Chemical attraction? (re-post)

By Hilary Miller 12/08/2010

The chicken or egg blog family is on holiday in Germany during August, so I probably won’t have a chance to write any new posts.  To keep you all entertained, I’ll be re-posting some of my earlier (pre-Sciblogs) articles.  This one is the very first post I wrote for this blog, from March 2009.

Research published in last month’s Chemistry and Biodiversity journal heralded the discovery of a new compound ’tuataric acid’. Yes, isolated from our very own tuatara.

Stefan Schulz and his colleagues at University of Braunschweig, and collaborator Paul Weldon at the Smithsonian Institution, have analysed the constituents of the cloacal secretions in tuatara and found an unexpectedly diverse array of compounds. As tuatara have no external sexual organs, the cloaca is the ’one stop shop’ opening at their posterior end, with prominent skin glands on either side of the opening that secrete a greasy white substance. When the tuatara secretions were analysed, Schulz and colleagues found over 150 different types of glyceride-based molecules, including one never-before seen compound, which they dubbed ’tuataric acid’.

Perhaps even more excitingly though (for me at least), was the finding that individual tuatara secrete specific mixtures of these glycerides and that the makeup of these individual profiles remains stable over years. This could provide a mechanism for chemical recognition of individual tuatara, a finding which ties in nicely with some behavioural work we have recently been doing on tuatara.

Our research group at Victoria University has been investigating mating behaviour and territoriality in tuatara, and we have found evidence to suggest that tuatara do indeed use some form of olfactory recognition in their social lives.  We analysed the genetic makeup of mated pairs at immune genes (called MHC genes). These genes are linked to odour recognition in many species, and several studies (mostly in mammals and fish) have found that individuals preferentially choose mates with a differing MHC genotype to their own. This theoretically produces offspring that have high genetic diversity at these immune genes, and are thus more resistant to disease. When we compared mated pairs of tuatara with randomly chosen pairs, we found that the mated pairs were more different from each other at their MHC genes than would be expected by chance, suggesting that odour recognition plays a role in how they choose their mates.

However, this MHC-related effect in tuatara is relatively weak, and is largely overshadowed by the fact that large males tend to be more successful in mating, regardless of their MHC genotype. In fact at the time of mating there appears to be little mate choice at all — it’s all about male machismo, with males fighting among themselves for access to the females that live nearby. Tuatara maintain relatively stable territories, and don’t move far to mate, so it’s possible that the apparent influence of MHC genotype on mate choice actually reflects choice of territory. Perhaps tuatara use their sense of smell, mediated by their MHC genes, when deciding who to shack up next to, and in doing so avoid mating with their close relatives.

The finding of individual differences in the makeup of cloacal secretions provides us with a mechanism for how tuatara may recognise each other. However, more field studies will be required to confirm the link between these two streams of research and nail down both the biological roles of the glycerides, and the extent to which tuatara use chemical communication in their social lives. Watch this space…

References: Flachsbarth, B., Fritzsche, M., Weldon, P. J. & Schulz, S. (2009) Composition of the Cloacal Gland Secretion of Tuatara, Sphenodon punctatus. Chemistry & Biodiversity 6, 1-37.

Miller, H.C., Moore, J.A., Nelson, N.J. & Daugherty, C.H. (2009) Influence of MHC genotype on mating success in a free-ranging reptile population. Proc. R. Soc. Lond. B, 276:1695-1704.