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A survey of koala road kills in New South Wales

Canfield, PJ 1991, Journal of Wildlife Diseases, vol. 27, no. 4, pp. 657-660.

With increasing fragmentation of remnant forest along Australia’s east coast, motor vehicle accidents have become a significant cause of injury and death in koala populations. In a survey of koala road kills conducted in the northern coast of New South Wales between 1984 and 1990, 75 koalas were found to have died from injuries relating to motor vehicle accidents. After analysis, it was identified that healthy, young to middle-aged male koalas were more prone to vehicular incidents, especially during the mating period.

  Koalas received for necropsy were examined externally and internally for injuries, with overall body condition evaluated based on fat deposits and muscle mass. Throughout the study period, deaths occurred more frequently between August and December and consisted predominantly of males in good body condition aged between two and seven years. The study identified head trauma (44 koalas) as the main injury resulting from motor vehicle accidents, with limb injuries (16) and hemothorax (15) also common. Twelve koalas examined showed signs of underlying disease, primarily prostatitis or periovarian cystitis. Only one of these koalas had poor body condition.

  Seasonal variation in deaths is a clear trend established by this study, resulting from greater male mortality between August and December. This can be explained by increased male activity, as individuals are more mobile during the mating season which occurs in the spring and summer. Koalas are thus more likely to traverse fragmented habitat where the probability of vehicle collision is higher. Similarly, the prevalence of deaths in males aged between two and seven years is a likely consequence of increased movement due to heightened sexual activity during the mating season. In addition, young male koalas aged between two and five years are establishing their independence, dispersing from their mother’s home range. Young males can travel great distances during this time in which they are more likely to encounter roads than koalas of other demographics. While some koalas possessed underlying disease, this is unlikely to have influenced the prevalence of road collisions, as individuals maintained good body condition.

  As the majority of koalas involved in vehicular accidents are healthy and sexually active, a number of implications arise for the management of local populations. Primarily, with the loss of young to middle-aged males, populations will inevitably develop changes in age structure and sex ratio. Such changes are likely to impact population viability by reducing reproductive output and genetic variation. Local road management and education are key tools in mitigating this issue.

 

Summarised by Julian Radford-Smith

 

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