Communal egg-laying is widespread among animals, occurring in insects, mollusks, fish, amphibians, reptiles, and birds, just to name a few. While some benefits of communal egg-laying may be pervasive (e.g., it saves time and energy and may ensure the survival of mothers and their offspring), the remarkable diversity in the life histories of the animals that exhibit this behavior presents a great challenge to discovering any general explanation. Reptiles and amphibians offer ideal systems for investigating communal egg-laying because they generally lack parental care--a simplification that brings nest site choice behavior into sharp focus. We exhaustively reviewed the published literature for data on communal egg-laying in reptiles and amphibians. Our analysis demonstrates that the behavior is much more common than previously recognized (occurring in 481 spp.), especially among lizards (N = 255 spp.), where the behavior has evolved multiple times. Our conceptual review strongly suggests that different forces may be driving the evolution and maintenance of communal egg-laying in different taxa. Using a game theory approach, we demonstrate how a stable equilibrium may occur between solitary and communal layers, thus allowing both strategies to co-exist in some populations, and we discuss factors that may influence these proportions. We conclude by outlining future research directions for determining the proximate and ultimate causes of communal egg-laying.

Although social behavior in vertebrates spans a continuum from solitary to highly social, taxa are often dichotomized as either ‘social’ or ‘non‐social’. We argue that this social dichotomy is overly simplistic, neglects the diversity of vertebrate social systems, impedes our understanding of the evolution of social behavior, and perpetuates the erroneous belief that one group—the reptiles—is primarily ‘non‐social’. This perspective essay highlights the diversity and complexity of reptile social systems, briefly reviews reasons for their historical neglect in research, and indicates how reptiles can contribute to our understanding of the evolution of vertebrate social behavior. Although a robust review of social behavior across vertebrates is lacking, the repeated evolution of social systems in multiple independent lineages enables investigation of the factors that promote shifts in vertebrate social behavior and the paraphyly of reptiles reinforces the need to understand reptile social behavior.

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Dating back to 255 Mya, a diversity of vertebrates created mysterious deep helical burrows, often called Daimonelix (devil's corkscrews). A consensus function for these unique structures has not been reached, but the recent discovery of deep helical nesting burrows created by (extant) monitor lizards provides a unique opportunity to interpret Daimonelix and morphologically similar fossil burrows. We excavated a communal nesting warren of the Sand Monitor (Varanus gouldii) to test hypotheses for nesting and emergence behavior. First, we hypothesized that the nests of V. gouldii (in desert) would be deeper than those of the closely related V. panoptes (in savannah), because lower annual rainfall in the former (~350 mm vs.~1000 mm) increases the threat of egg desiccation during the extremely long, dry season incubation period (~8 months). Second, we predicted that hatchlings would follow the nesting burrows of their mothers during emergence, because excavating their own emergence burrows would be energetically prohibitive due to the extreme depth and soil hardness. We excavated the warren to a depth of 4 m, finding 97 nests. As predicted, nest depth in V. gouldii (mean = 3.0 m, N = 73) was significantly greater than in V. panoptes (mean = 2.3 m, N = 52) from a previous study, and represents the deepest excavated ground nests of any vertebrate in the world. Contrary to our hypothesis, hatchlings ignored their mothers' nesting burrows, instead remarkably excavating their own emergence burrows. The communal nesting warrens and deep burrows of V. gouldii must have profound implications for the energetics of mothers and hatchlings, and perhaps for the social biology of the species. Although the function of the helix itself remains elusive, our results support the hypothesis that the extreme deep burrowing of Palaeocastor, Diictodon and other extinct animals was a response to arid conditions rather than temperature.