Measuring fitness loss in captive breeding programs: Implications for threatened species recovery

A project undertaken at the Zoos Victoria, and supervised by Dr Tim Jessop

Human impacts, both direct and indirect, are having an increasingly negative effect on the ability of many animal species to survive in their natural environment. As a consequence, captive breeding and reintroduction have become high profile and often controversial methods for species conservation. The major aims of captive breeding programs are to maintain healthy captive populations as insurance against extinction of endangered species and to preserve the option of species reintroduction as a way to re-establish free-living populations or to supplement breeding in declining populations . However, as many of the processes that have led to localised animal extinction are long-term, many endangered species may spend several generations in captivity before suitable habitat is available for their reintroduction. As a result, many of the high cost reintroduction programs implemented in recent years have met with limited success , as captive-bred animals often under-perform in both survival and reproduction compared with translocated or wild conspecifics. The failure of captive-bred individuals to successfully exist in the wild has important implications for conservation biology as the reduced fitness of these animals not only delays or prevents the overall recovery of the natural population, but inbreeding or supplementation of wild populations with captive-bred stock may even reduce a wild population’s overall fitness. It is therefore essential that conservation biologists gain a better understanding of not only how captivity alters fitness traits desirable in the wild, but also the time-frame in which these changes are likely to occur. Captive breeding and reintroduction can only be effective management tools if fitness traits essential for a life in the wild are maintained in captivity.

One possible explanation for the reduced fitness of captive-bred animals is that maintenance in captivity inevitably leads to some degree of adaptation or habituation to the captive environment and that this habituation is likely to become more pronounced the longer the animal is maintained ex situ. What is less understood is the timeframe within which this habituation occurs, as captivity-induced changes arise not only as a result of genetic changes occurring over generations, but also as a result of environmental experiences throughout an individual animal’s lifetime. This study had two aims; the first was to compare behavioural changes of wild house mice (Mus musculus) reared in captivity across multiple generations using a standardized open field test. The second aim was to examine the effects of short-term captivity on fitness proxies in reintroduced, first and third generation captive-born mice relative to translocated wild-bred mice under high and low population densities. Captivity appeared to have a significant effect on risk assessment and predator avoidance behaviour; with first and second generation captive mice spending less time exploring tunnels and more time hiding within tunnels and with second generation mice engaging in significantly more jumping bouts. Once released into a common ‘wild like’ environment, captive mice displayed increased reproductive fitness with more recorded pregnancies and lactations and were significantly heavier than wild born mice. Both captive and wild born mice increased weight throughout the experimental period while injuries generally decreased throughout the experimental period and were affected by population density. Overall these results indicate that significant changes in behaviour occur within only a few generations in captivity, which has important implications for many captive breeding programs that often maintain animals in captivity for several generations before release. Although a significant reduction in fitness proxies was not observed in this study, the experimental design used does indicate that the use of a soft-release protocol for reintroduction may confer several benefits including maintaining body weight and increasing reproductive performance in captive animals prior to full release.



Figure 1. Feral mice were used as a surrogate species to evaluate the fitness implications of captive breeding for reintroduction programs.


Figure 2. Experimental release enclosures enabled robust testing of the fitness implications of captive breeding
for species reintroduction programs.