Dedomestication series Pt. II: When domestic animals run wild

As I wrote in the previous post, I am neither an expert on developmental biology nor am I a geneticist, so I am just going to present my personal take-on to this subject here.

When domestic animals run wild, for whatever reason, they are opposed to the following selective pressures:

·      Climate

·      Food quality and quantity

·      Diseases and ability to recover from injuries

·      Predation (herbivores)

·      Intraspecific competition

o   Sexual selection

·      Interspecific competition

The ability to live on poor forage and to recover from injury as much as resistance to diseases are not visible traits. Adaptions to climate are not necessarily either, but some are. For example, the development of a sufficient winter fur, or the reduction of unnecessarily large appendages like large dewlaps, udders or ears. An interesting fact is that domestic pigs with sparse fur and pink skin are prone to sunburns [1].

There are behavioural adaptions too: Domestic animals tend to mate all the year round, which is not advantageous because both mother and juvenile may not find enough food during winter. Therefore, any wild animal exposed to such a climate has a seasonal mating circle.

Defending against predation requires morphological and behavioural changes as well. These chenges depend on the species and its defensive mechanisms. Herbivores living in groups show herding behaviour, taking the young individuals into the middle of the herd, and also form defensive circles around the youngsters. They must have the physical ability to defend themselves – that is the necessary strength, size, speed, manoeuvrability, and weapons (horns, antlers, kicks, tusks, whatever). Camouflage is a factor as well.

Intraspecific competition is basically about dominance and reproductive success. To be a  dominant animal in a herd means to have access to the best feeding and resting places, and not being chased by other, more dominant herd members (dominant individuals have to defend their status, on the other hand). Both the combats for dominance within a herd or mating rights should have an influence on the morphology, as I am going to outline later on. But also on behaviour – the more aggressive, energetic and more willingly to take risks should be in advantage. Free reproduction not influenced by man will also invoke sexual selection (of which the aforementioned mating combat is part), which is another important factor for certain traits.

So, what would happen if a population of domestic animals is released into the wild? Of course the same that evolution does with any population: shape it to fit the requirements given by nature (= intra- and interspecific, abiotic and biotic factors). How this is going to happen certainly depends on the area they are released on, but let’s assume it is the same evolutionary adaptive environment the wild type of these domestic animals inhabited. It probably sounds logical that those traits that were present in the wild type and were advantageous adaptions to live in this ecosystem will be fixated and united in these domestic animals as far as they are retained in the population. It is unlikely that the whole genome of the wild type is preserved within a domestic species, and that the released population contains all the wild type traits present in the domestic species itself, but they probably would still have such traits.  Furthermore, the animal’s genetic and developmental potential might enable them to evolve traits equivalent to an original wild trait that is lost but now needed again.   

Nevertheless, the transformative selection that would take place certainly would not just be pure regression towards the retained wild type traits. The requirements of the biome might have changed since the time the wild type has disappeared – some other species might have vanished, or new been introduced and the landscape might have been modified and the space restricted by man. The population has to adapt to these circumstances as well.

Many typical traits of domestic animals are useless or hindering: floppy ears, reducing the acuteness of the auditory sense and affecting their social behaviour (dogs); over-grown fur that felts or soaks full of water, dirt or parasites; or unnecessarily large appendages such as large dewlaps, udders, fat bumps, hypertrophied muscles, ears et cetera. Those would probably be eradicated by natural selection as far as possible.

As anybody familiar with the basic principles of population genetics will know, mutations are not always fitness-reducing or deleterious. They can also be advantageous  or at least be neutral. Perhaps some of the new mutations that occurred since the domestication event would now be advantageous to overcome the genetic bottlenecks they domestic lineages went through. Probably a number of new traits acquired since domestication would be neutral and disappear only slowly or only with the help of genetic drift, especially colour traits.

Natural selection would likely also influence the genes that were responsible for the developmental changes during domestication (outlined in the previous post) and change them in an evolutionary advantageous way, therefore removing some domestic artefacts such as paedomorphism.

Eventually the whole population would become more and more uniform. Wild animals usually are very uniform, and this is not only because of stabilizing selection that purges out traits that are “not as fit” as the others, but also due to genetic drift, which purely depends on coincidence concerning neutral traits. It would probably take a very long time until a variable population of domestic animals reaches the uniformity of wild animals (without severe bottlenecks).

Phenotypic plasticity probably changes the appearance of the animals in just the reverse way it does when taking place in husbandry: the animals should look more “trained”.

Based on these thoughts, I worked out the following hypotheses:

1. Domestic animals change under selective pressure in nature. These changes are

            a) a regression towards wild-type traits because they provide an immediate fitness advantage

            b) a response to the new and/or new old selective pressures, that might also enforce traits that are not necessarily wild-type traits

2. Mechanisms that cause morphological and behavioural changes in domestication, such as relaxed selection, alteration of developmental cascades and pleiotropy do the reverse during dedomestication

3. Not all new traits that emerged during domestication are necessarily fitness-reducing. Those will remain in the population for a long time or even become fixed per coincidence.

4. Eventually, the population gets as uniform or nearly as uniform as wild animals usually are through stabilizing selection and genetic bottlenecks, however long that process will be.

Now I am going to give a number of examples of dedomestication that might be models for what we can expect to see in a released population of cattle:

Feral rabbits

It is well-known that the native range of the common Rabbit, Oryctolagus cunniculus, was restricted to Iberia, North Africa and southern France before it was expanded by humans from the antiquity onwards. Now they inhabit great parts of Europe, Australia, New Zealand and also a bit of South America. Those outside their original range are very uniform in appearance and I do not know of any differences to those rabbits within the original range. I do not know how many of the rabbits that were originally released in non-native regions were wild or domestic, but there surely must have been domestic rabbits among them. Heinz Heck claimed all of them were of domestic origin (does anybody know further sources for that?).[2] Rabbits are small animals and have many predators, they cannot effort any deviant, eye-catching colour variants, floppy ears and so on.

Feral pigs

Pigs had escaped or been released on a lot of places on this earth. The most famous examples are Australia and the Americas. The North American “razorbacks” will be in the focus here. If you do a google search, you will find some specimen that resemble European wild boars very closely – that is because those have been introduced to several regions as game animals and hybridized with the feral pigs. But there are, luckily, still regions with un-hybridized feral hogs. Texas, for example. Have a look at these two videos showing Texan feral hogs: video 1, video 2. Although not identical, they bear a considerable resemblance to wild boars in looks, behaviour and movement. They have a body build for agility and strength, and that's how they move. Their tusks are well-pronounced as they have a social and defensive function. The skull is very elongated, as much as in the wild boar – perhaps this is an example of a “reversal” of paedomorphism as described above through developmental cascades [UPDATE: I was pointed out to a paper that suggests that the elongated snout of feral pigs is a result of phenotypic plasticity due to the chewing mechanism]. What is also striking is their (with a few exceptions) uniform fur colour, beautiful mud-coloured brown or very dark, almost black, brown (not as greyish as in the European wild boar) – very likely camouflage in forested environment.

Feral Horses

There are a lot of feral horse populations as well. But in this case I wasn’t able to trace down a population that was originally of a diverse origin (in which the effect of selection would be easier to spot than in a population that was rather homogeneous right from the start*), fully exposed to natural selection and without further influx by domestic horses. So I do not know of any feral horses that serve as a good role model. And as far as I can see, there are no populations that are homogeneous but not inbred at the same time. One of the reasons might be that horses, being large herbivores that do not have any natural enemies in most of the areas they run wild, do not have the same predative pressure than feral rabbits or pigs have (adult feral hogs may be untouched by native American predators, but juveniles and subadults probably not). Furthermore, I think that horses are far less domesticated in terms of morphology than (derived) cattle or pigs are. 

*The feral horses of the Namib Desert are very homogeneous. But I would explain that with their very low genetic diversity [3] and ancestral breeds that were much alike.

My dedomestication concept has empirical problems that am going to outline in a following post. But for now I am going to use it as a working hypothesis for the next post, which is going to explore what might happen in a cattle population becoming dedomesticated.

For the next part, go here. 

Literature

[1] Margret Bunzel-Drüke, Carsten Böhm, Peter Finck, Gerd Kämmer, Rainer Luick, Edgar Reisinger, Uwe Riecken, Johannes Riedl, Matthias Scharf, Olaf Zimball: ''„Wilde Weiden“. Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung.'' 2. Auflage. Arbeitsgemeinschaft Biologischer Umweltschutz im Kreis Soest, Bad Sassendorf-Lohne 2009, ISBN 978-3-00-024385-1.

[2] Heinz Heck: “Der neue Auerochse”, Internationales Zuchtbuch für Auerochsen, 1980.

[3] Cothran EG, van Dyk E, and van der Merwe FJ (March 2001). "Genetic Variation in the feral horses of the Namib Desert, Namibia". Journal of the South African Veterinary Association (J S Afr Vet Assoc) 72 (1): 18–22. PMID 11563711. R