Sensory ecology in conservation breeding programmes: an overlooked, but vital, consideration

Captive breeding programmes are key to species reintroduction strategies, but could potentially be associated with adaptations that are maladaptive in their natural habitat. In a recent paper, Chris Freelance and colleagues explored differences in sensory organ morphology between wild and captive-bred populations of a critically endangered insect.

Captive breeding programmes are a well-established part of threatened species conservation strategies. These programs create insurance populations against extinction in the wild and contribute to species reintroductions. However, as the aim is to produce as many healthy animals as possible, captive breeding environments are, by their very nature, benign: shelter and preferred food is in ready supply, potential mates are in close proximity, and there are no predators.

It is often overlooked that captive environments are like any other environment: populations will adapt to them over generations. With relaxed natural selection pressures, adaptations to benign captive environments are often maladaptive to the native environment – this may be problematic during reintroduction as they may be less efficient at performing behaviours required in the wild..

In captivity, male (bottom) Lord Howe Island stick insects likely rely less on sex pheromones to locate females (top) than they would in the natural habitat. Photo: Rohan Cleave Zoos Victoria

Sensory ecology and captive environments

Recent research from my colleagues and I explored whether adaptations to captivity may extend to sensory organs, which are adapted to detecting salient information (e.g. odour of food plants) against the “background noise” (e.g. odours from non-food plants) in the environment. In captivity, easy access to shelter, food, and potential mates means individuals can rely less on plant odours or sex pheromones to locate these resources. In this simplified sensory environment, natural selection should favour individuals with less sensitive sensory organs.

To explore this, we compared individuals from different populations of the critically endangered Lord Howe Island stick insect. The insect was extirpated from Lord Howe Island in the 1920s and rediscovered on Ball’s Pyramid in the early 2000s, since which a population founded from Ball’s Pyramid has been bred in captivity for over 14 generations at Melbourne Zoo.

The captive population is intended to be a source of individuals for reintroduction onto Lord Howe Island. We also obtained preserved specimens from the Lord Howe Island population (pre-extirpation), the generation on Ball’s Pyramid that founded the zoo’s captive population, and two generations of the captive bred zoo population.

As expected, we found that individuals from the captive population have smaller eyes and fewer odour receptors on their antennae compared to individuals from the extirpated wild Lord Howe Island population. The exact drivers of the pattern are unclear, but these results are consistent with sensory organs potentially becoming adapted to a benign environment.

In the wild, Lord Howe Island stick insects rely on plant odours to locate food and shelter. Photo: Rohan Cleave Zoos Victoria

Practical solutions

Experiments are still needed to quantify the impact on the ability of the captive bred insects to use visual and scent information to locate resources in a more natural habitat. Nonetheless, these findings raise interesting questions about how captive populations can be better managed.

In the context of sensory systems, there are some potentially easy solutions. Captive populations could be presented with a variety of both food and non-food plants so the insects must use plant odours to locate the food plants; additionally, diet itself directly influences the abundance of odour receptors on insect antennae. Lower density living may increase the need for males to use sex pheromones to locate a female. Introducing other sensory elements from the natural habitat, such as playing recordings of bird songs and other natural sounds, could also reduce the likelihood for sensory organ adaption to captivity.

Crucially, most of these measures are relatively cost effective and easy to implement and may also increase the expression of natural behaviours, which is important for captive animal welfare. These principles are relevant not only to insects but can be extrapolated to vertebrates as well: better matching the sensory complexity of the natural environment in captivity may reduce sensory adaptations to unnatural environments.

Beyond sensory systems, this research carries an important reminder: all organisms adapt to their environment – natural or artificial – through natural selection over time. If populations bred in captivity are to be released as part of threatened species reintroductions, minimising potential adaptations to captivity is essential to ensure those animals can efficiently express appropriate behaviours in the natural environment. Incorporating evolutionary ecology into conservation programme design is vital for achieving this.

Read the full paper Long-term captivity is associated with changes to sensory organ morphology in a critically endangered insect in Journal of Applied Ecology.

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