Cross-Fostering: a New Strain of Wolf Conservation

Sometimes legal protection, captive breeding and reintroduction aren’t enough to save the endangered wolf.

With a reduced gene pool it’s more vulnerable to diseases, deformities, and more likely to cross-breed with other species. Take the hybridisation of red wolves and coyotes, for example, or the Isle Royale pack, whose sickly cub has a hunched posture from in-breeding. Most Scandinavian grey wolves are also descended from just five individuals, meaning that many alpha pairs may now be siblings. Due to quarantine laws, importing or relocating wolves is not always an option, and difficult terrain and culling can make natural migration almost impossible. Thankfully, the above efforts may form a new solution.

Cross-fostering involves adding or swapping pups among packs so that there is more genetic diversity. It can also increase the survival chances of a captive-bred pup, as it would be nursed and socialised by a wild pack and gain a stronger immune system. However, the secretive and territorial nature of wolves make it a risky and challenging process, as Schultz et al. discovered in 2007.

Published in The Canadian Field Naturalist, their study attempted to place four wild 13-14 week-old pups with a neighbouring pack when their own was relocated after attacking livestock. They were introduced to one another via a shared feeding site, but all but one of the pups was eventually killed by the other wolves or by disease, and the lone survivor was later culled in adulthood.

Happily, the first cross-fostering attempt in Europe was a different story. Scharis and Amundin’s 2015 study in Zoo Biology placed eight 3-4 day-old pups among packs in Scandinavian zoos, and their survival rate was little different to that of the biological pups. Although it used a small sample size of captive-bred wolves, the study suggested that for greatest success the pups should be no more than eight days younger than their foster siblings, and less than three weeks of age – the period between 3-7 weeks is when wolves begin to recognise their young. Pups were also introduced directly into the mothers’ dens. This is easy enough for a pack in captivity, but would this work in the wild?

According to the U.S. Fish and Wildlife Service’s red wolf recovery programme, it’s a resounding “yes”. By 2010, they had introduced a total of 26 captive-bred red wolf cubs.

Thanks to strict monitoring of the wolves they were able to estimate when and where a pack had given birth, and when the mother was away, added the new pups to the litter. As in Scharis and Amundin’s study, they made sure that the pups were of similar age, were less than three weeks old, and as an extra precaution, smeared the foster pups’ urine on the wild pups for a consistent scent. In almost all cases, the mother accepted the new young.

Unfortunately, there may always be a risk of the fostered wolves being illegally shot or culled later in life, and timing is essential for cross-fostering to work. However, without the legal protection, captive-breeding and reintroduction programmes set up by wolf conservationists, this new method of conservation would not be possible, showing that diversity, both tactical and genetic, is essential for saving wolves from extinction.


Delene-Beeland, T. 2010. “Red wolf pups go into wild”. U.S. Fish and Wildlife Service Red Wolf Recovery Programme. Available at:

Mills, A. 2015. “Down to three wolves on Isle Royale”. Michigan Tech News. Available at:

Scharis, I., and Amundin, A. 2015. “Cross-fostering in gray wolves (Canis lupus lupus)”, Zoo Biology 34(3): 217-222.

Schultz, R.N., Wydeven, A.P., Winn, L.S., and Buller, S.A. 2007. “Attempt to cross-foster gray wolf (Canis lupus lupus) pups into another pack”, The Canadian Field-Naturalist 121(4): 430-432

This article was originally published in Wolf Print #57, magazine of the UK Wolf Conservation Trust.


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