Sex Ratio Bias of Tuatara and Risks of Extinction

This article deals with the sexual determination at play in tuataras and how this has a negative effect on their survival, especially in environments such as isolated islands or otherwise divided land.

 The primary issue is male bias in the animals that cause a relative lack of females, and how this has more of a negative affect than the reverse of the problem.  With more males than females, there are less reproducers overall than in comparison with an even split or with the female heavy equivalent.  Males also compete for females in tuataras, meaning during times of male bias there will be fiercer competition for mates resulting in some injuries and possibly loss of fitness leading to death.  In reptiles with temperature dependent sex determination, the process usually produced females at higher temperatures as the article states either male to female, or female to male to female as temperature increases.

The next issue with this bias is in the case of positive feedback from the process resulting in increasing the current trend.  In this case, majority male societies for tuatara generally continue the trend and produce more males as an effect.  In the case of the data gathered, three factors contributed to this gender skew and only increase said bias over time.  Decreasing body condition of adults, reduced adult survival, and shifts in hatchling sex ratio due to climate change.
The experiment to study these effects was largely data collection of tuatara, in this case on North Brother Island wildlife sanctuary.  Individuals were captured, measured and given permanent unique marks then released.  Furthermore this test went on for twenty years and it was hypothesized that unmarked adults after the first ten years of testing were likely children of the first generations captured.  One of the larger issues found during the experiment was the comparative speed and subtle nature of young tuatara meaning less were caught compared to adults.
 
The mark and recapture results show that male bias has further increased overtime in this area from 1.5:1 male to female sex ratio in 1988 to 2.3:1 male female sex ratio.  Also measured when caught, it seems over time the body condition (mass, length) have gone down during this period meaning reduced fitness.  In a problematic nature adding to the sex bias, males are typically larger than females and will out-compete their smaller rivals for food or other resources.  The reduction size, fitness, and nutrients have also shown in some cases a vastly reduced rate of reproduction in females.  Some solutions are proposed in the paper such as assistance of juvenile females to make sure they are large and fit before facing competition, or artificial incubation to ensure more females, but all are short term and won't help resolve the problem without constant work. 

As most in the class are aware, tuatara are the last remaining genus of the Rhychocephalia, and are a relatively ancient group at that indicating a unique section of reptiles.  The process involved in the paper also is similar to general methods that could be used for other endangered species as well in broader terms.

Citation:
Grayson, Kristine L.; Mitchell, Nicola J.; Monks, Joanne M.; Keall, Susan N.; Wilson, Joanna N.; Nelson, Nicola J. PLoS ONE. Apr2014, Vol. 9 Issue 4, p1-10. 10p. DOI: 10.1371/journal.pone.0094214.

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