Appearance Matters: Artificial Marking Alters Aggression and Stress

Abstract

Artificial marking of animals for identification is frequently employed by researchers in the behavioral, biomedical, agricultural, and environmental sciences. The impact of artificial marking on experimental results is rarely explicitly considered despite evidence demonstrating that changes in phenotypic appearance can modify animal behavior and reproductive success. Here we present evidence that artificial marking of individuals within a social group has frequency-dependent effects on the behavior and physiology of domestic fowl (Gallus gallus domesticus). We demonstrate that when only 20 or 50% of individuals within a group were artificially marked, the marked birds received more aggression and had lesser body mass than the unmarked individuals within the same group. Furthermore, in groups in which only a small proportion of the individuals were marked, we report altered plasma epinephrine and dopamine levels in marked individuals. These effects of marking were imperceptible when all birds in a group were marked. This finding has important implications for animal research because, when only a subset of group members is artificially marked and used for data collection, the results obtained may not be representative of the population.

The relevance of variations in group size and phenotypic appearance on the behaviour and movement patterns of young domestic fowl

“…Subsequently, for each group size and phenotypic appearance combination there were 3 replications, with all group size populations being either homogenous (100 or 0%), with marked (M) or unmarked (U) phenotypes, or heterogeneous (30M/70U, 50M/50U or 70M/30U) with both phenotypes coexisting in the same pen. The alteration of the birds' phenotypic appearance was carried out at one day of age by placing a black mark with a non-toxic dye on the back of the head of each bird, following previous experimental procedures (Dennis et al, 2008). All the marks were identical and birds were remarked as needed at the end of each data collection week.…”

“…For example, it has been reported that birds which are phenotypically different from their conspecifics, due to a natural or artificial variation in feather colouration, are at a higher risk of being pecked Dennis et al, 2008). In addition to appearance, familiarity has also been found to influence the frequency of aggressive interactions (increased fights in groups of unfamiliar birds, Lindberg and Nicol, 1996), which could be dependent on group size (D'Eath and , and their ability to recognize group mates individually or based on badges of status (or traits that may signal their status or fighting ability when individual recognition and pecking orders are not possible, Pagel and Dawkins, 1997).…”

Emergence of cooperation in phenotypically heterogeneous populations: a replicator dynamics analysis

“…Although the marks are clearly visible to our eyes, to what extend they serve chickens to perform morphological differentiation within their conspecifics is out of the scope of this work, but see Vallortigara (2009). Nevertheless, Dennis et al (2008) observed that marked fowls suffer more aggressive events and have less body mass than their unmarked pen mates.…”

“…However, individuals are not always capable of recognizing their own kin, or even their own species, although they can perceive a phenotypical difference between neighbor individuals. In an experiment in animal welfare in poultry industry carried out by Dennis et al (2008) with domestic fowls, two types of individuals are generated by artificially marking a given proportion of fowls on the back of their necks. Chickens behave under partial information conditions: they are unable to identify their own type (whether they have a mark or not) but observe their opponent's type.…”

The use of visual and automatized behavioral markers to assess methodologies: a study case on PIT-tagging in the Alpine newt

“…A major component of marking technique choice is that it should not affect the integrity of organisms (Dennis, Newberry, Cheng, & Estevez, 2008;Ferner, 2010;Gibbons & Andrews, 2004). This is important in order to avoid biases in analyses (Winter et al, 2005), but also for ethical considerations (May, 2004).…”