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Wednesday, 10 September 2014

Red Junglefowl (Gallus gallus) selected for low fear of humans are larger, more dominant and produce larger offspring. Animal: Sept 2014, Volume 8, Issue 9

Red Junglefowl (Gallus gallus) selected for low fear of humans are larger, more dominant and produce larger offspring

Volume 8 / Issue 09 / September 2014, pp 1498-1505
PubMed ID: 24910136


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B. Agnvall, A. Ali, S. Olby and P. Jensena

IFM Biology, Avian Behavioural Genomics and Physiology Group, Linköping University, 581 83 Linköping, Sweden

Many traits associated with domestication are suggested to have developed as correlated responses to reduced fear of humans. Tameness may have reduced the stress of living in human proximity and improved welfare in captivity. We selected Red Junglefowl (ancestors of all domestic chickens) for four generations on high or low fear towards humans, mimicking an important aspect of the earliest period of domestication, and tested birds from the third and fourth generation in three different social tests. Growth and plumage condition, as well as size of eggs and offspring were also recorded, as indicators of some aspects of welfare. Birds selected for low fear had higher weight, laid larger eggs and generated larger offspring, and had a better plumage condition. In a social dominance test they also performed more aggressive behaviour and received less of the same, regardless of whether the restricted resource was feed or not. Hence, dominance appeared to increase as a consequence of reduced fear of humans. Furthermore, egg size and the weight of the offspring were larger in the less fearful birds, and plumage condition better, which could be interpreted as the less fearful animals being better adapted to the environment in which they were selected.

In general, the results showed that chickens selected for a low fear response towards humans (L) secondarily had a higher weight, laid larger eggs and generated larger offspring than birds selected for a high fear response (H) and the unselected strain (U). They also had a less damaged plumage, indicating that they were less exposed to feather pecking. Furthermore, they were more socially dominant and received less aggressive behaviour in the SD. Finally, they also tended to be more feed motivated, both in the SD and in the ES. A possible interpretation of this is that birds selected for low fear were better able to cope with the experimental environment in which they had been selected.
Although we have only carried out selection for four generations in total, it was clear that this produced a significant difference in fear of humans. As this was the target selection criterion, it is of course the expected result and Agnvall et al. (2012) observed a significant genetic component to this trait after only two generations. In the present study, we found a number of correlated effects on traits related to ability to cope with captivity, which is a fast response to a simulated domestication event. In foxes, Belyaev (Trut et al., 2009) found that correlated responses to selection for tameness developed in a small proportion of the animals after 8 to 10 generations, which shows that domesticated phenotypes may start to occur within few generations of reduced fear of humans. The effects we found on social dominance were replicated in two consecutive generations, showing that this is probably a stable behavioural difference between the selection lines in the present experiment. Hence, our results indicate that the ability to obtain high social dominance (as assessed in this study) was affected by selection for high or low fear. Of course, there is a risk that our results are the result of genetic drift, since it comprises a relatively limited number of families. However, each selection line consists of 4 to 10 families, which can be regarded as independent replicates, since selection is done on individual basis. Furthermore, the fact that the results were replicated in two subsequent generation strengthens the impression that they were due to the selection imposed, as does the fact that the unselected birds generally fall in between the selected lines in the variables measured.
In a similar type of test on foxes, Harri et al. (2003) found that animals selected for tameness monopolized a feed source in 9 out of 10 tests. This was interpreted as a generalization of the reduced fear also towards conspecifics and the test situation, and a reduced stress susceptibility, which could perhaps also explain the effects observed in the present study.
The birds selected for low fearfulness grew larger than the other strains, and this may have confounded the social dominance results to some extent. As earlier reported (Craig et al., 1975; Cloutier and Newberry, 2000; Müller et al., 2002), BW seems to be a reliable predictor of a chicken’s position in a hierarchy. It is therefore not possible to distinguish cause and effects between the observed correlated selection responses, since high BW may either have been a consequence of the increased social competitiveness, or it may have been the factor causing it. Moore et al. (2002) proposed that social dominance is affected by additive genetic variation, which can lead to rapid changes. Fear of human traits may therefore have a shared genetic component with social dominance, but to disentangle the genetic mechanisms, further experiments are needed.
BW may also be an indicator of overall welfare and capacity to cope with the environment (Tachè and Selye, 1985; Broom, 1991), since a calm and confident animal may be better at obtaining feed and growing. However, to establish the overall welfare of the animal, variables such as behaviour disorders, physiological measurements and diseases need to be assessed (Broom, 1991) and our results therefore remain suggestive. In a similar study of rats, Albert et al. (2008) found no differences in BW between rats selected for tameness and those selected for aggressiveness, which indicates that correlated effects on BW may not be universally present, but may depend on species and environmental context.
The larger L birds also laid larger eggs, in accordance with the study by Müller et al. (2002), reporting a positive correlation between body size and egg mass. Increased body size in L birds may reflect a better ability to cope and grow in the captive environment used in this experiment. Further, in accordance with this, the L birds in this study also had better plumage condition, both on the head and on average on the whole body. Both exposure to abnormal feather pecking and aggressive pecking can cause deterioration of plumage condition (Jensen et al., 2005), and a larger and more dominant bird may be more resistant to this. In general, the plumage condition is regarded as good reflection of the welfare of chickens (Campo et al., 2001). It has previously been shown that offspring of barn swallow mothers with high corticosterone levels produce offspring with lower hatch weight and slower plumage development (Saino et al., 2005), and similar effects have been observed in chickens (Eriksen et al., 2003). The fact that the offspring of the H chickens weighed significantly less could therefore indicate higher stress levels in H chickens. Even though this is well in line with the overall impression, that L birds were better adapted to the environment, this remains an interesting hypothesis, since at present we have no data on levels of stress hormones in the different selection lines. Japanese quail selected solely on high plasma corticosterone response, experienced human contact more aversive than quails selected on low response (Jones et al., 1994), emphasizing the link between tameness and general stress susceptibility.
During domestication, chickens have been selected on maximizing the ratio between feed intake and egg laying (Kerje et al., 2003). Probably due to this, domesticated chickens show a higher general feeding motivation and less explorative foraging (Schütz et al., 2001). In our experiment, the animals selected for low fear response fed more both in the SD and ES tests, although the feed consisted of familiar and unfamiliar chicken feed in the ES and mealworms in the SD. This may imply that increase in feeding motivation could subsequently explain the difference in social dominance. As feeding motivation could be the confounding variable behind the effects observed in experiment 1, we performed the second experiment, which showed that regardless of resource, chickens from the L line were more dominant. Hence, it seems likely that the increased dominance resulting from selection was not merely caused by a generally increased feeding motivation. As there were no overall effect of selection on frequency of social behaviour the increased social dominance cannot be explained simply by a higher degree of social activity in the L line birds.

In conclusion, birds selected for low fear of humans grew to a higher BW and laid larger eggs, with consequently larger offspring and the plumage condition of these birds was better than in those selected for high fear. In spite of the fact that there was no difference in frequency of social behaviours between the selected strains, birds selected for low fear were more dominant in test situations both with and without feed. Overall, this indicates that the L birds from the two tested generations were better adapted to the experimental captivity environment.

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