Fused eggs and twinning in the Australian Jacky Dragon (Amphibolurus muricatus)

While twinning in oviparous species is often considered a rare event, this phenomenon is well known, and anecdotal observations have been reported frequently in the literature (see reviews by Piovano et al. 2011;Di Marzio et al. 2023).Although less common, many cases of twinning involve bicephalic individuals or those that are conjoined in a variety of different ways (e.g., Cooper 2009;van Schingen and Ziegler 2014;Di Marzio et al. 2023).Most observations of twinning occur during research or captive husbandry programs that typically involve artificial incubation of large numbers of eggs (Hartdegen and Bayless 1999;Di Marzio et al. 2023), which increases the likelihood of observing these rare events.Nevertheless, the rate of twinning relative to the production of singleton offspring (i.e., a single hatchling from an egg) is rarely reported, and difficult, if not impossible, to quantify in nature.Herein we report observations of fused eggs and conjoined hatchlings from wild-caught Australian Jacky Dragons (Amphibolurus muricatus) during four years of egg incubation studies on this species.Our observations were made during a series of research projects in 2003-2007 that focused on maternal effects and developmental plasticity (e.g., Warner and Shine 2005, 2011; Warner et al. 2008).Our most notable observations come from offspring produced by a single wild-caught female from Lane Cove National Park in Sydney, Australia (-33.7912, 151.1512).This female was collected at 1100 h on 13 September 2005, and housed in an outdoor field enclosure at Macquarie University located ~1 km from the collection site.This female was kept in captivity (housed with one male and four other females) and monitored for egg production until the end of the reproductive season in 2007 (see Warner et al. 2008 for details on captive husbandry).This female produced two clutches during the 2005-2006 Austral summer, which resulted in normal viable offspring.In the following summer (2006)(2007), this female was housed with three males and two other females, and she produced three more clutches that resulted in several instances of fused eggs and one case of conjoined offspring (Table 1; Fig. 1).