Genetics and genomics of animal behaviour and welfare—Challenges and possibilities
Introduction
Ever since the release of the Brambell report (Brambell, 1965), applied ethology and animal welfare science has mainly concentrated on investigating problems occurring in the interaction between animals and their environments. This has led to a significant increase in our understanding of the needs and wants of farm animals, and in many cases has deeply affected the development of farming systems. For example, many countries, including those in the European Union (EU), are phasing out battery cages for laying hens and tethering and individual stalling of sows, largely as a consequence of scientific evidence from applied ethology. Extensive research has been carried out to study the natural behaviour of domestic animals, and in many cases, this has shown that fundamental aspects of behaviour differ only marginally from that of the wild ancestors. For example, pigs released into natural settings will perform the full range of wild boar behaviour around farrowing, including removing themselves from the group and building an elaborate nest (Jensen, 1988). Detailed studies have shown that frequencies as well as sequences of nest building behaviour are virtually indistinguishable between domestic sows and wild boars (Gustafsson et al., 1999).
However, over the last few decades, new challenges to animal welfare have appeared in animal farming. A rough estimate shows that the average production levels of farm animals have more or less doubled since the publication of the Brambell report, and a number of undesired side-effects on animal welfare have been documented (Rauw et al., 1998). Selection during this period has been intensely focused on production, but it is clear that behaviour and welfare have been affected as well. It has been suggested that some of these side-effects can be explained by resource allocation theory, which suggests that animals make adaptive adjustments in the allocation of resources to different life processes when facing changing selection pressures (Beilharz et al., 1993). There is some experimental evidence that this happens and may be important for the sake of understanding animal welfare under intense selection for increased production (Beilharz and Mittpaiboon, 1994, Rauw et al., 2000). This raises the issue of how such trade-offs are controlled genetically, and how this relates to the process of domestication in general.
Domestication is the process whereby populations of animals change genetically in order to adapt to an environment where reproduction is controlled largely by man (Price, 1997). Even though many fundamental behaviours are largely unaffected by the process, there are several important changes induced by domestication (Jensen and Andersson, 2005); ontogenetic processes are changed, social tolerance has increased, sexual and reproductive behaviours have been altered, and adaptive ability has been affected in different ways. All these changes are possible to understand in terms of resource allocation. Of course they lead to the question what constitutes normal behaviour for modern, highly productive strains of domesticated animals. One example of this is the highly reduced activity levels of modern broilers. Is this an adaptive modification of behaviour in order to allocate energy chiefly to growth, or is it an abnormal side-effect caused by inability to move and behave in accordance with the motivation of the animal (Bizeray et al., 2000)?
Questions such as this can only be solved by a deeper understanding of the genetic processes involved in controlling behaviour and other welfare related traits. This review paper aims at outlining some of the challenges and possibilities offered by genetic selection, and by the modern revolution in genetics and genomics. We are rapidly moving towards not only understanding the genetics of welfare, but also of manipulating it dramatically with molecular technology. The ethical implications of this are beyond the scope of the present paper, but any ethical statement should be based on thorough knowledge of the biology involved.
Section snippets
Classical selection experiments on behavioural traits
Classical selection experiments for specific behaviours have been done using a variety of animal species. The main objectives of these experiments are typically to gain knowledge of the genetic mechanisms underlying the trait as well as investigating changes in other behavioural traits and physiological variables due to co-selection (correlated responses). The single genetic parameter of greatest interest is the heritability (h2) which gives information on the probability of changing a trait
Finding the genes—principles of QTL mapping
As seen in the previous section, selection for behaviour traits in farm animals can successfully be conducted without knowing the underlying genetic architecture (Kjaer et al., 2001, Mills et al., 1997, Muir, 1996). With the discovery and mapping of polymorphic DNA markers, dense marker maps have been developed for most farm animal species, including cattle, pigs, and chickens (Barendse et al., 1997, Groenen et al., 2000, Rohrer et al., 1996). These dense marker maps are essential tools to
Molecular genetics of stress responses
Stress is central to animal welfare, and is usually defined as the non-specific response of an organism to any demand upon it (Selye, 1973). It covers the behavioural and biological responses to a wide range of stimuli such as social interactions or rough handling, common farming practices such as castration, dehorning or teeth clipping, but also exposure to extreme climatic conditions, or restricted feeding, just to cite a few. Stress-triggering stimuli are not necessarily painful but can also
Stress, welfare and gene expression
Since the expansion of molecular biology, there has been an emphasis on finding mutations in genes or regulatory regions, which may explain variation in phenotypic traits such as behaviour. As seen in the previous sections of this paper, mapping of genomic areas, identification of candidate genes and characterisation of causative mutations have been important elements of this research process. However, the modern insights into genome science has made it increasingly obvious that much phenotypic
Genetics of reproductive traits and the welfare of domestic animals
Reproductive success is a fundamental component of evolutionary fitness, and variation in reproductive performance is under intense Darwinian selection (natural and sexual). In many sexually reproducing dioecious species, diverging fitness interests of males and females foster sexual conflict and the evolution of contrasting male and female reproductive strategies (Parker, 1979, Parker, 2006). In these species reproductive success is to an important extent mediated by antagonistic interactions
Genetics, behaviour and welfare—future perspectives
As already mentioned, the focus of applied ethology has been on the interaction between the behaviour and the environment of the animals (e g, Jensen, 1993). The intense selection of farm animals for increased production forces us to place more emphasis on the role of the genotype in relation to animal welfare. As seen from the present review, there are many burning issues in relation to this, where we have only begun to understand the underlying biology. Correlated responses to selection can
Conclusions
Classical genetic selection experiments have shown that a range of behavioural traits can be changed dramatically in a range of animal species. This selection on behaviour often induces changes in the morphology, physiology and immunology simultaneously. Knowledge of the genetic relationship between these parameters will help to apply selection for behavioural traits in production herds. Combining the classical selection techniques with new tools like QTL-analysis and marker-assisted selection,
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