Ligers, Zorses and Bottlenose Dolphins

Most people do not realise that there are numerous species of Bottlenose Dolphins; the exact number is a subject of strong debate and depending on the background of whom you ask will strongly influence the number they give. For example, a conservationist, a geneticist and a taxonomist are unlikely to agree on such matters. However, there are at least three species formally recognised by everybody, each with their own behavioural and physical characteristics. Of the three recognised species Tursiops truncatus, known as the Common Bottlenose Dolphin, is the species that most people are familiar with. This is owing to it’s near global distribution (it is found in every sea except those in the polar regions), its prevalence in popular culture – be it films or TV and of course its popularity in aquariums during the latter half of the 20th century. The second of the three species, Tursiops aduncus, is more often referred to as the Indo-Pacific Bottlenose Dolphin and as its name suggests is found in the Indian and Pacific Oceans only. Whereas T. truncatus is found in both coastal and offshore environments T. aduncus is principally only found in coastal waters. The third species, Tursiops australis, known as the Burrunan Dolphin is found only in coastal waters of parts of Australia.

One of the most controversial topics in in the field of biology is the subject of what defines a species. Certainly in most high schools, pupils are taught that animals belong to the same species if they can reproduce and form fertile offspring. This is undoubtedly complete fallacy and a recent topic of research that I have been involved in significantly proves this. This research, led by Dr. Tess Gridley of the University of Cape Town, has just been published and I provided the genetics elements included in the paper. The research focuses on the production of fertile hybrids by two species of Bottlenose Dolphin when kept together in captivity. Hybrids are the offspring of two different species – famous examples include the Liger (the offspring of a Lion and Tiger) and Zorse (yes you guessed it, the offspring of a Zebra and a Horse).

Hybrids
Hybrid animals are remarkably common. Here we see a Liger (Lion and Tiger hybrid) on the left and a Zorse (Zebra and Horse hybrid) on the right.

First of all, let’s deal with the elephant in the room. Yes, this research is based on dolphins kept in captivity. Let me be absolutely clear that I am in no way an advocate for keeping any species of cetacean in captivity. When the individual dolphins on which this research focusses were taken into captivity it was the 1970s, at which time our understanding of cetacean biology, in particular their emotional intelligence, was significantly inferior to our understanding today. Like all areas of knowledge, our understanding progresses through time and moral humans adapt their behaviour and actions to take account of this improved understanding. Flipping this on its head, we should be reticent to judge people who made decisions in the past with which we would normally condemn when judging by todays understanding, morality and societal will. Times and understanding were different then and as long as we are willing to, pragmatically and sensibly, adapt our actions today to take account of our improved understanding then we should look forward and not back. No, we should not be taking new cetaceans into captivity but those that currently are kept in aquaria, like those in this study, provide an opportunity to expand our knowledge of cetaceans such that we can continue to improve our decision making in the future; thus, having greater benefit for the conservation of wild cetaceans.

Our research focussed on two dolphins and their offspring. The first, a male Tursiops truncatus by the name of Gambit, and the second a female Tursiops aduncus by the name of Frodo. As well as physical characteristics (Frodo has speckling on her underside, a feature common in older Tursiops aduncus), we confirmed their species identity genetically. This is done using DNA extracted from blood taken from routine veterinary check-ups. The principal finding of this study revealed that hybrid and backcross offspring were fertile – proven by a second generation in both cases.

Backcross fertile
One of the apparently fertile backcross offspring featured in this research.
Picture1
The underside of Frodo, showing her belly speckling that is a common feature in mature Tursiops aduncus individuals.

This finding is important for two reasons. Firstly, it adds further weight to current scientific thoughts on evolution as a process. We like to think that evolution is a linear process and that once a new species is formed it is permanent until such time that it may go extinct due to some natural disaster or change in environment. We know, however, that this is not the case at all. There are a number of emerging examples that show species emerged in the past, likely as a result of physical separation, but then disappeared again when the physical barrier was removed because they simply merged and interbred with their parent species. A great example of this comes in the form of the Common Raven. We also know that reticulation, or the interbreeding of species during speciation is common and demonstrating the production of fertile hybrid offspring in this study provides a mechanism for this to happen.

Perhaps more importantly, however, this study demonstrates the potential resilience of Bottlenose Dolphins to adapt to changing environments. By producing fertile offspring, the success of gene flow events between different species of Bottlenose Dolphin may allow them to adapt to a more coastal or more pelagic way of life more readily should the need arise. We should take encouragement in this new understanding; although life in our oceans is currently under a great many threats it is likely that, thanks to the plasticity of evolution, the famous smile of a Bottlenose Dolphin will continue to greet us for many generations to come.

 

 

This research is published in PLoS ONE, Sepember 2018. You can download a copy of the paper here.

Full paper citation:

Gridley, T., Elwen, S.H., Harris, G., Moore, D.M., Hoelzel, A.R. and Lampen, F., 2018. Hybridization in bottlenose dolphins—A case study of Tursiops aduncus× T. truncatus hybrids and successful backcross hybridization events. PloS one, 13(9), p.e0201722.

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Current research

My current research is focussed on developing molecular markers for deep water sharks and is being conducted at Edge Hill University with fellow early career researchers David Goodson, Thom Dallimore and Carl Barker. Deep water sharks are poorly understood relative to their shallow water counterparts but are often even more vulnerable to anthropogenic pressures. Deep water fishing fleets are increasing in size every year and it is well known that many deep water sharks aggregate in single sex or age cohort groups meaning a single trawl can have a very significant impact to localised populations. It is vital that we understand more about these deep water species in order to effectively manage conservation efforts. Key to this understanding is knowledge on population structure and connectivity. The vital first step on this road is the development of molecular markers to enable comparisons.

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The shark samples I am working on all come from the Rockall Trough, an area of deep water off the coast of Scotland, and were collected whilst on board the FRV Scotia during a Marine Scotland deep water survey. I am principally targeting the sharks Galeus melastomus and a selection of sharks from the genus Apristurus, including some additional samples very kindly supplied by Dr Ana Verissimo of CIBIO – University of Porto. I am also attempting to replicate the excellent work completed by Helyar et al (2011) who developed microsatellite markers in Centroselachus crepidater. Whilst initial thoughts were to jump straight to population examination through the use of Amplified Fragment Length Polymorphisms (AFLPs) we were encouraged to attempt to develop microsatellite markers by Dr Jim Provan (Queens University Belfast) whilst presenting a poster at the BES Ecological Genetics Group 59th Annual Conference. With his guidance that is exactly what myself, fellow Technician David Goodson and PhD students Carl Barker and Thom Dallimore are now attempting. Please follow me on Twitter for updates and other exciting Marine Vertebrate news.

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Helyar, S., Coscia, I., Sala-Bozano, M., & Mariani, S. (2011). New microsatellite loci for the longnose velvet dogfish Centroselachus crepidater (Squaliformes: Somniosidae) and other deep sea sharks. Conservation Genetics Resources, 3(1), 173-176.

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