The evolution of crocodilian nesting ecology and behavior

Abstract

Crocodilians comprise an ancient and successful lineage of archosaurs that repeatedly raises questions on how they survived a mass extinction and remained relatively unchanged for ~100 million years. Was their success due to the change-resistant retention of a specific set of traits over time (phylogenetic conservatism) or due to flexible, generalist capabilities (e.g., catholic diets, phenotypic plasticity in behavior), or some combination of these? We examined the evolution of reproductive ecology and behavior of crocodilians within a phylogenetic perspective, using 14 traits for all 24 species to determine whether these traits were phylogenetically constrained versus (ecologically) convergent. Our analysis revealed that the ancestral crocodilian was a mound nester that exhibited both nest attendance and defense. Nesting mode exhibited 4-5 transformations from mound to hole nesting, a convergence of which habitat may have been a driving factor. Hole nesters were more likely to nest communally, but this association may be biased by scale. Although there were exceptions, mound nesters typically nested during the wet season and hole nesters during the dry season; this trait was relatively conserved, however. About two-thirds of species timed their nesting with the wet season, while the other third timed their hatching with the onset of the wet season. Nest attendance and defense were nearly ubiquitous and thus exhibited phylogenetic conservatism, but attendance lodging was diverse among species, showing multiple reversals between water and burrows. Collectively, our analysis reveals that reproductive trait evolution in crocodilians reflects phylogenetic constraint (nest attendance, nest defense), ecological convergence (seasonal timing of nesting, nest attendance lodging), or both (mode of nesting). Some traits (e.g., communal nesting and mode of nesting) were autocorrelated. Our analysis provides a framework for addressing hypotheses raised for why there has been trait convergence in reproductive ecology and behavior in crocodilians and why some traits remained phylogenetically conserved.

Keywords: character evolution; crocodilians; eggs; nesting; phylogeny; reproduction.

Figure 1

Character optimization of mating vocalizations using accelerated (a) and delayed (b) transformations

Figure 2

Character optimization of nesting mode (a) and communal nesting (b). Accelerated and delayed transformations recovered unequivocal results for both characters

Figure 3

Character optimization of nest attendance (a) and nest defense (b). Accelerated and delayed transformations recovered unequivocal results for both characters. Additionally, character optimization of nest attendance lodging using accelerated (c) and delayed (d) transformations, as well as unequivocal character optimization for hatchling attendance (e)

Figure 4

Character optimization of clutch frequency using accelerated (a) and delayed (b) transformations

Figure 5

Character optimization of hatching timed for wet season using accelerated (a) and delayed (b) transformations; nesting season using accelerated (c) and delayed (d) transformations, and hatching stimulus (e). Accelerated and delayed transformations recovered unequivocal results for this character

Figure 6

Character optimization of incubation duration using accelerated (a) and delayed (b) transformations; size‐adjusted clutch mass using accelerated (c) and delayed (d) transformations, and size‐adjusted clutch size using accelerated (e) and delayed (f) transformations

Figure 7

Principal components analysis by which clutch mass, clutch size, incubation period, and deposition site (categorical) arrange species in ordination space. This elucidates that hole‐nesting crocodilians had larger clutch sizes than mound nesters