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The Guts of the Captive Breeding Problem

Captive lions have different skull shapes compared with their wild counterparts, possibly due to a high-quality diet and fewer bones to chew. Credit: Cat Bell/Adobe

Captive lions have different skull shapes compared with their wild counterparts, possibly due to a high-quality diet and fewer bones to chew. Credit: Cat Bell/Adobe

By Stephanie Courtney Jones

The discovery that captivity affects the internal organs of animals has significant implications for their successful reintroduction to the wild.

Captive breeding first came about with the domestication of animals and plants thousands of years ago, followed by the keeping of animals in royal menageries and zoological gardens for amusement, and only as recently as the 1960s with the development of captive breeding programs for the conservation of threatened species.

However, breeding and raising animals in captivity presents challenges for animals once they are reintroduced back into the wild, potentially reducing their ability to survive and to find mates. To this end, a wave of conservation research has investigated ways to improve the success of captive breeding programs, with researchers now attempting to predict how an individual animal will perform following reintroduction.

Much of this work has focused on the behaviour of animals in captivity, with the absence of predators and competition for mates leading to behaviours that contribute to their failure at reintroduction. However, morphological changes that occur in captivity are often unnoticed.

In captivity, animals face changes in various environmental conditions, such as diet, nutrition and cognitive stimulation. These changes in the captive environment can lead to changes in external morphological traits. For example, captive lions and tigers have different skull shapes compared with their wild counterparts, possibly due to the high-quality diet that lacks bones for captive big cats to chew.

However, changes in morphology aren’t always easy to see. Captivity can also cause changes in soft tissues and organs, with research revealing changes in the size and shape of the brain and digestive system. For instance, captive-reared mallards look exactly the same as wild mallards, but the captive birds have lighter gizzards. This affects their ability to reach the body conditioning required for their successful reintroduction.

Since undetected changes in anatomy may impact the success of reintroductions, we wanted to further explore the effects of captivity on morphology. The difficulty lies in getting to the guts of the problem. How do you measure morphological changes in captive animals?

One simple approach is to compare captive-bred and wild animal morphology to determine if there are any differences, and determine what they are. The benefit of this comparison is that the wild individuals will provide a baseline for what animals should be like in the wild. Any difference from the wild baseline is likely to be unfavourable for captive-bred species being reintroduced to the wild. This approach may be useful for predicting how well an animal will survive and reproduce following reintroduction, and potentially be used as criteria to select captive animals for reintroduction.

We have compared the external and internal morphology of mice bred in captivity with mice caught in the wild. Because females and males are often sexually dimorphic, we also factored in sex-based morphological differences. We measured the body shape and size of mice, and then dissected them to examine and measure the digestive system, brain, heart, lungs, and other accessory organs.

We found that captive-bred and wild mice superficially looked the same, with no obvious differences in body size or shape. Only females and males had morphological differences.

It was only after dissection that we saw morphological differences between captive-bred and wild mice. The kidneys and spleen were lighter, and the small intestine was shorter.

Changes in the digestive system of captive mice could be a result of the quality and quantity of food in captivity. With good quality food, a long intestine is excess to requirements, and the kidneys and spleen may be smaller because there are less challenges such as disease and parasites in captivity.

The old adage “use it or lose it” comes to mind. In this case, captive animals don’t need to invest energy in larger organs because of the comfortable, disease-free environment maintained in captivity. We know that reintroduced animals don’t always survive, and perhaps these morphological changes might be one of the contributing causes of mortality.

However, organs can be plastic, adjusting to suit their environment. While the organs of wild animals being rehabilitated can adapt to conditions in captivity, it’s not known how quickly their organs can adapt when they are reintroduced back into the natural environment. They may have the right instincts and behaviours for foraging and finding food, but if their guts aren’t able to digest food efficiently they may remain hungry or be unable to fend off disease, competitors and predators.

A potential approach to improving the success of captive breeding programs may therefore be to prepare animals for the conditions they will encounter in the natural environment, including a poorer diet and exposure to disease and parasites, to enable their digestive system to adapt. For example, the survival of pheasants is higher if they’ve been exposed to more natural diets in captivity prior to release. One of the mechanisms that increases their survivorship is altered intestine and caecum lengths to suit a natural diet.

We have found that there is more to what meets the eye when looking at the impacts of captivity on animal morphology. Some anatomical changes, and their impacts, would be unknown without further investigation. Finding out which morphological changes occur in captivity, and why they occur, is an important step towards improving and refining captive breeding and reintroduction programs aimed at saving endangered species from extinction.

Stephanie Courtney Jones is a Conservation Wildlife Officer at Tidbinbilla Nature Reserve, and conducted this research at the University of Wollongong. It has been published in Royal Society Open Science (