Table 1.

Trait–risk relationship hypotheses tested in previous mammalian studies.

	trait	hypothesis	references
intrinsic traits	body mass	larger bodied mammals are more likely to have an elevated risk of extinction as they are targeted for hunting and persecuted. In addition, larger species usually have lower reproductive rates and thus slower population growth, making them more sensitive to exploitation, and low population density	[6,7,11–16]
	head body length	head body length has not been tested as a predictor of extinction in previous studies; however, it is strongly correlated with body mass	not tested previously
	inter-birth interval	species with slow life histories are less able to compensate for increased mortality through increased fecundity and are therefore more vulnerable to population extinction; a longer inter-birth interval is thus associated with an elevated extinction risk	[12,17]
	litter size	species with a low reproductive output, mediated by life-history attributes such as litter size, are more likely to be at greater risk of extinction	[5,7,11,18]
	age at first birth	the predictions regarding inter-birth interval above are also applicable here, as both are important components of reproductive rate; some studies have looked at the closely related age at sexual maturity	[7,11,14,19]
	geographical range size	small geographical range size is associated with elevated extinction risk because of its relationship with low population size and the concomitant risks from demographic and environmental stochasticity. A small geographical range indicates habitat specificity, i.e. small habitat breadth, which is likely to affect a species' ability to withstand habitat modification, and increases the risk that the entire species range is in the firing line	[6,7,11,12,14,17,19–21]
	home range size	larger home range size, which is a necessary product of larger body size, is also associated with higher extinction risk; this is because species where individuals have large home ranges, reflecting high resource demands, are particularly vulnerable to habitat loss and degradation and, in particular, to reserve edge effects in fragmented habitats; by contrast, small home ranges suggest lower individual energetic requirements and higher population densities, and thus a lower risk of extinction in low-productivity environments or remnant habitat patches	[5,12,14,22–24]
extrinsic factors	average human population density	higher human population density within a species' range means more competition for resources, and more opportunity for conflict and exploitation, and thus a higher risk of extinction; in addition, habitat degradation, fragmentation and destruction are more likely to occur in densely populated locations	[6,14,20]
	precipitation	precipitation, along with temperature, plays a complex role in its effect on mammal population sizes; in times of drought and in regions of low productivity or resource availability, populations may be highly variable, leaving them more vulnerable to extinction; precipitation is also related to latitude, and is likely to become an important factor in light of continuing climate change	[14,15]
	temperature	mean annual temperature is used as a measure of available ambient energy; temperature and precipitation are closely related, and predictions are as above	[25]
	AET	actual evapotranspiration (AET) reflects the joint availability of energy and water, and is regarded as an index of primary productivity; a low mean AET is therefore associated with an elevated risk of extinction	[14,19]
	PET	mean potential evapotranspiration rate (PET) is strongly influenced by temperature and reflects the potential availability of energy and water; a low PET is therefore associated with an elevated risk of extinction	not tested previously
	habitat breadth	narrow habitat breadth suggests specialization and therefore poor ecological adaptability and flexibility; specialists are more susceptible to habitat modification and loss, and thus at an increased risk of extinction; in addition, narrow habitat breadth is associated with a small geographical range, which can put an entire species at risk	[15,22,26,27]