Why do female lizards lay their eggs in communal nests?

Summary

• 1
  In many reptile species, females oviposit communally (i.e. many clutches are laid within the same nest). This behaviour might result from constraint (scarcity of nest-sites offering suitable incubation conditions) or adaptation (direct fitness benefits accruing from the proximity of other eggs).
• 2
  To test between these alternatives, we gathered field and laboratory data on montane scincid lizards Bassiana duperreyi from south-eastern Australia. Our data support the adaptationist hypothesis.
• 3
  In the field, communal vs. solitary clutches were laid in similar sites, and the relative frequency of communal nesting was not predictable from nest-site availability. Thermal regimes for incubation did not differ between communal vs. solitary nests, nor between eggs at the core vs. periphery of a communal nest. In the laboratory, females selectively oviposited beside existing eggs rather than in otherwise identical potential nesting sites.
• 4
  From cycling-temperature incubation in the laboratory, eggs incubated within a cluster of other eggs took up less water, but produced hatchlings that were larger and faster-running than were hatchlings from eggs incubated alone.
• 5
  Hydric modifications of incubation conditions within a cluster of tightly packed eggs thus may provide a direct fitness benefit to communal oviposition.