Its 4am, pitch black and I wake feeling tired and sore after a week spent trekking up and down steep slopes in unforgiving landscape. It’s a privilege to be here in Bawangling National Nature Reserve on Hainan Island, China, but it’s tough and testing work.
An hour’s hike uphill takes me and my team of forest wardens to an elevated listening post where we wait and listen for the song of a rare and elusive animal. As the sun rises, we hear it, the call of a male Hainan gibbon rising above the racquet of the early morning cicada chorus; it’s the rarest song on Earth.
About the Hainan gibbon
The Hainan gibbon (Nomascus hainanus) is the world’s rarest ape, and probably the rarest mammal, with only one surviving population of just 25 individuals.
Historically, the species was widespread across Hainan, numbering in the thousands, but intensive hunting for Traditional Chinese Medicine and rapid, aggressive forest clearance for industry led to a precipitous decline in the 1950s-1980s.
The remaining population is constrained to Bawangling, and with no captive individuals, conservation of this species is an urgent priority for mammal conservation globally.
Heading towards the rarest song on Earth
Getting a fix on the song direction, we tear off towards the gibbons, racing against time, thorny undergrowth and slippery slopes to reach the group before they move on. After some hairy moments, we finally locate the group in a large fig tree.
Conventionally, gibbons are known to live in small, monogamous family groups consisting of one adult male, one adult female, and 1-2 offspring. However, for the Hainan gibbon, we see this polygynous (multi-female to one male) structure.
However, it’s not just one male, one female and their offspring that make up this group; there is also a second female with her offspring.
This is not the traditional picture of a gibbon social group. Conventionally, gibbons are known to live in small, monogamous family groups consisting of one adult male, one adult female, and 1-2 offspring. However, for the Hainan gibbon, we see this polygynous (multi-female to one male) structure in all of the existing groups. We also see larger overall groups, consisting of six or more members.
By tracking the groups as they range through the landscape, it’s also clear that the Hainan gibbon has a much larger home range (estimates between 1.5-10 km2) than those of most gibbon species (around 0.40 km2).
The drivers behind these apparently anomalous ranging and social habits are not clear, but one explanation is that they may be a response to current external pressures. The large home range could be the result of limited and/or poor quality habitat, and the large, polygynous groups could be due to a lack of available mates produced by the tiny current population size.
This ambiguity makes it difficult to know what the best course of conservation action is for the species. This is a concern because for species that are on the very brink of extinction, time is of the essence – decisive action can mean the difference between a conservation success story and a species’ extinction.
About the research
To clarify things, we conducted a comparative analysis employing data from 39 populations of the 19 currently recognised gibbon species to identify intrinsic versus extrinsic drivers of variation in home range size, social group size, and mating system across the gibbon family.
We found that all three traits showed a strong phylogenetic signal, which means that gibbon species which are more closely related have similar home ranges, group sizes and mating system than expected just by chance.
But, by contextualizing what we see for the Hainan gibbon within the variation in these traits across all gibbons, we were also able to determine that the remnant groups at Bawangling do have larger home ranges than expected (relative to all other gibbon species). It seems that this is likely a result of the critically low population density at this site, rather than issues of habitat quality.
Conversely, although the Hainan gibbon has large group sizes, no observed gibbon group size (for any species /population) is significantly different from what we would expect on the basis of their phylogenetic relationships alone.
Furthermore, there is no evidence that the polygynous mating system observed for the Hainan gibbon is driven by any existing external factors. This suggests that large, polygynous groups may be the norm for this species.
What does this mean for conservation?
By teasing out the Hainan gibbon’s natural ecological and behavioral traits, we have revealed important considerations for conservation planning.
Any potential conservation activities must consider the complex, polygynous social structure of the species as an intrinsic component of its biology.
First, it’s clear that actions designed to improve habitat quality alone will be unlikely to address the current constraints to Hainan gibbon population recovery, with more intensive management likely required.
Second, any potential conservation activities must consider the complex, polygynous social structure of the species as an intrinsic component of its biology.
We are currently working with Bawangling management staff to try to ensure these findings are incorporated into conservation management for the Hainan gibbon, in the hope that the last surviving population can be preserved and recovered, and ultimately additional populations can be established.
Perhaps, with careful but proactive conservation action based upon this sort of sound scientific evidence, in the near future the Hainan gibbon’s song can echo through the valleys of many Hainanese mountains once again.