Changes in the Life-History Disease of the Tasmanian Devil Populations



Abstract

Changes in life history are expected when new sources of extrinsic mortality impact on natural populations. We report a new disease, devil facial tumor disease, causing an abrupt transition from iteroparity toward single breeding in the largest extant carnivorous marsupial, the Tasmanian devil (Sarcophilus harrisii), in which males can weigh as much as 14 kg and females 9 kg. This change in life history is associated with almost complete mortality of individuals from this infectious cancer past their first year of adult life. Devils have shown their capacity to respond to this disease-induced increased adult mortality with a 16-fold increase in the proportion of individuals exhibiting precocious sexual maturity. These patterns are documented in five populations where there are data from before and after disease arrival and subsequent population impacts. To our knowledge, this is the first known case of infectious disease leading to increased early reproduction in a mammal. The persistence of both this disease and the associated life-history changes pose questions about longer-term evolutionary responses and conservation prospects for this iconic species.

Table of Contents

Abstract2

1. Introduction4

1.1 Identifying the issue4

1.2 Describing the issue6

1.3 How biologists addressing the issue9

2. The implication of method12

3. Results15

4. Discussion18

5. Conclusion19

End Notes22

Tasmanian Devil

1. Introduction

Ever since the last lonely dodo expired on Mauritius, in the Indian Ocean, islands have taught us much of what we know about the process of extinction. Conservation biology coalesced as a discipline, during the 1970s and 1980s, from efforts to understand the special problems that threaten small populations; and small populations are most commonly found on islands and island-like patches of habitat. Now another island phenomenon is delivering stark confirmation of one of conservation biology's cardinal precepts: that small populations, having small gene pools and little genetic diversity, face large dangers from unpredictable environmental assaults, including disease. This latest news, weird and sad, comes from Tasmania, Australia's island state, where the Tasmanian devil (Sarcophilus harrisii) is suffering an epidemic of contagious cancer.

1.1 Identifying the issue

The devil, a marsupial carnivore native only to Tasmania, has had an up-and-down demographic history since Europeans and their livestock colonized the island. Perceived as inimical, like coyotes in the American West, it suffered bounty killing in the 19th century and poisoning with strychnine in the early 20th. By the time it became appreciated as an iconic element of Tasmania's indigenous wildlife, it had passed through severe (but not precisely measurable) population bottlenecks — that is, periods of badly reduced abundance, during which devils were scarce on the landscape. Thanks to its high reproductive rate and opportunistic behavior, it had recovered nicely — at least in sheer numbers — and by the 1990s, according to one estimate, the wild population stood at about 150,000. But of course genetic diversity rebounds much more slowly than population size.

Fig 1: Tasmanian Devil

The first hint of a new kind of trouble came in 1996, when a Dutch wildlife photographer named Christo Baars noticed strange lumps on the faces of devils he photographed at ...