Life-history change in disease-ravaged Tasmanian devil populations
Jones et al (2008) PNAS July 22, 2008 vol. 105 no. 29 10023–10027
Tasmanian devils (Sarcophilus harrisii) are the largest extant marsupial carnivore – male specimens can weigh up to 14kg, while females can reach 9kg. They have long been extinct on the Australian mainland. Recently, an infectious tumour (Devil Facial Tumour Disease, or DFTD) has afflicted the remaining population of Tasmanian devils, resulting in considerable population decline.
DFTD is a peculiar disease: it’s an infectious tumour which is spread via injury. Technically, and unusually, transmission follows transfer of tumour cells via injury – the tumours are said to be allografts – the tumour cells afflicting an individual are Tasmanian devil cells (albeit abnormal) but they are genetically distinct from the afflicted individual. Tasmanian devils are a bit rough, and frequently injure each other by facial injuries – this is the route by which infection occurs. A similar disease, Canine Transmissible Venereal Tumour (CTVT) is found in dogs – it is transmitted during copulation.
DFTD was first reported in 1996 – by 2007 it had reached about half of the range occupied by Tasmanian devils, and has resulted in declines of up to 89% in some populations. As one might imagine, the appearance of this condition results in a strong selective pressure, This paper investigates the consequences of this DFTD on reproductive strategies in Tasmanian devil populations. I’ve come to this paper as a non-specialist (I’m a Drosophila geneticist, and this paper is essentially an ecology paper).
Prior to the appearance of DFTD, Tasmanian devils would live to abut 5 or 6 years old, generally breeding from their second year (breeding in successive years – iteroparity). It seems these marsupials are adapted for a “breed quick, die young” kind of strategy: reproduction is a tough business, with males for example losing about 25% of body mass in a breeding season. It is in light of this that the authors have investigated the consequences of DFTD on breeding age in infected populations, and find a shift to semelparity.
The authors compare the frequency of animals aged 3+ years in populations sampled before and after DFTD invasion. It is pretty clear that not only has the frequency of devils of age 3+ years dropped markedly (in some locations older animals are entirely absent), but that the frequency of one year old females that are breeding has increased – in some locations from 0% to over 50%. So what are the authors’ conclusions? Principally, they conclude that this is an adaptation to a strong selective pressure – a possible mechanism being that reduced population density permits more rapid growth of individuals and therefore permitting earlier reproduction. An alternative hypothesis, that the cause of earlier reproduction is a reduction in interference competition is given short shrift (though I don’t have the expertise to understand why!).
An interesting paper, on a fascinating disease!
semelparous – breeds once before death (see this article on reproduction)
iteroparous – breeds in several successive seasons (see this article on reproduction)
M. E. Jones, A. Cockburn, R. Hamede, C. Hawkins, H. Hesterman, S. Lachish, D. Mann, H. McCallum, D. Pemberton (2008). Life-history change in disease-ravaged Tasmanian devil populations Proceedings of the National Academy of Sciences, 105 (29), 10023-10027 DOI: 10.1073/pnas.0711236105