This is post 7 on “Evolution and Domestication” of:
Towards a common conceptual framework and illustrative model for feather pecking in poultry and tail biting in pigs – Connecting science to solutions
Marc B.M. Brackea, T. Bas Rodenburgb, Herman M. Vermeera, Thea G.C.M. van Niekerka
a Wageningen Livestock Research
b Wageningen University, Dept. of behavioural ecology
Reading guide
This is one of 8 blog posts under the heading of: “Towards a common conceptual framework and illustrative model for feather pecking in poultry and tail biting in pigs – Connecting science to solutions”. It contains the following sections/posts:
- Introduction, specifying the need to compare feather pecking (fp) in layers and tail biting (tb) in pigs
- Terminology, specifying the various concepts involved in fp/tb.
- Overview of main similarities and differences between feather pecking and tail biting
- Farmer as a risk factor, emphasising, perhaps for the first time, that the farmer is a kind of ‘animal’ that is part of the problem (and the solution).
- Models, reviewing available conceptual models of fp and tb, as well as presenting a new ‘face model’.
- Disease framework, arguing that fp/tb may be regarded as a medical disorder, over and above being an abnormal behaviour per se.
- Evolution and domestication, emphasising the need to view fp/tb as a phenomenon an evolutionary and genetic background.
- References
The entire text (8 posts) can be downloaded as one pdf here.
7 Evolution & domestication
This final post/section aims to emphasise that adopting a disease framework for feather pecking (fp)/tail biting (tb) does not imply discarding the common science-based and evolutionary perspective on fp/tb. In order to show why this may be important we will first consider non-scientific reasoning to deal with fp/tb.
From a non-scientific and non-welfare perspective it may make perfect sense to farmers and veterinarians to prevent or treat fp/tb using respectively beak treatment (i.e. removing the means for fp) and tail docking (i.e. removing the object of tb). Similarly, measures like spectacles to prevent accurate vision (preventive measure in poultry) and teeth cutting (treatment measure in pigs) have been used, as has been the keeping of animals in the dark (thus blocking the animals’ vision). Along these lines one may also propose breeding poultry without feathers and pigs without tails, hens with blunted beaks, and pigs without incisor teeth, or perhaps blind animals (Ali and Cheng, 1985) (e.g. without eyes). Similarly, physical restrictions may be imposed in theory, e.g. solitary confinement/individual housing would be highly effective in stopping fp/tb. A related ‘solution’ is a limited physical ability to move (rather than lack of motivation (Bokkers and Koene, 2004)), as appears to be the case in heavily selected broilers. In fact, this may (partly) explain why fp is much less of a problem in broilers compared to laying hens. When comparing broilers to pigs, another reason, besides the limited physical activity, may be age. Broilers are slaughtered at 5-6 weeks of age, while egg-laying (puberty) starts at around 17 weeks (when severe fp normally develops). In pigs slaughter age and puberty are around 6 months and tb may be seen roughly in the period between 4 weeks and 6 months. Perhaps the situation in pigs, where tb is frequently seen in weaned and young growing pigs, and in rearing gilts but not in pregnant/farrowing sows, is somewhat more comparable to turkeys, where fp is a problem (5-8% in untreated turkeys; 10-16% in beaktreated turkeys) at around 4 days of age and around 8-10 weeks of age, at which age egg-laying/puberty may also start, while slaughter age is around 16-20 weeks (Van Niekerk and Bracke, 2016; van Niekerk and Veldkamp, 2017). Turkeys, like pigs, have been bred less intensively for muscle growth compared to broilers (Van Niekerk and Bracke, 2016). However, it may also be noted that fp in turkeys does not seem to respond as favourably to enrichment as does fp in laying hens (Van Niekerk and Bracke, 2016) and tb in pigs. In line with these considerations, the comparison between fp in poultry (laying hens and broilers) and tb in weaned/growing pigs, would raise the tentative suggestion that while fp is less prevalent in fast-growing broilers because of their very young age and limited physical activity, slower-growing broilers, in virtue of the older age and enhanced physical activity, should be expected to have an enhanced propensity to show fp behaviour. An anonymous poultry-welfare expert (pers. comm.) indicates that this may indeed be the case.
A risk of using breeding for inactivity to reduce fp/tb, of course, could be that in addition to reducing the propensity of the actor to show harmful social behaviour, inactivity may also reduce the propensity of the victim to avoid being pecked/bitten. Another breeding goal may thus be to select for animals that do not have the (cognitive) capacity to ‘discover’ fp/tb, and/or to breed against the ability to acquire the behaviour through social transmission (i.e. to learn from conspecifics who have become actors). Such selection for ‘stupidity’ is also unlikely to be effective, because both laying hens and pigs need a certain level of cognitive functioning and synchronisation, e.g. to regulate access to limited resources like nest boxes and feeders (Boumans, 2017).
A major factor in causing fp/tb is the animals’ motivation to explore/forage. Would it then make sense to select against this motivation per se?
In applied ethology it is commonly assumed that fp/tb are caused or at least mediated by a deprived motivation to forage. The idea is that poultry and pigs still have behavioural needs originating from evolution in a natural environment. Domestication is not perceived to have had a major attenuating effect, i.e. modern pigs and poultry are not (yet) adapted to intensive farming. The motivation to forage is still considerable because it was essential to survive in a natural environment spending considerable periods of time searching for food. Being generalist omnivores also implied these animals had relatively inquisitive natures to investigate a wide variety of potential food items under variable circumstances encountered in nature.
In fact, this attraction to novelty and eagerness to learn may well be sufficient to explain one of the most characteristic features of fp/tb, namely that a kind of irreversible state change occurs once the first fp/tb has taken place, and also that the problem has a certain tendency to escalate and is much more difficult to counteract later than it is to prevent it from occurring in the first place. A normal learning process can thus explain the difference in set point between animals who have never experienced fp/tb and those that have. No pathology need to be involved here.
Esp. pigs that are provided with novel enrichment materials clearly show ‘fanatic’, almost compulsive behaviours, except that the behavioural intensity tends to wear off readily (it is mostly a matter of a few quarters rather than hours that pigs spend on interacting with new enrichment materials). However, when the enrichment is slowly destructible (like soft wood), designed to fit the needs of the animal (e.g. branched chain design, (Bracke, 2017)) or provides (irregular) food rewards, e.g. as in the case of the Edinburgh foodball in pigs (releasing food pellets upon being rooted and thus moved around the pen (Young et al., 1994)), much more persistent (and less fanatic) interest may be observed. Pigs and poultry also clearly appreciate the taste of blood and (tail/feather/skin) tissue.
Note also that in addition to being potentially explained as a cognitive (learning) process, the escalation of fp/tb and (subsequent) state change may also be related to cognition and a tendency to show synchronised (feeding/exploration/activity) behaviour. A related potentially-involved mechanism could be the supposedly powerful tendency to show conformism, as suggested by De Waal in the case of primates (De Waal, 2016).
E.g. van de Waal et al. (2013) showed that green monkeys that had been trained to prefer maize of one colour, would unlearn their previous colour preference and acquire the colour preference of the group they had been introduced into. Similarly, mixing a less friendly primate species with a more friendly species, made the former much (4 times) more friendly (De Waal and Johanowicz, 1993). Uitdehaag et al. (2009) found that mixed housing of a more and less fearful strain of laying hens negatively affected fp and fear-related behaviour. Perhaps conformism may play a role in fp/tb in that once more and more individuals start to show the behaviour, other individuals may have a strong tendency to do the same. Thus conformism may explain (part of the escalation) by potentiation, but it cannot explain its origin (though it may explain why there is a reluctance to show fp/tb in a group that has never experienced it before). (Note: the origin may also be more or less accidental, e.g. McAdie and Keeling (2000) showed that (artificially) damaged feathers may trigger (outbreaks of) fp in laying hens.)
Counteracting the motivation to show fp/tb by genetic selection may simultaneously counteract the animals’ motivation to consume feed and thus (efficiently) produce under commercial conditions. Pigs and poultry need to be eager to consume food and they must readily accept novel feeds (e.g. when moved from rearing farm to finishing/egg-laying farm).
Thus, the motivation to forage may not only be a remnant of evolution in a natural environment, it may also be a product of selection for maximised production efficiency. In other words, domestication and genetic selection may have been co-shaping the current problem underlying fp/tb in intensive pig and poultry farming.
Traditionally, pigs and poultry have been selected using individual selection, i.e. the fastest growing individuals were selected to breed the next generation, perhaps even when individuals were showing high levels of production at the expense of pen mates (e.g. due to excessive aggression or the performance of fp/tb). In particular, when fp/tb occurred the (most heavily affected) victims were unlikely to be used for reproduction, but the actors in a pen could partly go unnoticed (unless they were detected and eliminated from the group). Group selection has been proposed as an alternative to individual selection, where the production efficiency of pen mates is also taken to load on an individual’s selection potential (Muir, 2003; Bijma et al., 2007a; Bijma et al., 2007b). Group selection has thus been suggested as a potential solution for fp/tb by selecting for peaceful pigs/poultry. Such peaceful pigs, however, may be less motivated to forage, and thus be less efficient for production.
Genetic selection probably has made use of the evolutionary tendency of animals (esp. males) to grow fast so as to have a higher likelihood of reproduction (as the largest individuals of a generation tend to win fights for access to females). Fast growth (as required for pigs and broilers), however, requires a persistent appetite to sustain growth. Compared to egg-laying in hens, which similarly require a substantial appetite to be able to sustain a high egg production, the biological prioritisation is different. Resource allocation theory suggests that animals make adaptive adjustments in the allocation of resources to different life processes when facing changed selection pressures (Beilharz et al., 1993). Hens must prioritise allocating energy to their offspring (eggs), whereas pigs and broilers must (i.e. have been selected to) allocate energy to their own growth (so as to produce meat). Thus, both types of farm animals are also likely to have been selected to prioritise (as much as possible) those processes that are preferred by man (be it the production of meat or eggs). Thus, when dealing with (mild) disease states it is possible that farm animals have been selected to (tend to) prioritise production over the (energetically costly) activation of the immune response. Farm animals that continue ‘functioning’ in an economic/zootechnical sense, however, may not be the most productive overall, e.g. when enhanced appetite has a negative side-effect in increasing the likelihood of fp/tb.
Several observations may be in line with the suggestion that in modern farm animals appetitive foraging motivation may have originated in a discrepancy with the natural environment, but may also have been co-determined by genetic selection for maximised production efficiency. A main indicator is that adult animals, esp. pregnant sows and broiler breeders, are known to experience high levels of feeding motivation and a tendency to become obese when given ad lib access to feed. (In laying hens, however, fp rather seems to be associated with hyper-mobility, also called a hyperactivity disorder (Kjaer, 2009), perhaps related to an (over-)activated foraging motivation.) In addition, when growing pigs are feeding they seem to be focussed so much on feed intake that vaccinating them with a rather painful (large diameter) needle seems to go (largely) unnoticed. Also, growing pigs whose front teeth have been cut so as to counteract an outbreak of tail biting don’t seem to show a clear reduction in feed intake. When their teeth have been cut, however, the pigs are much less inclined to continue tail biting and they also show much less interest in manipulating enrichment materials like chains, wood and ropes, suggesting that they do feel pain while maintaining a relatively high motivation to feed.
A final aspect where a science-based evolutionary understanding of fp/tb behaviour has clear added value over a classic medical framework may be the observation that not all kinds of stressors are equally likely to contribute to fp/tb. This may be an important aspect of the pathophysiology/mechanism underlying fp/tb. In laying hens, for example, organic farmers say that pullets with access to an outdoor range are less fearful later in life (e.g. in using the outdoor range), and therefore are less likely to develop fp (TvN, pers. comm.). Also in laying hens, the stressor of being moved from the rearing farm to the layer facility appears to be a trigger of stress and fp (even though it may also be a ‘revival’ of fp that originated at the rearing farm). In pigs, by contrast, mixing is not typically eliciting tail biting, despite the fact that it is highly stressful for the pigs. E.g. Holinger (2017) found no effect of mixing and isolation stress on tail and ear manipulation in pigs. Note also that mixing in pigs typically occurs at 25kg body weight (10-12 weeks of age), which is long before puberty at slaughter age, i.e. about 5-6 months of age, whereas laying hens are transferred to the laying facilities shortly before egg-laying starts (i.e. puberty). In pigs this compares to the rearing of breeding gilts, who are also particularly prone to tb (probably because they are fed on more restricted diets than slaughter pigs are), but only at a younger age (i.e. before the gilts are inseminated). Tb in pigs is hardly seen in (first or older parity) pregnant sows and in this respect tb in pigs differs from fp in laying hens. Such differences must be kept in mind, but they should not overrule the striking similarities between fp in poultry and tb in pigs, nor should they be regarded as a counterargument to the proposition that fp/tb would certainly benefit from being regarded as a medical/mental health disorder, provided the existing science-based and evolutionary framework is maintained to understand the behavioural as well.
Reading guide
This was blog post nr. 7 under the heading of: “Towards a common conceptual framework and illustrative model for feather pecking in poultry and tail biting in pigs – Connecting science to solutions”. It contains the following sections/posts:
- Introduction, specifying the need to compare feather pecking (fp) in layers and tail biting (tb) in pigs
- Terminology, specifying the various concepts involved in fp/tb.
- Overview of main similarities and differences between feather pecking and tail biting
- Farmer as a risk factor, emphasising, perhaps for the first time, that the farmer is a kind of ‘animal’ that is part of the problem (and the solution).
- Models, reviewing available conceptual models of fp and tb, as well as presenting a new ‘face model’.
- Disease framework, arguing that fp/tb may be regarded as a medical disorder, over and above being an abnormal behaviour per se.
- Evolution and domestication, emphasising the need to view fp/tb as a phenomenon an evolutionary and genetic background.
- References
The entire text (8 posts) can be downloaded as one pdf here.
Acknowledgements
These blog posts have been made possible by the Hennovation project (HORIZON 2020 ISIB-02-2014 project, Grant no. 652638).
