Introduction

Currently, all extent wild populations are estimated to consists of fewer than 100 breeding tigers that are restricted to isolated patches of suitable protected habitat in India (Ranganathan, 2007 and Walston, 2010). In combination with a reduction in the amount of habitat, overhunting of tiger prey has reduced carrying capacity of much of the remaining tiger habitat (Karanth, 1999; Jhala, 2011). In small fragmented populations, the occurrence of inbreeding is inevitable (Spielman, 2004). Moreover, tigers are subjected to survive only by human conservation efforts, like keeping them in Zoo habitat, National Parks, Wildlife Sanctuaries under the human guidance. But for survival of tigers, small populations inside zoos are bred among themselves resulting inbreeding. The greater the degrees of inbreeding, the greater loss of hybrid vigor will result. Inbreeding also reduces the genetic diversity and viability of the individual. Genetic diversity is defined as the variability within a species due to genetic differences between individuals (Lawrence, 2008). Larger genetic diversity will increase chance of individuals surviving in changing environments while a reduced genetic diversity may result in an enhanced susceptibility to diseases and other environmental challenges (Snyder et al., 1996). It is clear that a high genetic diversity is of great importance for species survival. Small populations with limited dispersal probabilities and endangered species have problems with maintaining a high genetic diversity because inbreeding which is difficult to avoid in these populations. Heterozygous genotypes, with a recessive deleterious allele, will not express the deleterious genotype, but due to inbreeding the homozygous genotype increases, it can have several negative effects. Inbreeding can also have negative effects on skeletal growth (Pucek et al., 2004) and even on competitive abilities (Gillingham et al., 2013).

The fitness traits affected most adversely due to inbreeding. Some of the important fitness traits of the tiger are number of young surviving upto the age of weaning, age at first parturition, age at mating, litter size, gestation period, sex ratio, inter-parturition period, etc. However, some metric traits (e.g. birth weight, disease condition, life span, etc) are indirectly associated with fitness and are therefore, affected by increased level of inbreeding. Inbreeding depression leads to viability in the expression of phenotypical character by producing the difference in behavioral, morphological, physiological and molecular level (Lande, 1994). Close inbreeding leads to reduction in fitness (Keller and Waller, 2002; Sarre and Georges, 2009). An interesting finding with brother-sister, father-daughter and mother-son mating was birth of white tigers. But deformities and deficiencies begin to surface very soon in white tiger population e.g. higher cub mortality in tiger population (Warrick, 2010; Xu, 2013). Some white tigers also show strabismus, probably due to the reduction of pigment in the retinal epithelium and iris during eye development. Therefore, the present study was conducted in tiger population (live and dead) with the objectives to estimate the inbreeding coefficient of each and every tiger from 1964-2015 of zoological park, to find the correlation values between inbreeding coefficient and fitness traits and to find the dependency of inbreeding with different disease condition.

Materials and Methods

Detailed pedigree information of 342 tigers was collected from stud book of Nandankanan Zoological Park, Bhubaneswar, Odisha, India. The Zoological park is situated at a longitude of 85˚ 48′ 09″ to 85˚ 48′ 13″ E and latitude of 20^˚ 23′ 08″ to 20˚ 24′ 10″ N. The region belongs to tropical climatic zone with average rainfall of 1200 mm to 1400 mm during monsoon and rainy season. During summer average temperature was found to be 40^˚C which drops by 10˚C in autumn. However, temperature falls to an average of 15^˚C during winter season. Out of the 342 tigers, 161 tigers were male, 178 tigers were female and 3 tigers sex were not detected as they died immediately after birth without proper development of their genital organs. Presently, twenty seven tigers were present in the zoological park with 3 white colored male, 5 white colored females, 5 normal colored male and 14 normal colored females. So, all total with 8 male animals along with 19 female animals are present in the zoological park. According to the zoo guideline, animals were kept with well organized managemental and animal husbandry practices. The information on sire, dam and date of birth, date of death, sex and colour were collected for the period 1964 to 2015. The path of pedigree for each tiger was determined and inbreeding coefficient (F) of each tiger was calculated (Falconer, 1981). The following traits were measured on each tiger i.e. birth weight, age at first parturition (females only), age at first mating, parity, total number of cubs born in life time, number of cubs live up to weaning (weaning age of cubs is one year), age at death, litter size, number of cubs born, number of normal colour cubs born, gestation period, average inter parturition period, causes of death (death due to rejection by mother, cardiac failure, shock, accidental injury, still birth, chronic nephritis, septicaemia disease, inanition pneumonia, hepatitis, respiratory failure, trypanosomiasis, debility, senility, endometritis, peritonitis, haemorrhaegic gastroenteritis, anaemia, tumor, paralysis, dyspnoea, jaundice, parasitic disease were collected from post mortem registers).

Correlation between inbreeding coefficient and birth weight, age at first cubing, age at first mating, parity, number of normal cubs born in life time, number of white cubs born in life time, number of cubs live upto weaning, age at death, litter size, number of cubs born in life time, gestation period, average intercubing period were calculated (Becker, 1975). The traits that were affected with inbreeding coefficient were tested by chi-square test and t-test at 5% level of significance. The animals were classified into different groups according to range of F values. Chi-square test of heterogeneity (Snedecor and Cochran, 1967) between inbreeding coefficient groups and twenty four different said diseases was conducted to find the effect of inbreeding on such disease conditions.

Results and Discussion

The inbreeding coefficient of 342 tigers was calculated by pedigree path analysis that ranges between 0.00-0.325. More than 50 % of the tiger have taken birth with a range of inbreeding coefficient 0.175-0.30. Number of death of the tiger with low inbreeding coefficient i.e. 0.00-0.20 is much less as compared to the death of animal with higher inbreeding coefficient i.e. 0.20-0.30 (Fig.1).

Fig. 1: Line diagram comparing total number of cubs born and death due to diseases at each level of F

But when proportion of animal died due to disease condition was compared with the total number of animal taken birth was found to be less susceptible within inbreeding coefficient range of 0.00-0.05 and 0.20-0.25 (Fig. 1). We may predict that in captive breeding of tiger, the inbreeding coefficient should maintain within these range for less disease susceptibility. However, when percentage of death due to aged and disease condition was calculated with respect to the inbreeding coefficient it was found that only 6.667 % of tigers were died due to increased age but remaining 93.333 % were died because of any disease condition. It was also found that the percentage of death due to any disease condition increases with the increase in the level of inbreeding coefficient (Fig. 2). It was determined that with the low range of inbreeding coefficient (0- 0.10) the percentage of death due to any disease condition was low i.e. 12.585 %. But at higher range of inbreeding coefficient (0.20- 0.30) the percentage of death increases to 43.197 %. These indicate that the animals with higher inbreeding coefficient are more susceptible to any disease condition.

Inbreeding is defined as the mating of a pair of animals who are related to one or more common ancestors. The animals more closely related to each other shows greater degree of inbreeding and maximum loss of hybrid vigor. There is a direct link between inbreeding depression and loss of genetic variation or population viability (Westemeier, 1998; Madsen, 1999).

Fig. 2: Column bar diagram between disease death and normal death at different level of F

A population with a history of inbreeding is likely to exhibit less inbreeding depression as purged most of the deleterious allele as compared to an outbred population with a more recent and brief history of inbreeding (Templeton, 1984). An interesting example to this was the probability of inbreeding in New Zealand species was expected to be much higher than that of other continental species but they were less affected to inbreeding depression as compared to species elsewhere (Craig, 2000). The inbred population could reach the same population size as the non-inbred population or might be more susceptible to new disease conditions and be slower to recover from any human setback or disaster (Frankham, 2003). Previous study by Montali, 1980 established a significant correlation between heterozygosity and traits determining fitness, such as weight, fecundity and developmental stability. Subsequently, the overall health of a population could be inferred by examining the heterozygosity of the population in question. Populations with lower heterozygosity were also at greater risk for disease acquisition. A classical example of decreased genetic diversity was studied in the cheetah, Acinonyx jubatus which occurred as a result of a historic bottleneck. This lack of genetic diversity had led to difficulties in captive breeding due to abnormalities of the spermatozoa (O’brien et al., 2008). Furthermore, the major histocompatibility complex (MHC) was found to be identical in cheetahs making the population susceptible to pathogens (O’brien et al., 2008). The inbreeding depression caused a variety of fitness reduction in a wide range of animal species which were kept in captivity. The number of observed fitness-related factors affected by inbreeding was determined ranging none in Indian tiger and Asiatic lion (Shivaji et al., 1998) to five in Lord Howe Island Stick Insect (Honan, 2008). This suggests that the level of inbreeding depression is not that easy to quantify and it will be difficult to come to an overall conclusion whether it poses a big problem for captive animal populations or not.

Age at death was established to be negatively and significantly (p < 0.05) correlated with inbreeding. That means the animals with higher inbreeding coefficient were subjected to early death might be due to any disease susceptibility condition. This is in agreement with previous findings that inbreeding affected various components of fitness traits in animal (Wright, 1977). However, litter size was found to be positively and significantly (p < 0.05) correlated with inbreeding. The dams with high inbreeding coefficient developed a good maternal behavior which resulted in increased survival rate of the litter with reduced reproductive success (Dwyer, 2008).

In the present work it was observed that litter size was positively correlated whereas; age at death was negatively correlated with inbreeding coefficient. It is in agreement with the earlier study where maternal behavior of the dam increased because of the enhanced progesterone level which favours the survival of the offspring (Dwyer, 2008). The maternal inbreeding of 119 zoo populations had a negative effect on fitness (Boakes, 2006). Litter size increases with inbreeding coefficient value, but age at death decreases with the inbreeding coefficient value. This might be due to the fact that litter size is governed by additive effect of genes, but age at death is influenced by both heredity and environment. The result was supported by studying the significant association between total inbreeding coefficient in dams with mortality at days 7 (p <0.05), 30 (p <0.05) and 90 (0.10) indicating that it would decrease the mortality risk of the litter (Quilicot, 2009). Similar findings were observed with reduced litter size in wolf (Canis lupus) (Laikre, 1999), Mexican wolf (Canis lupus baileyi) (Fredrickson et al., 2007) and red wolves (Canis rufus) (Lockyear et al., 2009, Rabon and Waddell, 2010). The litter size reduction of the captive tiger in Nandankanan zoo is comparable with the brown bear (Laikre, 1999). A trend of lower litter size was visible in captive than free-ranging animals. Furthermore, with increased dam age, litter size was decreased in free-ranging populations (Lockyear et al., 2009). A study on Mexican wolf lineage revealed that inbreeding between mother and son resulted into development of abnormal testicles in three inbred male pups (Hedrick et al., 1997). Similar type of results was also observed in the present study where three highly inbred cubs died immediately after birth without proper development of their genital organ. Another study was done on the Mexican wolves that focused on the relationship between inbreeding with sperm quality and reproductive success (Asa et al., 2007).

The study found a highly significant negative correlation between inbreeding coefficient and percentages of normal sperm cells in the ejaculates and motile cells. Because sperm quality can affect fertility and reproductive success (fitness) these findings can be addressed as inbreeding depression effects. In the present study, the survivability of cubs upto weaning (i.e. one year of age) and total number of cubs born in the lifetime were found to be negatively and significantly (p < 0.05) correlated with inbreeding coefficient was as explained in Table 1. The inbreeding of an individual had a negative effect on mortality upto weaning with significant value which indicate that inbreeding in an individual influencing its survivability.

Table1: Correlation coefficient of different fitness and reproductive traits with inbreeding coefficients in tiger population

* P < 0.05

The similar trend was also reported earlier in mammals (Ralls et al., 1988; Cassell et al., 2003). Inbred animals cannot survive in a harsh environment, but the survivability was not affected if, the environment was conducive. However, age at first cubing, average inter-cubing period, gestation period and age at first mating were found to be positive but non-significantly correlated with inbreeding coefficient whereas parity, birth weight, number of white cubs born in life time and number of orange cubs born in life time were negatively but non-significantly correlated with inbreeding coefficient. The increase in new inbreeding were more associated to the ancestral inbreeding coefficient with the increase in calving interval (p < 0.05) and age at first calving (p < 0.001) (Ralls et al., 1988; Sinead et al., 2009). In contrast to this, ancestral inbreeding had an unfavorable effect on calving interval, age at first calving and survival (Ballou, 1997; Hedrick and Kalinowski 2000).

The tigers under different inbreeding range face the problem of death due to twenty four different diseases were identified. The diseases that were most affected due to inbreeding with a significant value leading to death of an animal were shock stress, exertions, accidental injury, nephritis and senility. Inbred gazelles were more prone to death due to reduction in resistance to pathogenic diseases (Cassinello, 2005). Inbred animals cannot cope up with the changing environment like fluctuating temperature, limited feed, unpredictable rainfall etc. (Falk and Holsinger 1991). In the present research work, the four diseases were the product of improper environment. Thus death due to disease conditions was associated with inbreeding of tiger. It was also discovered that the number of tiger death at higher inbreeding decreases in some particular period of time because of better managemental practices and optimization of developed health care unit by zoological park. Supporting to this finding, a study was earlier conducted on Black Robin population with no evidence of viral, protozoan or bacterial infections could still be susceptible to novel infection due to increase in inbreeding coefficient (Miller and Lambert, 2004). But one surviving population of Black Robin was said to be in a vulnerable condition (Frankham, 2003). Based on these findings, it was illustrated that the inbreeding not only resulted lower survival rate in tigers but also reduced juvenile survival rate and longevity by decreasing resistance to pathogens.

Conclusion

This research showed that inbreeding depression can cause several negative effects in captive zoo populations, especially by reducing reproduction chances and survival rate of inbred animals. Increased litter size with abnormal cubs and decreasing longevity were two of the most observed indicators of inbreeding depression was the most important finding of our study. Other traits that were found to be associated with inbreeding are albinism, reduces birth weight and skeletal size. Reduced fitness was a indicator of inbreeding depression but may also affected by some other factors such as poor husbandry or managemental practice. Inbreeding depression is generally consider to be the immediate threat to population persistence. Inbreeding increases the frequency of homozygous loci in a genome which increases the potential expression of recessive deleterious or lethal alleles. Dispersal, the movement of individuals between isolated populations, and subsequent success in breeding may reduce the frequency of recessive homozygotes, thereby reduce the deleterious effects of inbreeding on population viability. The genetic variability is an important criterion to sustain the danger of extinction. It is recommended for zoos and other captive institutions to reduce inbreeding and inbreeding depression in their captive animal populations as much as possible and should be limited within the range of 0.20-0.25 by using artificial insemination, species-specific breeding plans and using minimising kinship as captive breeding strategy, so that the animal would be less susceptible of diseases. Also, recommended to adopt good managemental practice to reduce the incidence of disease occurrence, thereby reducing the number of uncertain death of captive wild animal within the weaning period.

Acknowledgement

The authors are thankful to authority of Nandankanan Zoo, Bhubaneswar, Odisha for providing necessary support to conduct the research work.

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