The little-known downsides to captive breeding programs
The answer is: California condors, Black-footed ferrets, and Kihansi spray toads. The question is: what species would surely be extinct if not for captive breeding efforts? And they’re not the only species whose very existence is thanks to the hard work of zoos and other conservation institutions as they implement captive breeding and reintroduction programs in an effort to stave off extinction. But captive breeding, for all the good it has done, is not without its drawbacks.
Breeding over multiple generations in captive settings will inevitably lead to small biological changes between the captive and wild lineages, and those tweaks can become magnified over time. But what happens when those changes in phenotype – that is, the way genes are expressed – impact reproduction-related behaviors?
If reintroduced animals are more likely to mate with other reintroduced animals and their descendants – even after being returned to the wild – then the captive bred animals aren’t actually supporting the wild population. In effect, there would simply be two relatively independent populations of similar-looking animals, and given enough time, they could diverge into two separate species.
How much of this concern is conjecture and how much of it reflects a real possibility? That’s what University of Melbourne zoologist Brendan Slade wanted to know. So and his colleagues from the University of Melbourne and Zoos Victoria put together an experiment to find out. Their results were reported in the journal Biology Letters.
Slade turned to an animal that’s become a workhorse in modern biomedical research – so we know a lot about its genetics – and that reproduces rapidly: the house mouse. They captured some wild mice from an open field at Victoria’s Werribee Open Plains Zoo and reared them in captive conditions until three generations of their descendants were born in captivity.
Then, the third-generation captive-bred adult mice were released into semi-wild outdoor pens along with wild adult mice. There were nine pens and a total of twelve mice per enclosure: six captive-bred (three males, three females), and six wild (three males, three females). The enclosures were covered with shade structures to avoid losing the mice to their natural avian predators. For twenty weeks, the researchers observed the mice going about their business, keeping watch for any pairs successfully giving birth to baby mice.
First, the researchers needed to see whether the captive bred mice really were genetically distinct from their wild counterparts, even if only slightly. There were some outwards signs that the two groups differed. Captive mice were generally heavier in body mass than the wild mice, for example. There were also detectible genetic differences between the groups. The captive-bred group had slightly lower allelic diversity, for example.
By the end of the twentieth week of their reintroduction, there were a total of 59 litters of 189 baby mice, from 40 pairings. A staggering 83 percent of these litters resulted from same-source pairings, meaning both parents were either captive-bred or wild. Put another way, fewer than one out of five litters was the result of a mixed marriage between captive-bred and wild mice.
This phenomenon is called “assortative mating,” and it means that there were strong preferences among the mice to mate with their own kind. “Our findings reveal a potential concern for release programmes [sic] that appears to have received very little previous consideration,” write the researchers. “After just three generations in captivity, we found that captive–captive and wild–wild pairs accounted for 83% of all offspring produced, inferring strong assortative mating between animals in relation to their origin.”
What could have resulted in these strong mating preferences? It could be that the captive environment altered traits that were important to mouse mate preferences, like body size or odor, making captive mice unattractive to wild ones, or vice versa. It’s also possible that the “managed” pairings while being bred in captivity to retain maximal genetic diversity could make captive mice more likely to prefer mating with other captive mice simply because their genes were more complementary. (That’s possible if the “compatible genes hypothesis” is true.) Third, it’s possible that there were fewer opportunities for captive and wild mice to breed simply because they encountered each other less often. That could be due to differences in activity schedules each day, or in which parts of the habitat they preferred to hang out.
It’s a worrying trend because if these findings apply more broadly than just mice, it could impact the long-term success of captive breeding programs. Just because reintroductions are successful doesn’t mean that the genes of reintroduced individuals mix in to the wider population of at-risk animals. And that’s a problem, according to Slade, when “there is an urgent need to improve fitness of wild individuals by the integration of new genes to bolster genetic diversity.” For species with parental care, mixed pairing may be even more critical, as the wild-reared parent would be necessary to teach the offspring important skills related to foraging and avoiding predators.
Slade and colleagues conclude by calling on researchers to report the mating outcomes following reintroductions, since so little is known about the potential pervasiveness of this problem. They argue that conservation managers ought to consider the issues related to assortative mating as they design and manage their breeding and reintroduction programs, so that “changes…could be implemented to address the problem.” Just what those changes might be, though, is an exercise left for the reader’s imagination. – Jason G. Goldman | 03 December 2014
Source: Slade B., A. Paproth, M. J. L. Magrath, G. R. Gillespie & T. S. Jessop. (2014). Assortative mating among animals of captive and wild origin following experimental conservation releases, Biology Letters, 10 (11) 20140656-20140656. DOI: http://dx.doi.org/10.1098/rsbl.2014.0656
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