Aggression and Competition for Space and Food in Captive Juvenile Tuatara (Sphenodon Punctatus)

Abstract

Intraspecific competition is of importance in the wild and captivity, as the interaction among individuals for resources can affect growth, survival, and ultimately fitness. Tuatara, Sphenodon punctatus, are endemic to New Zealand and the sole representatives of the reptile Order Rhynchocephalia, and their recovery plan outlines the importance of head-started individuals to supplement existing populations and provide stock to start new populations. Head-starting is a widespread conservation tool for raising juveniles in captivity prior to release in the wild, with the aim of reducing juvenile mortality and providing populations with more individuals. However, mortality differs between sexes and juvenile tuatara show enormous variation in size in captivity. I investigated aggression and competition for space and food in a tuatara head-starting facility to determine whether intraspecific competition may affect mortality and growth. Pairs of one-year-old tuatara, S. punctatus, were chosen according to sex and relative size, e.g. a big male and a small female or two similar sized females. Seven scenarios were replicated four times with different pairs. Behaviour (including two feeding trials) was recorded over a six day period via security cameras and direct observations. The number of aggressive conflicts differed among scenarios, and male-male dyads were significantly more aggressive than female-female dyads. Dominance hierarchies were established in 18 of 28 experimental pairs, with bigger animals being dominant. Conflicts include chasing, biting or colliding at full speed. One year old juveniles did not compete for space. They did not use space exclusively, but stopped clustering and had developed aggressive behaviour, suggesting that they are not territorial yet but in an early stage of transition towards territoriality as seen in older juveniles and adults. Space use and avoidance in space and time did not differ among social scenarios and the latter were negligible, but they marked a novel enclosure with urine and faeces. Juveniles competed directly and indirectly for food. Dominant individuals were likely to secure more food than submissive individuals. Females acquired less food when paired with males of bigger or similar size, and acquired about equal shares when paired with a smaller male. While bigger males acquired slightly more food when paired with smaller males, this was not the case in differently sized females. Interference behaviours such as chasing and food stealing were mostly directed from bigger towards smaller individuals. Captive group housing has consequences for competition and aggression, and may directly influence survival. As juvenile tuatara mortality is female-biased, and aggression against females in bigger male-biased groups common, I recommend keeping sexes separate, and assorting groups by size with more spacious enclosures for male groups. These modifications should improve health and numbers of juveniles for release, improve recruitment into the reproductive adult population, and ultimately create more successful head-starting facilities.