Photoperiodic Hatching Rhythms Suggest Circadian Entrainment of Anolis sagrei Eggs

—Synchronous hatching occurs in many reptiles that lay multi-egg clutches, but this phenomenon and its potential environmental cues are poorly documented for species that lay their eggs singly, such as Anolis lizards. We tested for a circadian hatching rhythm in 196 Brown Anole (Anolis sagrei) eggs maintained in social isolation under a 12 : 12 L : D photoperiod with constant temperature and humidity. Hatching occurred exclusively in the morning and was approximately normally distributed around the transition from dark to light, with most (94%) eggs hatching 61 hour from the onset of light. Hatching times differed significantly between sexes, such that most males hatched after the onset of light (71%), whereas only half of all females hatched after the onset of light (52%). That a substantial proportion of eggs (38% across sexes) hatched in the dark suggests circadian entrainment to photoperiod, rather than a direct behavioral response to light. Whether this reflects a natural circadian rhythm of hatching or an artifact of increased light exposure in our artificial incubation environment is presently unknown. If this circadian rhythm of hatching is a natural phenomenon, then its potential fitness benefits (e.g., optimal timing for predator avoidance, prey detection, water balance, or thermoregulation) warrant further attention. Synchronization of hatching in response to light or photoperiod occurs in diverse taxa, including insects (Minis and Pittendrigh, 1968; Lazzari, 1991; Sakamoto and Shimizu, 1994; Itoh and Sumi, 2000), crustaceans (Branford, 1978; Forward and Lohmann, 1983; Forward et al., 1986; Forward, 1987), and monogenean parasites (Gannicott and Tinsley, 1997). This can arise from circadian entrainment of an endogenous biological clock to exogenous light cues (Minis and Pittendrigh, 1968; Lazzari, 1991; Sakamoto and Shimizu, 1994; Itoh and Sumi, 2000) or from a direct response to exogenous light cues without establishment of an endogenous rhythm (Gannicott and Tinsley, 1997). Hatching is often synchronized at or near the light-phase transition, although this can occur at the transition into either light or dark, depending upon the ecology of the species (Gannicott and Tinsley, 1997; Itoh and Sumi, 2000). Despite its prevalence in other taxa, photoperiodic regulation of hatching has received relatively little attention in reptiles, presumably because the eggs of many species are buried at depths that shelter them from light. Among oviparous reptiles, environmental cues influence hatching in turtles, crocodilians, lizards, snakes, and tuatara (Doody, 2011); however, evidence for precise temporal synchronization of hatching is restricted primarily to turtles and crocodilians (Doody, 2011). Because most turtles bury their clutches at considerable depths, hatching and emergence tend to be synchronized by temperature, hypoxia, and cues from other eggs and hatchlings, rather than light cues (Spencer et al., 2001; Colbert et al., 2010; Doody et al., 2012; Spencer, 2012). Likewise, crocodilians synchronize hatching via vocal cues from nestmates, which also signal the mother to excavate the nest (Lee, 1968; Vergne and Mathevon, 2008). In contrast to turtles and crocodilians, Anolis lizards often bury their eggs at shallow depths or in sites potentially exposed to light cues, such as the undersides of decaying bark and logs, in leaf litter, under small rocks, in tree holes and rock crevices, or in bromeliads and other vegetation (Rand, 1967; Andrews, 1982; Losos, 2009). All anoles lay one or two eggs at a time, although some species deposit their eggs in communal nesting sites, and emergence from these communal nests may be synchronous (Rand, 1967; Andrews and Rand, 1974; Losos, 2009). Collectively, these observations suggest that light cues often may be perceptible to Anolis eggs, whereas cues from other eggs or hatchlings are typically reduced or absent, at least in species that do not nest communally. Also, anoles have a photosensitive pineal complex (pineal gland and parietal eye) that produces a circadian rhythm of melatonin secretion (Underwood and Menaker, 1976; Menaker and Wisner, 1983; Underwood, 1983, 1985; Tosini et al., 2001). The pineal complex develops early in lizard development and, at least in other oviparous vertebrates, is capable of regulating circadian rhythms in melatonin secretion well before hatching (Akasaka et al., 1995; Faluhelyi et al., 2004). Therefore, light cues could potentially regulate the timing of hatching in Anolis eggs through circadian entrainment