Individual life histories: neither slow nor fast, just diverse

Abstract

The slow-fast continuum is a commonly used framework to describe variation in life-history strategies across species. Individual life histories have also been assumed to follow a similar pattern, especially in the pace-of-life syndrome literature. However, whether a slow-fast continuum commonly explains life-history variation among individuals within a population remains unclear. Here, we formally tested for the presence of a slow-fast continuum of life histories both within populations and across species using detailed long-term individual-based demographic data for 17 bird and mammal species with markedly different life histories. We estimated adult lifespan, age at first reproduction, annual breeding frequency, and annual fecundity, and identified the main axes of life-history variation using principal component analyses. Across species, we retrieved the slow-fast continuum as the main axis of life-history variation. However, within populations, the patterns of individual life-history variation did not align with a slow-fast continuum in any species. Thus, a continuum ranking individuals from slow to fast living is unlikely to shape individual differences in life histories within populations. Rather, individual life-history variation is likely idiosyncratic across species, potentially because of processes such as stochasticity, density dependence, and individual differences in resource acquisition that affect species differently and generate non-generalizable patterns across species.

Keywords: demography; individual heterogeneity; intra-specific variation; pace-of-life syndrome; slow–fast continuum; trade-off.

Figure 1.

Geographical location of the study populations for the 17 species included in the analyses. Our study includes 10 bird species: white-throated dipper (Cinclus cinclus), house sparrow (Passer domesticus), great tit (Parus major), blue tit (Cyanistes caeruleus), Eurasian oystercatcher (Haematopus ostralegus), wandering albatross (Diomedea exulans), black-browed albatross (Thalassarche melanophris), snow petrel (Pagodroma nivea), southern fulmar (Fulmarus glacialoides) and red-billed gull (Larus novaehollandiae) and seven mammal species: bighorn sheep (Ovis canadensis), mountain goat (Oreamnos americanus), golden-mantled ground squirrel (Callospermophilus lateralis), yellow-bellied marmot (Marmota flaviventer), human (Homo sapiens), and Weddell seal (Leptonychotes weddellii).

Figure 2.

The slow–fast continuum of life histories across species. (a) Pattern of covariation among life-history traits across 17 bird and mammal species revealed by a phylogenetically informed PCA (pPCA) using species-specific trait values. The pPCA was calculated using an average phylogenetic tree for birds and mammals over 100 trees. (b) Projection of individual scores (i.e. rotated individual data according to species' means) on the pPCA performed on species' mean life-history traits. Different colours for points and 95% data ellipses correspond to different species. Length of eigenvectors was increased by a factor of (a) 2 and (b) 5.5 for ease of visualization.

Figure 3.

Relative importance of principal components, PC, across species and among individuals within populations. The height of bars labelled ‘across' correspond to the proportion of variation explained by each PC of the pPCA across species (table 1). The height of the bars labelled ‘within' was calculated for each PC by summing the proportion of variation explained by this PC for all species and dividing by the total number of species. Each colour-coded rectangle in the ‘within' bars represents the importance of that axis for a given species, standardized by the total importance of that axis across species (i.e. the height of the bar). Within populations, we present only the 13 species for which four distinct components could be extracted. The great tit, the blue tit and the golden-mantled ground squirrel were excluded because their specific PCAs were based on less than four PCs due to the absence of variation in one or two life-history traits. The Weddell seal was also excluded because two life-history traits (breeding frequency and fecundity) were confounded in this species.

Figure 4.

Species' positions on the slow–fast continuum of life histories across species and the species-specific individual patterns of covariation in life-history traits. Individual values (grey dots) and scores of life-history traits on the first and second axes of variation (arrows) extracted from individual-level principal component analyses (PCA) are shown for each species. Species' positions on the slow–fast continuum correspond to the species' scores on the first axis of a phylogenetic PCA at the species level using the species' mean value for each life-history trait. For ease of interpretation and comparison among species, the orientation (sign) of the scores of individual and life-history traits were reversed to show the green arrow (adult lifespan) pointing towards the left-hand side of the plots, when necessary.