While this indicates bloodstain ageing has clear, measurable, and interpretable degradation patterns, there was a high degree of variation among studies -this is reflected by the random effect of study explaining an additional 35% on average (Table ). Likewise, the within-study individual variation of TSD signals can be strikingly high . This leads to the TSD paradox: the overall effect time has on blood is large, but the predictive power of TSD models is small. Our meta-analysis suggests near uniformity in effect sizes related to the class of biomolecule studied, analytical technique used to measure degradation, the temperature the bloodstain was aged in, and the type of blood source used. While differences exist in the molecular properties of mammalian blood sources , the findings of our meta-analysis suggest that they do not appear to hamper the timewise physicochemical degradation pathways of the biomolecules. This leads to a first key takeaway: most readily available mammal blood sources can be used as a human blood substitute in proof-of-concept TSD studies. Moreover, addition of anticoagulants allows for important experimental flexibility, mirrors human properties 106 and in at least one TSD study, had minimal effect on the recorded metrics ( 107 but see 17 ). While a variety of suitable mammal alternatives exist 106,108 and would facilitate expanded TSD research, we still recommend that for integration into forensic casework, human blood will be required for validation.