MORPHOMETRIC CHARACTERISTIC OF THE CAECUM IN LONG-TAILED DUCKS CLANGULA HYEMALIS WINTERING ON THE POLISH BALTIC COAST

Ewa Działa-Szczepańczyk
Department of Zoology and Apiculture, West Pomeranian University of Technology, Poland

ABSTRACT

The caeca of 140 long-tailed ducks (Clangula hyemalis) (87 males:9 immature and 78 adults; 53 females: 13 immature and 40 adults) collected in 1993-2000 in the western part of the Polish Baltic coast were examined. A particular attention was paid to relationships between 4 metric characters: left caecum length (CLL); right caecum length (CRL); left caecum weight (CLW); right caecum weight (CRW) and three characters describing body size: weight (BW), length (BL), and sternum length (SL). The fluctuating asymmetry (FA) in the caecum length and weight was explored by means of the fluctuating asymmetry coefficient (FAC = 1-r^2,; where r is the correlation coefficient of a correlation between parameters selected). Fluctuating asymmetry of paired organs may reflect changes in the homeostasis of wild animals that are affected by various environmental factors; hence FAC may be a valuable proxy of the habitat quality.

The Long-tailed Ducks examined showed mean CLL, CRL, CLW, and CRW to be 90.7 cm; 78.4 cm; 0.47 g; and 0.42 g, respectively. No significant correlations between mean values of BW, BL, and SL with any of the caecum character analysed. Length and weight asymmetries of the caecum showed a pronounced pattern: the left caecum in 127 individuals (90.7%) was longer and in 115 individuals (82.1%) heavier than the right one. The FAC values for the caecum length and weight were 0.472 and 0.437, respectively.

Key words: Long-tailed Duck Clangula hyemalis , caecum, asymmetry.

INTRODUCTION

More than a half of all described bird species up to now is characterised by the presence of two caeca in their digestive system. A great abundance of shapes and sizes of these organs observed in various species is a result of the evolutionary adjustment of birds’ alimentary system to digesting and absorbing food of a given kind [7, 27, 28]. Bird’s caeca is also characterised by great morphological individual variability, which is a consequence of a diet, composition and quality of which considerably vary in time, both as a result of external results – environmental ones (e.g. seasonal changes of quantity and kind of available food, presence of endoparasites) and physiological results (e.g. the influence of hormones). The environment where birds live may significantly influence not only a periodical change of size, shape and colour of caeca, but also the frequency of the occurrence of disproportions in the structure between the left and right caeca. The asymmetry of even internal organs of animals is a commonly observed phenomenon in nature and is called the bilateral asymmetry. There are three kinds of it: directional asymmetry, when one side always dominates the other; anti-symmetry, in which at one time the left side is dominant and at the other time the right one dominates; and fluctuation asymmetry, in which differences between the right and left sides are random and are results of the deviation from genetically conditioned symmetry [49]. Many studies prove that the increase in fluctuation asymmetry may indicate changes in a body’s homeostasis occuring as a result of the influence of unfavourable environmental factors [6, 21, 22, 36, 37, 48, 50, 51]. Therefore, the degree of such asymmetries in the structure of caeca of birds living in the wild may become an indirect indicator of changes in the natural environment of those animals.

The purpose of this study is to analyse selected morphometric features of caeca of the Long-tailed Duck Clangula hyemalis and the description of the degree of fluctuation asymmetry within those features, taking the birds’ sex and size into consideration.

The Long-tailed Duck Clangula hyemalis is a common nesting bird of Arctic part of Eurasia and North America. Birds of the European population winter on the Baltic, North, and Black Seas [18]. The Long-tailed Duck stays on the Baltic from October to March [10, 17, 25]. The species is rated among the group of absolute number dominants [47]. During the nesting period, when the species lives on lakes and slowly flowing rivers of tundra, fjords and small sea islands, it mainly feeds on animal food with addition of plant stuff (algae, grass blades, seeds, and other parts of tundra’s plants). In winter it eats animal food – mainly crustaceans, to a lesser extent – fish, insects and their larvae, and sporadically – parts of plants [18].

According to the classification used by McLelland [27] in relation to birds’ caeca, based on the size of those organs, the Long-tailed Duck is included in the group of species with well-developed two caeca.

METHODS

The study included caeca of 140 individuals of the Long-tailed Duck Clangula hyemalis – 87 males (9 immature and 78 adult) and 53 females (13 immature and 40 adult). Dead birds were collected in winter-spring seasons in the years 1993-2000 in the seashore areas situated in the western part of the Polish Baltic Coast. The birds were divided into two age groups – immature birds, under 1 year of age, and adult birds, over 1 year old. Three measurements describing the birds’ body size were made: body length (BL), measured from the beginning of a beak to the end of a rump (with soft tape, exact to 0.5cm); sternum length (SL), measured with a vernier calliper gauge, exact to 0.5mm – according to Dziubin and Cooch’s suggestion [14], and body weight (BW), exact to 10g. The caeca’s morphometry included: left caecum (caecum sinister) length (CLL) and weight (CLW) and right caecum (caecum dexter) length (CRL) and weight (CRW). The measurement of caeca was made with a vernier, exact to 0.1mm. The weight of those organs was determined exact to 0.01g after cleaning them off chyme and drying with filtration tissue. Moreover, selected elements of a digestive system were included in the measurements. Five parameters were determined: oesophagus length (EL), small intestine length (SIL), and terminal intestine length (exact to 0.5cm), using Leopold’s method [25]; empty gizzard weight (GW) and liver weight (LW) (exact to 0.01g) were also established.

The obtained results were processed statistically. Basing on t-Student test, the significance of differences between males and females was established in respect to the birds’ body parameters, selected parameters of digestive system and the analysed parameters of both caeca. Owing to the small number of immature individuals, no such comparisons were made in respect to age groups. t-Student test were also used to establish the significance of differences between left and right caeca in respect to the average values of the measurement of weight and length of those organs – separately for the group of males and females, and in the relation to the whole group of birds. In order to examine relationships between the values of caeca parameters and parameters describing the birds’ body size, as well as between individual parameters of those organs and parameters of selected digestive organs, appropriate Pearson’s linear correlation coefficients (r) were calculated. The size of fluctuation asymmetry in the length and weight of caeca was characterised with fluctuation asymmetry coefficient (FAC), suggested by Van Valena [49]: FAC = 1 – r², where r means correlation coefficient.

RESULTS

An average length of caeca for the examined group of long-tailed ducks reached the value of 90.66mm for the left caecum and 78.43 for the right one; similarly, an average weight of those organs reached the values of 0.47g and 0.42g. Females and males differed significantly in the body size – in all three body parameters the drakes reached higher average values than the ducks. No statistically significant differences between the groups of males and females in relation to all four analysed caeca parameters were noticed (Table 1).

Table 1. Differences in absolute body and caeca parameters between males and females of the Long-tailed Duck Clangula hyemalis, M – males, F – females, – mean, SD – standard deviation, CV – coefficient of variation, t – value of t-Student’s statistics, NS – difference non-significant, p – level of significance, BW – body weight (g), BL – body length (mm), SL – sternum length (mm), CLL – left caecum length (mm),CRL – right caecum length (mm), CLW – left caecum weight (g), CRW – right caecum weight (g)

No significant correlations of caeca parameters with any of the parameters describing the birds’ body size were found. The value of coefficient for all correlations occurring between caeca measurements (CLL, CRL, CLW, CRW) turned out to be positive and statistically significant (from 0.365 to 0.726 at p< 0.001). Out of five digestive organs’ parameters, only gizzard weight did not show any relationship with any of the caeca measurements. Remaining parameters of digestive organs positively correlated with at least one caeca parameters. The most number of significant relationships were found in relation of small intestine with the caeca parameters. Liver weight was significantly correlated only with right caecum parameters – CRL and CRW (Table 2).

The birds’ post-mortem revealed clearly visible asymmetry in the structure of caeca. The asymmetry in length and weight of those organs had clearly indicated direction. Out of 140 examined birds, in 127 (91%) longer left than right caecum was found; and in 115 (82%) of long-tailed ducks it was heavier. Only in one individual the caeca were the same length, and in seven birds both organs had the same weight (Table 3). The noticed difference in the length and weight between left and right caecum turned out to be statistically significant. Fluctuation asymmetry coefficient FAC for caeca weight was 0.470; and 0.437 for their weight.

DISCUSSION

Long-tailed Ducks have well developed two caeca shaped as long blind bags with quite even diameter on the whole length. They are situated opposite each other along the ileum, directed with the tops towards the head, and their opening is situated in the zone of the small intestine transition into terminal intestine (Fig. 1, Fig. 2). The obtained results concerning caeca length and weight in the examined group of long-tailed ducks were similar to the ones obtained by other authors. In studies by Barnes and Thomas [3] an average weight of both caeca of fourteen long-tailed ducks with an average body weight of 716 g was 0.61g. In studies of Goudie and Ryan [16] twenty Long-tailed Ducks with an average body weight of 758.8 g had both caeca with an average length of 20.1cm. Caeca of 26 Long-tailed Ducks (with an average body weight of 873 g) had the following lengths – left one 92 mm, and right one – 79 mm [43].

The shape and structure of caeca of the Long-tailed Duck is clearly connected with the kind of food the birds feed on. Many studies indicate that morphological features of the digestive system of birds reflect their diet [9, 23, 30, 31, 32, 46]. The studies conducted by Kehoe and Ankney [20] on five representatives of genus Aythya indicate a great dependence of caeca size on food the individual species of Anatidae eat. The authors [20] noticed that a similar relationship may be observed in Galliformes birds – the more is this order’s representative’s food loaded with indigestive elements, mainly fibre, the bigger caeca of a given species. Similar comparisons were made with 18 individuals of the family Anatidae and they showed that herbivorous species had heavier caeca than omnivorous and carnivorous species [3]. The Long-tailed Duck is a predatory species and it takes small amounts of plant food only in summer. Cottam [8], examining 190 adult long-tailed ducks during 11 months, stated that animal food constituted 87.93% of food mass in this species. According to Stott and Olson‘s studies [40], conducted on 40 long-tailed ducks wintering on the north-west coast of the USA, Gastropoda from the species Laguna vincta constituted 54%, and shrimps (Caridea) from the species Crangon septemspinosus – 31% of the contents of the oesophagus and both stomachs of this species. Next, studies conducted on 127 Long-tailed Ducks in autumn and winter on Canadian cost showed that crustaceans constituted as much as 89% of wet food mass of the oesophagus and glandular stomach, and the specific structure of the crustacean varied depending on a feeding ground, i.e. on its distance from the seashore [19]. Bustnes and Systad [5] indicated that Long-tailed Ducks examined by them significantly changed feeding preferences of eaten animal species in different months of winter. In November, Gastropoda dominated in their diet (60% of wet food mass content), and in April – capelins Mallotus villosus (Pisces), which constituted almost 100% of wet food contents. Stempniewicz [39], in his studies on the Long-tailed Duck wintering on the Bay of Gdańsk, found that the birds’ main food were bivalves, whose weight participation in the food mass of the oesophagus and both stomachs was 77%. Presumably, the preferences concerning the amount of individual food elements may also influence the digestive organs’ parameters. In Bustnes and Systad’s studies [5] wintering Long-tailed Ducks most often chose Gastropoda with the shells’ height of 2.6-5.0mm. According to some authors, not only the kind and amount of food can influence the parameters of birds’ digestive organs, but also time devoted to feeding. According to Goudzie and Ankney [15], the Long-tailed Duck, with relatively small body weight, feeds long comparing to bigger ducks, such as the Common Eider Somateria mollisima or Common Scoter Melanitta nigra, and its main food is molluscs – ca. 70% (mainly Mytilis eddulis and Laguna vincta).

No relationship of caeca length and weight with the birds’ body parameters was found in the examined group of long-tailed ducks. Similar results were obtained in the studies on 26 long-tailed ducks, in which the lack of correlation between both caeca length and the birds’ body weight was proved [43]. The lack of correlation between the parameters describing body weight and parameters of caeca was also recorded in case of the Common Scoter Melanitta nigra [13], Velvet Scoter Melanita fusca [41] and Mallard Anas platyrhynchos [11].

No dimorphic differences were noticed in caeca parameters, although body size differences between males and females of the examined Long-tailed Ducks were statistically significant. Similar results were obtained in relation to 26 individuals of the Long-tailed Duck – despite a clear difference in body weight in favour of males (however, not confirmed statistically), no significant differences in caeca length between the group of drakes and ducks were recorded [43]. Also in the studies carried out on the Common Scoter Melanitta nigra no significant differences in caeca length and weight between males and females of this species were found, despite the clearly proved sexual dimorphism in those birds’ body size [13]. The studies with similar character conducted on the Mallard Anas platyrhynchos showed that males had longer right caecum than females of this species [11]. In relation to the domesticated form of the Mallard, it was noted that bigger drakes had lighter left caecum than smaller ducks [12]. Dimorphic differences in the structure of digestive organs observed in many bird species may be influenced by the diversity of feeding between males and females [2]. The results of Bengston’s studies [4] showed that Long-tail Ducks females in their diet preferred Chironomid larvae Chironomidae (86% of wet food mass of the oesophagus), and males preferred Chironomid larvae Chironomidae (62% of wet food mass) and fish eggs (35% of wet food mass of the oesophagus). In the studies carried out on the Tufted Duck Aythya fuligula, the recorded differences in drakes’ and ducks’ diet concerned both eaten species of mussels and the size of their shells – males ate bigger mussels [34]. Ankney and Afton [1] noticed that males and females of the Shoveler Anas clypeata in the nesting period considerably differed in food preferences, which was reflected in their caeca weight and length. The differences between sexes in caeca sizes may also have genetic grounds. Miller’s [29] experiment performed on wild Mallards Anas platyrhynchos kept in captivity proved that females’ intestines had bigger adaptation abilities in relation to the diet changing in time than males’ intestines of this species.

Both the lack of relationship of long-tailed ducks’ caeca sizes with body parameters and the lack of dimorphism connected with sex in those organs’ parameters may indicate their great morphological flexibility. In the representatives of the family Anatinae the change of the amount and kind of food causes faster reaction of caeca in comparison to other birds [9, 29, 30, 32, 35].

In the examined group of Long-tailed Ducks, no significant correlation of caeca parameters with gizzard weight was found. Presumably, the dynamics of change in caeca structure is bigger than in gizzard, which results in the lack of correlations between those organs’ measurements. Moreover, positive value of correlation coefficient for the relationship of most caeca parameters with small intestine length and terminal intestine length may suggest a similar reaction of those two organs to changes in diet.

In overwhelming majority of individuals of the examined group of Long-tailed Ducks the asymmetry of length and weight between left and right intestine was found, and the majority of birds had longer or heavier left caeca than right ones. Similar asymmetry in length (or weight) in favour of the left caecum was noticed in the Mallard Anas platyrhynchos – both in the domesticated form [12] and the wild one [11], and in the Common Scoter Melanitta nigra [13]. In the studies conducted on the caeca of 24 individuals of the Long-tailed Duck, clear number domination of birds with longer left caecum than right one was noticed [43] 1999). In case of the Red-necked Grebe Podiceps griseigena, all individuals had longer left caecum than the right one [44]. An opposite situations, when individuals with longer right caecum outnumbered the ones with longer left caecum, were recorded in case of the White-fronted Goose Anser albifrons [42] and the Bean Goose Anser fabalis [45]. In Sitna’s [38] studies, a half of the examined individuals of the Coot Fulica atra had longer left caecum than the right one (Table 4).

In the examined group of Long-tailed Ducks a statistical significance in the length and weight between left and right caecum was proved. The value of fluctuation asymmetry coefficient for caeca length and weight of the examined group of Long-tailed Ducks was different for the same characteristics in the Common Scoter. In the mentioned species, the value of fluctuation asymmetry coefficient in case of caeca length was lower than in the examined Long-tailed Ducks and reached 0.368, and for the weight of those organs it was higher and reached 0.502 [13]. The obtained result may be connected with the difference in the structure of caeca between the mentioned species. The Common Scoter has very short caeca, and the symmetry of their structure is especially visible in the weight of those organs.

CONCLUSION

1. No sexual dimorphism was found in all analysed caeca parameters, despite noticed differences in body size between males and females.

2. No relationship was found between caeca measurements and Long-tailed Ducks’ body measurements.

3. The most numerous relationships of caeca parameters’ values with the parameters of remaining elements of digestive system were recorded in small intestine and terminal intestine lengths. Gizzard weight was in no relationship with caeca measurements.

4. The asymmetry between left and right caecum was visible both in caeca length and weight

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Accepted for print: 16.11.2006

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