Testosterone in the Gum Trees! You Yangs Koala Update – October 2010

Springtime is breeding season for many animals – but for koalas, whose long breeding season goes from October to March, these early days seem to be the time for males to sort out their rivals.   I’m sure they would prefer to be breeding, but those pesky females are just not ready yet so the boys have to do something!

In Springtime in 2006 we watched young male Vegemite oust Tim Tam from his Big Rock Track home range.  Then in springtime 2009 young Anzac displaced Vegemite from the same home range that he’d taken off Tim Tam three years earlier.  Now, in October 2010 Merle – the dominant male of Red Gum Gully – has disappeared.  And his nearest rivals Ngallo and Anzac have appeared in northern Red Gum Gully, in an area very much “owned” by Merle.  Oops!  There’s trouble brewing!

Merle has been the number one fella in Red Gum Gully since we first started monitoring koalas in the You Yangs in January 2006.  His reign has been absolute and unwavering – only old males Calvin (in 2006 & 2007) and Tim Tam (in 2008) really dared to cross Merle’s boundary for long.  Any other male trespassers speedily disappeared.  Koalas are not considered strictly territorial – that is, they don’t defend their home range against intruders all the time.  In other parts of Victoria dominant males are known to share parts of their home ranges with up to ten other males*  But in the You Yangs, in a less densely populated area, we have noticed that the level of overlap by males is little.  Merle’s domination of Red Gum Gully has been almost exclusive for four years.

So how does he keep other males away?  We know that male koalas fight, but in 12 years of monitoring koalas I have never seen a fight between males.  Maybe it doesn’t happen often in low density populations.  Perhaps it happens at night in Spring, when we are not watching.

What we have seen is a fair bit of intimidation.

In early December 2009, Merle’s northern neighbours Vincent and Ngallo were seen sharing a tree in Merle’s home range.  Younger Vincent was perched high at the far end of the branch and was looking very uncomfortable.  He was facing outwards, back to Ngallo, sitting up stiffly, hardly moving but wide awake the whole time we watched. Ngallo was in the lowest fork, relaxed and sprawling, eyes closed – but in such a location that Vincent would have to pass him to get out of the tree.  Merle could not be found that day, but guess what?  The next day he was back!  Merle was exactly where the two boys had been, Ngallo had moved east and was looking nervous, and Vincent was nowhere to be found!

In the Brisbane Ranges in 2004 we saw the same behaviour – this time it was dominant male Blaze and younger male GP. It was 10 October, and dominant male Blaze was in a tree that had been one of his favourites for years.  Young GP had just moved into the area and decided to take on the old fella.  GP was sitting in a lower fork so that Blaze would have to pass him to get out.  Blaze had his back turned to GP, but didn’t seem nervous.  Actually, we think Blaze won that day, as he stayed in his home range until June 2005.  GP disappeared for a while, then returned in August and October 2005 after Blaze had left.

And just yesterday (October 22, 2010) we saw Arne intimidating young Eugen at Big Rock Track North.  Eugen has visited Arne’s home range recently – we first saw him in late September.  Then yesterday Eugen was sitting stiffly at the furthest end of a branch gazing nervously outwards.  Like Vincent, he was never seen to rest.  He even climbed around on the branch as if looking for a way out.  Arne, like Ngallo, was sleeping unconcernedly in the main fork of the same tree.

Close interactions between adult koalas are few.  Just seeing two koalas share the same tree is really unusual.  We’ve only recorded male koalas sharing trees on six occasions over six years – these three above, and another two in the You Yangs – one in Spring, another in Summer.  Another one was immediately after a bushfire, so we’re taking that out of the equation.  Considering that we observe around 1000 koalas every year, that is a pretty low incidence.   If you ever see two adult koalas sharing a tree, we would love to know – email me pictures of each koala and one photo of the whole tree showing both koalas, date, time and a GPS location, if you can!   E:  janine@echidnawalkabout.com.au

*Martin, R & Handasyde, K: The Koala, Natural history, conservation and management. UNSW Press Second Edition 1999. page 54


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