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].