Paralysis efficiency (PD₅₀) scales linearly with lethality (LD₅₀) in spider venoms

Historically, venom potencies have been assessed using measures of lethality, such as the median lethal dose (LD₅₀). However, venoms may be selected primarily for their ability to rapidly incapacitate rather than cause mortality, meaning LD₅₀ may not capture the efficacy of venoms in an ecological and evolutionary context. To capture this context, recent studies have adapted measures that assess venoms' ability to rapidly incapacitate, such as the median paralysis dose (PD₅₀). However, while PD₅₀ values are expected to provide a more proximate assessment of ecological variation in venom potency, it is unknown whether historically available LD₅₀ values are still useful proxies of ecologically relevant potency or whether they capture independent axes of venom variation. Here, we test the relationship between LD₅₀ and PD₅₀ in spider venoms by experimentally estimating LD₅₀ and PD₅₀ for 12 species and collating additional potency data for 46 species retrieved from the literature, producing a dataset of 55 species spanning 26 families when combined. We observed a linear isometric relationship between LD₅₀ and PD₅₀, showing these potency measures are both strongly correlated, with an increase in paralysis efficiency associated with a similar increase in lethality. Our results suggest that due to the correlation between functional aspects of venom potency, paralysis and lethality, historically available LD₅₀ values may be used to compare general venom potencies in spiders, provided that they are based on the same prey model.