In my previous article, ‘Understanding Street Dogs’, I had mentioned that all species belonging to the dog family (Canidae) – wolves, coyotes, jackals, dingoes, domestic dogs, etc. – are interfertile (can mate with each other and produce offspring). There are numerous examples around the globe of canids mating across species lines. For instance, coyote-wolf hybrids have become common in North America in the last 100 years or so (Rutherford, 2018). In Australia, Dingoes – a dog species native to the continent – have been hybridizing with domestic dogs that arrived with the British, giving rise to the current mixed population (Leonard et al., 2013). More recent was a study in 2015 which, for the first time, provided genetic evidence for hybridization between European golden jackals and domestic dogs in Croatia. Jackal-dog and wolf-dog hybrids have also been documented in various parts of India.
In this article, I will discuss ‘hybridization’ from an ecological and evolutionary point of view, while also presenting some of the possible impacts of hybridization in canids.
The Ecological and Evolutionary Basis of Hybridization
Hybridization refers to “the interbreeding of individuals from 2 distinct populations or groups of populations” (Harrison & Larson, 2014). However, interestingly, even if hybridization results in viable, fertile offspring, it may not always result in introgression or the transfer of genetic material from one species (or population) to another (Leonard et al., 2013). Although morphological changes may or may not be visible in hybrid offspring, genetic changes (introgressive hybridization) will occur only when the hybrid offspring backcrosses to one or both of the parent species (Rhymer & Simberloff, 1996). In the case of non-introgressive hybridization, it usually means that the hybrid offspring is sterile (Rhymer & Simberloff, 1996), and therefore incapable of bridging the gap between the parental gene pools. However, when this gap is bridged and there is a transfer of genetic material across “species boundaries”, there are larger implications. But before I discuss some of these implications, a pertinent question to ask is, why does hybridization occur?
[Henceforth, the term ‘hybridization’ will be used to refer to introgressive hybridization, unless stated otherwise.]
According to Leonard and colleagues (2013), although the reasons for hybridization, specifically in the case of canids, are varied and complex, most hybridization events around the globe have been driven by two scenarios – i) the population of one or more species is so small that it becomes difficult for individuals to find mates within the population, or ii) the population(s) is subject to anthropogenic pressures in the form of landscape changes and human-induced mortality. An example of the first scenario is in Africa where at least one small population of the Ethiopian wolf – an endangered species – hybridized with free-ranging dogs, and the hybrids backcrossed and integrated into the population, thereby threatening the genetic integrity of the Ethiopian wolf population. Coywolves or coyote-wolf hybrids are a result of the second scenario playing out in North America where early European settlers wanted to clear large swathes of land for industrial and agricultural use (Rutherford, 2018). While agricultural expansion allowed coyotes to extend their range into former grey wolf territories, the range of grey wolves began to severely contract as a result of landscape change, hunting and poisoning (Stronen & Paquet, 2013). Consequently, the removal of geographical barriers between the two species made it possible for the suppressed grey wolves to hybridize with coyotes.
Impacts of Hybridization
From an ecological and evolutionary point of view, the impacts of hybridization in canids are both positive and negative depending on whether the cause for hybridization is natural or anthropogenic. The mixing of gene pools of formerly distinct taxa as a result of anthropogenic pressures has been variously referred to as “genetic assimilation”, “contamination”, “infection”, “genetic deterioration”, “genetic swamping”, “genetic pollution”, “genetic takeover”, and “genetic aggression” (Rhymer & Simberloff, 1996). The term ‘hybrid’ has negative connotations (Stronen & Paquet, 2013) because of the possibility of impacts such as “loss of reproductive potential, lowered fitness of individuals that hybridize, introduction of maladaptive alleles into wild populations, loss of genetic integrity, potential for disease transfer, and legal consequences that may affect the individual or population’s conservation status” (Leonard et al., 2013). Besides, hybrids may exhibit certain behaviours that are threatening to both animals and humans. For instance, coywolves have a blend of morphological and behavioural characteristics where their size falls somewhere in between that of coyotes and wolves, and their behaviour is such that, while they hunt in packs like wolves, they are fearless around humans like coyotes (Rutherford, 2018).
Although the negative impacts of hybridization are concerning for many scientists, there are others who view hybridization as a strategy for wild organisms to coexist with humans and to avoid extinction, especially in rapidly transforming environments (Galov et al., 2015; Stronen & Paquet, 2013). As landscapes change and novel habitats are formed, hybridization may lead to rapid evolutionary change in existing taxa by the formation of novel genetic variations which may increase fitness and adaptation to new environments (Rhymer & Simberloff, 1996). Therefore, depending on whether hybridization decreases or increases fitness and adaptation to an environment, hybridization is considered ‘genetic pollution’ or ‘genetic rescue’, respectively (Stronen & Paquet, 2013).
To Love or Not To Love?
The preservation of wild canid hybrids is a highly contentious issue. While some researchers and activists highlight the ecological, evolutionary and cultural significance of hybrids, others, mainly from conservation circles, underscore the importance of preserving the genetic integrity of wild species (Donfrancesco et al., 2019). Because people generally enjoy variety (Stronen & Paquet, 2013), the potentiality of hybridization to homogenize distinct populations (Servedio et al., 2013) and make them all look the same is perhaps unsettling. Further, behavioural manifestations such as lack of fear and aggressiveness towards both animals and humans make hybrids extremely unlovable pests (Rutherford, 2018). On the other hand, pure-bred dogs make such fantastic pets and garner so much love and attention for the simple reason that they are products of thousands of generations of artificial selection by humans for “tame” animals (Leonard et al., 2013). In other words, people are generally comfortable around creatures that are predictable and manageable.
Anthropogenic hybridization in wild canids is a by-product of suppression and control of these animals by humans. However, these animals have managed to overcome this suppression by crossing species lines and blurring species boundaries. Paradoxically, although hybrids may possess the greatest morphological and behavioural adaptability to modified environments, they are amongst the least recognized animals because of their liminal or in-between status (Donaldson & Kymlicka, 2011). Recent findings suggest that hybridization is a widespread phenomenon across the globe, necessitating a better understanding of hybridization dynamics and the ecological role of hybrids in human-modified landscapes (Stronen & Paquet, 2013).
As Rutherford (2018) states, “animals are individuals with lifeworld’s of their own” and therefore, co-existence and not control may be the way forward.
- Donaldson, S., & Kymlicka, S. (2011). Zoopolis: A Political Theory of Animal Rights. Oxford: Oxford University Press.
- Donfrancesco, V., Ciucci, P., Salvatori, V., Benson, D., Andersen, L. W., Bassi, E., … Mukherjee, N. (2019). Unravelling the scientific debate on how to address wolf-dog hybridization in Europe. Frontiers in Ecology and Evolution, 7(May), 1–13. https://doi.org/10.3389/fevo.2019.00175
- Galov, A., Fabbri2, E., Caniglia, R., Arbanasić, H., Lapalombella, S., Florijančić, T., … Randi, E. (2015). First evidence of hybridization between golden jackal (Canis aureus) and domestic dog (Canis familiaris) as revealed by genetic markers. Royal Society Open Science, 2(12). https://doi.org/10.1098/rsos.150450
- Harrison, R. G., & Larson, E. L. (2014). Hybridization, introgression, and the nature of species boundaries. Journal of Heredity, 105(S1), 795–809. https://doi.org/10.1093/jhered/esu033
- Leonard, J. A., Echegaray, J., Randi, E., & Vilà, C. (2013). Impact of hybridization with domestic dogs on the conservation of wild canids. In M. E. Gompper (Ed.), Free-Ranging Dogs and Wildlife Conservation (pp. 170–184). Oxford University Press.
- Rhymer, J. M., & Simberloff, D. (1996). Extinction by hybridization and introgression. Annual Review of Ecology and Systematics, 27, 83–109. https://doi.org/10.1146/annurev.ecolsys.27.1.83
- Rutherford, S. (2018). The Anthropocene’s animal? Coywolves as feral cotravelers. Environment and Planning E: Nature and Space, 1(1–2), 206–223. https://doi.org/10.1177/2514848618763250
- Servedio, M. R., Hermisson, J., & Van Doorn, G. S. (2013). Hybridization may rarely promote speciation. Journal of Evolutionary Biology, 26(2), 282–285. https://doi.org/10.1111/jeb.12038
- Stronen, A. V., & Paquet, P. C. (2013). Perspectives on the conservation of wild hybrids. Biological Conservation, 167, 390–395. https://doi.org/10.1016/j.biocon.2013.09.004