Intestinal development directly affects the digestion, absorption and metabolism of nutrients, which is very important for health and growth potential. The increase peak of small intestine length of broilers is earlier than their daily intake and daily gain, and the relative weight can reach the peak only after the relative growth speed of digestive organs reaches the peak, suggesting that the quality and index of intestine can reflect the biological and ecological conditions and functions of animals in the growth and development stage. Intestinal transit tends to be slower in female for the steroidal hormones would promote intestinal hypomotility and inhibit gastric emptying by acting as a smooth muscle relaxant , and progesterone also could decrease gastrointestinal motility through an inhibitory effect on motilin. However, a completely different angle is provided in this experiment rather than hormone. Previous studies have shown that postprandial changes in intestinal volumes are higher in males than females by using a single photon emission computed tomography , and the volume of post-mortem fluid in the intestine were higher in men than in women after standardization by body weight, indicating that different gender had certain influence on the capacity value of intestine. According to the results of this experiment, greater RL of the female duck lead the longer intestinal emptying time, while the greater ID of the male duck contribute the more effective peristalsis, suggesting that the digestion cycle of male duck is shorter than females.
The digestion and absorption ability of the intestine to nutrition depends on the joint action of pancreas, enzyme activity, surface area and nutrient transport carrier [23, 24], and the surface area of intestinal villi is the key factor limiting the growth of poultry[25, 26]. Another structure of the mucosal layer, crypt, transports nutrient molecules from the digestive tract to the blood for use, and the value of CD can reflect the renewal speed of the small intestine epithelium. Therefore, the higher CD value of the female ducks meaned faster update of epithelial cells in intestine. VH is adjusted correspondingly with the change of animal function and intestinal nutrient demand under normal circumstances, which is realized by the increment rate and intestinal cell turnover rate of intestinal crypt cell . The intestinal epithelium is a complex multicellular system, closely related to the growth potential. Sexbased differences of infection and inflammation were found in the intestine of 2-week-old C57BL/6 mice . In this study, the digestive capacity of the male duck’s jejunum was lower than females in the early stage due to VH/CD, and then higher than females in the later stage with the development of VH, which is similar with intestinal microbial diversity, suggesting there is consistency between intestinal development and intestinal microorganisms in ducks.
The balance and stability of intestinal microorganisms play an important role in maintaining the health of the body. Gender-related differences in intestinal microbial communities have been observed in many animals, such as mice, macaques and humans[29–31]. Samples from same gender wer found clustered together by Principal Component Analysis of T-RFLP and DGGE profiles from either human flora associated rats or specific pathogen free rats. Androgens could impact intesyinal microbial composition in lupus-susceptible mice and protect males against the development of lupus. Besides, males have higher serum SCFAs level come from the microbial fermentation by comparing with females, such as butyrate, propionate and acetate,which have a myriad of anti-inflammatory effects[34, 35]. Likely causes not only includ differing sex hormone levels in males and females but also influenced by the intestinal microbiota.
The evenness and diversity of the intestinal microflora of the female duck were higher by comparing with the male duck, and the distribution of the intestinal microflora were significantly separated. Proteobacteria is the largest family of bacteria, including many pathogens and free-living species, such as E.coli, Salmonella and many bacteria that can carry out nitrogen fixation, covered with lipopolysaccharide which can cause strong pro-inflammatory effect and secrete pro-inflammatory factors including interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) , and γProteobacteria increase significantly in intestinal in NAFLD and NASH patients. Bacteroides was initially known for its pathogenicity, and then found it was a large number of normal bacteria in the body, which played an important role in the host's nutrition absorption, fat accumulation, gastrointestinal flora balance and body immunity. The difference of the abundance of Proeobacteria and Bacteroides between the male and female ducks suggested stronger metabolism ability weaker anti-inflammatory ability in male ducks. It has been found that the abundance ratio of Firmicutes to Bacteroides in intestinal microorganisms can reflect the degree of obesity to a certain extent [37, 38], and the relative proportion of Bacteroides in obese and thin people is low , and the decrease of Bacteroides was accompanied by the increase of Firmicutes . In this experiment, the difference of the intestinal Firmicutes/Bacteroides and body wight between the male and the female ducks appeared at 10 weeks, suggesting that the fat deposition potential of male ducks was stronger females.
Microorganisms in the intestine affect the health and growth performance of the host through participating in different functional pathways. These was no significant difference in functional pathways although there were significant differences between male and female ducks at 2, 5 and 10 weeks. Only jejunum showed significant differences of function pathways between the two sexes, which was consistent with the value of VH/CD. Lipid metabolism is an important and complex biochemical reaction [42, 43], processed into substances needed by the body through digestion, absorption, synthesis and decomposition with the help of various enzymes and bile salts to ensure the normal operation of physiological functions [44, 45]. It has been found that the upper part of small intestine was the mainly place where fat digested and hydrolyzed to glycerol and fatty acids . The higher abundance of lipid metabolism pathway in male duck’s jejunum in this experiment may be related to the higher nutritional absorption capacity of male ducks. Neverthless, The disease pathway of the male duck is significantly higher by comparing with the female duck, which noteworthy in today's intensive aquaculture and it is necessary to strengthen the vaccine requirements and prevention and control measures for the male duck. Different from the male duck, the females have higher abundance of gene aspect. Nucleotide is the precursor of synthetic biological macromolecular RNA and DNA, which plays a dominant role in the growth, development, reproduction and heredity of organisms. Congenital whey aciduria would caused by the damage of pyrimidine nucleotide metabolism, and ventilation, Lesch Nyhan syndrome and immune deficiency by the damage of purinine nucleotide metabolism[48, 49]. It has been reported that the male to female ratio of gout patients is 20:1 and the incidence of obesity is higher in male, which is consistent with the high abundance of nucleotide metabolic pathway and the weak ability of fat deposition in this experiment.