Diel niche variation in mammals associated with expanded trait space

Abstract

Mammalian life shows huge diversity, but most groups remain nocturnal in their activity pattern. A key unresolved question is whether mammal species that have diversified into different diel niches occupy unique regions of functional trait space. For 5,104 extant mammals we show here that daytime-active species (cathemeral or diurnal) evolved trait combinations along different gradients from those of nocturnal and crepuscular species. Hypervolumes of five major functional traits (body mass, litter size, diet, foraging strata, habitat breadth) reveal that 30% of diurnal trait space is unique, compared to 55% of nocturnal trait space. Almost half of trait space (44%) of species with apparently obligate diel niches is shared with those that can switch, suggesting that more species than currently realised may be somewhat flexible in their activity patterns. Increasingly, conservation measures have focused on protecting functionally unique species; for mammals, protecting functional distinctiveness requires a focus across diel niches.

Fig. 1. Ecological strategy surfaces associated with mammals occupying different diel niches.

a Nocturnal (n = 3580, moon and stars silhouette), b crepuscular (n = 126, sunrise/sunset and stars image), c cathemeral (n = 467, moon, stars and sun image) and d diurnal (n = 931, sun image). For each diel niche, projections show the species (dots) on the surface defined by principal component axes (PC) 1 and 2. Solid arrows indicate the direction and weighting of vectors representing the five continuous traits analysed, and thus represent the major gradient of each trait (Supplementary Tables 1 and 2 for trait variance and loadings). Percentage values represent the proportion of the total variation explained by each PC. The colour gradient specifies regions of highest (red) to lowest (white) occurrence probability of species across the ecological strategy, with contour lines indicating 0.5, 0.95 and 0.99 quantiles. Thus, red regions correspond to functional hotspots and circled numbers denote the hotspots in each diel niche, as described in the main text. Silhouettes represent species characterising the hotspots. Large hotspots display two silhouettes, representing species at the hotspot extremes (silhouettes were freely downloaded from PhyloPic www.phylopic.org, under CC0 1.0 Public Domain Dedication and from Adobe Stock Images under Standard License).

Fig. 2. Two-dimensional representation of the overlap in five-dimensional trait space of species occupying different diel niches.

Nocturnal species are shown in blue (full volume 375 SD⁵, moon and stars silhouette), crepuscular species by the dash oval (full volume 43 SD⁵), cathemeral species in peach (full volume 95 SD⁵, moon, stars and sun image) and diurnal species in green (full volume 226 SD⁵, sun image). Hypervolumes for each diel niche were constructed on the basis of five z-transformed traits: body mass (log₁₀), litter size (log₁₀), diet, foraging strata and habitat breadth (square root transformed). Comparative analyses on paired hypervolumes were carried out, where numbers in bold specify the unique volume for each diel niche and numbers not in bold give the overlapping volumes of diel niches. The percentage of the hypervolume that is unique to each diel niche is given in parentheses. Statistical approaches are not available for comparing more than two hypervolumes, and so overlapping volumes are estimated based on paired overlaps (See Supplementary Table 3 for paired hypervolume analyses with unmatched sample sizes). The units of the unique and overlapping fractions are SD⁵. Note that one crepuscular species had a unique combination of traits that was not possible to represent in the figure.

Fig. 3. Diel flexibility in functional trait space.

Hypervolumes were constructed of diel obligate (red) and diel flexible (blue) species, before assessing comparative statistics across a all species, b nocturnal species only (moon and stars silhouette), c crepuscular species only (sunset/sunrise and stars image), d cathemeral species only (moon, stars and sun image) and e diurnal species only (sun image). Numbers give the volume of the unique diel obligate hypervolume (red), the unique diel flexible hypervolume (blue) and the overlapping volume between hypervolumes (purple; Supplementary Table 3). The percentage of the hypervolume that is unique to obligate and flexible species is given in parentheses. Mammal silhouettes give examples of diel obligate and diel flexible species and were freely downloaded from PhyloPic www.phylopic.org, under CC0 1.0 Public Domain Dedication.