in fixed and unfixed states. The length shortened about 8.5% after fixation in 10% formalin solution.
As explained by Cozzi et al. [
26] and Marigo and Groch [
44] for other species, for the Franciscana, it has been observed that pancreatic juices flow into the duodenum via the pancreatic duct. In most odontocetes, this process is individual, but in some cases, it joins the hepatic duct, thus flowing into what is called an hepatopancreatic duct similar, in this aspect, to equines [
34,
57], a continuous consumer.
Regarding the arrangement of the jejunal loops in adults, there is a hypothesis that this is in relation to the need to maximize efficiency when taking advantage of the available space. In this way, a longer intestine can be accommodated as time goes by in almost the same abdominal cavity, making space more efficient. And, even as the animals advance in age, their diets become of better quality due to learning how to capture prey and the ecological niches they occupy [
51]. This could be consistent with a greater prey metabolism, and therefore less gas production, which allows for a more orderly arrangement of the loops. However, there is no literature to support this hypothesis.
As mentioned in the results, there is not a particular distinction between the small and large intestines, with no caecum or vermiform appendix present. According to the results of this study, the authors agree with Cozzi et al. [
26], as this apparently simplified disposition results from the diet, which includes mostly proteins of animal origin (generally fish and cephalopods). The exception to the rule is the Ganges River Dolphin, belonging to the family Platinistidae, which possesses a cecum.
The mucosa of the colon shows one or two longitudinal folds that increase its inner surface and disappear as they move caudally. The Gangetic dolphin, on the other hand, has numerous longitudinal folds that begin in the distal colon and continue to the rectum; in the Amazon river dolphin, the folds appear along the entire colon [
28,
58,
59] Although this fact may be disadvantageous with respect to having circular folds for them to widen the absorption area, the extraordinarily long intestines may compensate for the lack of these type of folds [
20,
60].
There are differences between our results and previously published data on the total length of the large intestine of the Franciscana dolphin. Apart from the absence of cecum and the similar thickness of both intestines at the junction point, these variances could be related to the criteria used to define the beginning and end of the large intestine, as there are no clear macroscopic signs. In the study by Mead and Brownel, the values obtained were 40–48 cm [
61], which were within the range of those described by Yamakasi et al., 25–58 cm [
20], and the results of the present study were slightly higher, with 62–81 cm for adults, 45–71 cm for juveniles, and 36–44 cm in infants.
The peculiar morphological characteristics of the gross intestine of dolphins, and specifically the shortage of any bowel-like structure in the gut, could suggest minimal or no storage capacity of undigested residue [
62]. In fact, evacuated feces are generally rather liquid and may contain unprocessed materials, especially when shrimps are part of the diet [
26]. However, the observation made in some of the studied animals that were under human care was of a more consistent fecal matter than that of other dolphins. This may be related to the greater length of the intestines and therefore the greater surface area of water absorption of the feces, which is consistent with less watery feces.
Regarding the relationship between liver weight and body weight, the average did not include the neonate with hepatomegaly that was found stranded alive and transferred to the rescue center for rehabilitation. In this specimen, the liver weight was 7.5% of the body weight. In association with this high percentage, generalized jaundice was observed in the mucous membranes of this animal. Interestingly, this condition has been observed in many of the neonates that were undergoing rehabilitation at the rescue center Fundación Mundo Marino. This finding is not described in the literature. It is currently being investigated by several Latin American organizations collaborating in the conservation of marine fauna. In the rest of the individuals, the relative values were 2.43%, which coincides with those already described by other authors, where the weight of the liver averages 2–3% of the total body weight in dolphins [
21,
26,
63]. However, these values are higher than the relative values of other cetaceans, with the exception of estuarine dolphins [
63], which makes the authors think of a higher metabolic rate in this type of dolphin. These higher metabolic rate data also coincide with the hypothesis expressed by Helm (1983) that the higher the metabolic rate, the longer the relative length of the intestines [
64].
The liver of the Franciscana, like the liver of cetaceans and like that of many terrestrial species, lacks a gallbladder, and the bile produced in the hepatic stroma is carried into the duodenum by a common hepatic duct, as mentioned above. The absence of an extra-hepatic storage organ for the bile is possibly related to the continuous ingestion of food and the consequently frequent presence of food in the proximal intestine [
26], as is the case in continuous consumers.
5. Conclusions
The anatomical characteristics of the digestive system of the Franciscana dolphin are similar to those present in other marine mammals. However, our results show that this dolphin shows several peculiarities, especially in relation to the tongue, the teeth, the stomach, and the small intestine.
Although further studies are needed to confirm the results of the present paper, it could be assessed that the digestive system of the Franciscana dolphin is adapted to a small prey diet, considering the size of its oral cavity, the scarce development of its tongue, and the absence of a pre-stomach that allows for the shredding of larger prey. These animals are continuous consumers. This fact is consistent with the absence of a forestomach and a gall bladder, as well as the presence of a hepatopancreatic duct similar to that of other continuous consumers. The composition of crustaceans and squid in their normal diet could explain the length of their intestine, as it would allow the food to be inside the digestive tract for a longer time to facilitate its enzymatical degradation and absorption.
The importance of this work is clear due to the following reasons. First of all, it is important for anatomists, as there are very few previous descriptions of the anatomy of the digestive system of this dolphin. Then, it is important for physiologists and nutritionists, as anatomy is the basis of a better comprehension of other basic sciences. It is not possible to explain any physiological process without knowing the anatomy of an organ, apparatus, or system. For example, anatomy, digestion, and nutrition are closely related. Once again, the knowledge of the digestive system’s anatomy is critical to understanding certain processes related to the absorption of nutrients, the balance between proteins, fats, and carbohydrates, and metabolism. Therefore, our work supports the idea of anatomy in constant adaptation to nature or external factors to facilitate other processes. And, finally, this paper is also important because the species of interest is in danger, and any knowledge or data about these animals are of relevance and will increase their chances of survival.
In summary, it seems that the protein-based diet of the Pontoporia blainvillei could determine the anatomy of its digestive system, although certain phylogenetic adaptations should not be ruled out. Of course, further studies are needed to clarify this hypothesis.