Plant seeds are subject to severe predation pressure and therefore under selection for predator defense, and show both physical defenses (e.g. hardness and thickness of seed coat) and chemical defenses (e.g. tannins).  

Tradeoffs arise because limiting resources cannot be allocated simultaneously to all fitness-enhancing traits. Although it is intuitive that a tradeoff could exist between physical and chemical defenses because of finite defense resources, many studies have failed to detect this tradeoff. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences tested whether there was a tradeoff between physical (i.e. fiber content, which has proved to be a good indicator of investment into seed coat) and chemical defenses (i.e. total phenolics, which are abundant chemical defenses in plant seeds). 

They also hypothesized that the tradeoff between seed defensive traits declines with increasing seed mass. 

The researchers used seeds of 163 species, belonging to 121 genera and 67 families, collected from Xishuangbanna tropical forest, southwest China.  

They employed a model of phylogenetically corrected trait correlations to separate mechanistic tradeoffs from phylogenetic signal in those traits, to quantify the role of seed mass in mediating the tradeoff between chemical and physical defensive traits. 

They found that there was large interspecific variation in seed mass, total phenolics and fiber content of the 163 species. Across all species, they detected a tradeoff between total phenolics and fiber content. 

Across a taxonomically broad sample of plant species, they showed a clear trend that the magnitude of the correlation coefficient between physical and chemical defenses decreased (became less negative) with increasing seed mass.  

The results demonstrated that the tradeoff between physical and chemical defense in plant seeds was mediated by seed mass, such that the tradeoff between physical and chemical defenses was stronger among small seeds than among large seeds.