MULTIPLE AXES TO GRIND
The fast-slow continuum is almost consistently supported as the dominant but by no means the sole factor structuring life history variation, including in mammals [3–5], birds [3], insects [6,7], fish [8] and plants [9] (although the extent of support is sensitive to the life history traits analysed [31]). Exceptions to the fast-slow ‘rule’ exist, with species having strategies characterised by both high survival and reproduction, such as marine turtles or trees [32], being especially common. Many such organisms have vital rates closely related to size and development, for which age is a poor predictor [20]. These life history strategies are instead defined using a second reproductive axis of variation: how species allocate to reproduction, rather than their reproductive outputper se . The second most dominant axis of variation thus usually relates to reproductive and developmental tactics, such as fromaltricial vs. precocial offspring [5]; from high to low recruitment rates [33], high to low frequency of reproduction [7] or from semelparity toiteroparity [3,9]. In some cases, axes other than reproduction explain substantial life history variation, such as age-specific distributions of mortality and reproduction [34]. The exact meaning of additional axes is hindered by the vast heterogeneity of life history traits analysed in this context [31].
Additional axes may be required to explain additional peculiarities in post-maturation distributions of age-specific survival and reproduction, such as species that reverse development using shrinkage orretrogression , or pause development using seed banks or diapause [9].