Lizard Tail Autotomy

Tail autotomy is a common defense mechanism found in lizards (Bely & Nyberg, 2010; Hill et al., 2012; Russell et al., 2015). The autotomized tail will continue to twitch for up to 30 minutes due to the event of anaerobic metabolism (Dial & Fitzpatrick, 1983; Higham & Russell, 2010). This event will divert the attention of predator, giving a chance for the lizard to escape (Daniels, 1983). Even though autotomy may save their lives, lizards performing autotomy have to deal with the loss of tail vital functions in movement (Bateman & Fleming, 2009), establishing social status (Fox & McCoy, 2000; Meyer et al., 2002), and as fat-storage organ (Clark, 1971). Lizards which are using their tail to store fat most likely return to the site where they lost their tail and eat the autotomized tail to make up for the loss of the major fat reserve which comprises half or more total body fat (Congdon et al., 1974; Lin et al., 2006; Naya et al., 2007; Sanggaard et al., 2012). Moreover, autotomy also causes the decrease of somatic growth and reproductive output leading to a smaller home range and fewer access to female (Martin & Salvador, 1993; Fox & McCoy, 2000; Simou et al., 2008; Boozalis et al., 2012). The autotomy usually occurs right in front of the segment or no more than three segments anterior to where the tail was grabbed by the predator. Lizard cannot afford to lose unnecessary portion of its tail as the tail has significant functions in its life (Sheppard & Bellairs, 1972; Sanggaard et al., 2012). Therefore, regeneration is important in restoring all of those lost functions.

Autotomy can be either intravertebral, which is commonly found in most lizard species and the presence of preformed breaks (fracture planes) in the postpygal vertebrae is needed, or intervertebral, which is less common (Sanggaard et al., 2012). Intravertebral autotomy takes place in the area where the fracture planes lie, as it has been previously mentioned, in G. gecko it is situated posterior to the transverse processes of each postpygal vertebra (Rumping & Jayne, 1996). Intervertebral autotomy involves intervertebral planes as the site of autotomy (Higham et al., 2013), and all species capable of performing this pattern of tail loss do not show any obvious modifications in their tail structures compared to the non-autotomizing species (Bateman & Fleming, 2009). 

Autotomy is facilitated by anatomical modifications of structures found only in lizard tail (McLean & Vickaryous, 2011; Gilbert et al., 2015). Fracture planes, the pre-existing planes of weakness in each caudal vertebra formed by connective tissue, serve as one of the prominent structures responsible for the occurrence of autotomy (Pratt, 1920; Cox, 1969; Delorme et al., 2012; Kusumi & Fisher, 2012;). Other supporting structures playing important roles in autotomy are the interdigitation arrangement of tail muscles (Sanggaard et al., 2012) and modified structure of spinal cord and caudal vasculatures (Bellairs & Bryant, 2001; Gilbert et al., 2013). Although fracture planes do not reach the neural canal, the spinal cord at the level of each fracture plane is thinner in Podarcis sicula, and this structural adaptation is not found in the non-autotomous region (Alibardi, 2009). The caudal artery thickens around each preformed break, this thickening resembles the structure of a sphincter. The caudal veins are equipped with valves situated just right in front of each fracture plane (Bellairs & Bryant, 2001). These vessel sphincters and valves prevent excessive blood loss after tail amputation (Abdel-Karim & Michael, 1993; Gilbert et al., 2013; Sanggaard et al., 2012), then it is followed by rapid clotting response (Fernando et al., 2011; Mescher et al., 2016).