Zoo breeding: Sumatran tiger secrets unravelled using ASReml

Zoos have come a long way since the original Zoological Garden in Regent’s Park, London. They are not just a collection of animals for people to look and scientists to study; evolving into centres of research and play a vital part in wildlife conservation and protection. Certainly in the UK you would be hard pushed to find a zoo that doesn’t have some form of conservation programme.

And there can be no doubt of the importance of such programmes to the Mongolian Przewalski horse, which for around 30 years was bred in captivity under a project led by the Institute of Zoology at the Zoological Society London, and has been successfully reintroduced into the wild. Without the captive breeding programme this wild horse would have become extinct, indeed for many years was classed as such.

Since the 1970’s captive breeding programmes within zoos have become more important as wildlife populations decreased and the shipping of endangered animals became more restricted. Zoos needed to be able to successfully breed their own animals to maintain animal stocks, leading to the creation of “studbooks” listing information on the captive species and enabling zoos to recommend pairings and help maintain genetic diversity by avoiding “in-breeding”. This data can be very detailed, with birth, sex, parentage, source and the animal’s movement between institutions, as well as size and weight information.

Despite the increasing importance of captive breeding programmes to manage zoo stocks and animal conservation, very few studies have been carried out to understand selection and adaption to captivity. Admittedly quantitative genetic studies can be labour intensive which may help to explain this, however a recent study published in “Trends in Ecology and Evolution” suggests that the use of zoological records obtained from captive bred animals (such as data found in the “studbooks”) may be useful in studying selection and adaptation to captivity. The researchers concentrated on using a statistical technique known as the “animal model” –  a mixed effects model and a form of linear regression, where the explanatory terms are both fixed and random.

The animal model is already widely used in studies for the genetic improvement of livestock, and is increasingly being used in evolutionary studies, so conservation is the next logical step.

What is the animal model?

The animal model allows a phenotypic trait to be split into its additive genetic and other non-genetic variance components; it requires data that already exists or is easy to obtain for captive populations, for example information on the pedigree and phenotypic data on quantitative traits, such as body weight. Additional factors which highlight the suitability of this approach include the ability of the animal model to analyse unbalanced datasets, common with captive populations due to missing phenotypic data or pedigree links.  Using the data in existence and appropriate statistical tools could lead to a far greater understanding of the effects of captivity on the genetic variation of different traits. Although there are a host of other issues of interest to evolutionary biologists and conservationists, in terms of a captive breeding programme the heritability of reproductive traits, such as litter size or birth mass would be of vital importance.

As an example the researchers looked at the heritability estimate for the litter size in Sumatran tigers. Using ASReml, they were able to deduce that environmental factors appeared not to play a part in litter size; hence if female tigers producing large litters are bred by zoos, then it’s likely that large litters will continue to be produced in captivity, because this is a heritable trait.

To take this to the next step and achieve the goals of the reintroduction of endangered species to the wild, scientists need to be able to understand which traits are beneficial to both captive and wild populations. The use of existing data and statistics means that we could be closer to this. Certainly the researchers behind this study suggest that fine-tuning the data collection for studbooks could assist with very detailed analyses and understanding of genetic traits within captive populations, leading to more successful captive breeding programmes.

ASReml is a highly respected animal model software package and is perfectly suited to researchers working in genetics and animal breeding; it is already widely used in animal breeding programmes, due to its flexibility, speed and accuracy. As well as being specifically designed for the fitting of linear mixed models using the Residual Maximum Likelihood (REML), it is ideally suited to large datasets with complex variance methods. Please email VSNi with your name and organisation details quoting reference ZOOCB09 to obtain a free 60 day trial.

Our thanks to the researchers, especially Fanie Pelletier for their help in this piece. For more information on their research see the pages at the Université de Sherbrooke.

The original paper can be accessed from Trends in Ecology & Evolution, Volume 24, Issue 5, 263-270, 06 March 2009.

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