The mystery of the Dark and Light Beetles on Possession Island

New paper out! Espel, D.; Coux, C.; Pertierra, L.R.; Eymar-Dauphin, P.; Lembrechts, J.J.; Renault, D. Functional Niche Partitioning Occurs over Body Size but Not Nutrient Reserves nor Melanism in a Polar Carabid Beetle along an Altitudinal Gradient. Insects 2023, 14, 123.

The southern Indian Ocean is home to the little known archipelago of Crozet. Despite its harsh weather and lack of substantial human presence, the island is a haven for penguins and elephant seals. Among the island’s inhabitants is a seemingly ordinary beetle, the Amblystogenium pacificum. It crawls over and under the rocks, hunting for smaller things to eat, and just lives his normal beetly life. There is just one thing with this beetle that sparked the interest of the (unusually persistent colony of) French scientists on the island: there are two colour versions of it. A light and a dark one.

Star of the show: the endemic Carabid beetle Amblystogenium pacificum

Now, we wouldn’t be scientists if we didn’t want to dig deeper into this.

Scientists have proposed several hypotheses to explain this phenomenon, including Gloger’s rule, which states that darker coloration is a common response among insects exposed to cold conditions. If Gloger’s rule were at play, we would expect to see a fitness advantage in the darker insects when it’s colder. However, it’s rarely that simple in ecology. It’s worth noting that previous studies have observed that the proportion of dark beetles increases by about 8% for every 100 meters in altitude on the island. This results in a clear distinction in the spatial distribution of the two morphotypes. More recent observations made in fellfield habitats have reported that the proportion of brown to black beetles ranges from a 2:1 ratio of brown insects at low altitudes to a 2:1 ratio of black adults at higher altitudes. This further supports the idea that Gloger’s rule may be at play. But as scientists, we can never jump to conclusions too quickly, and we must always keep an open mind to other possibilities.

Find the beetle, happily running across a soft ‘moss forest’. Picture by Diane Espel

Nevertheless, there were both dark and light individuals across the entire elevation gradient on the island. This raises the question of whether the dark beetles fare better at higher elevations (in terms of being bigger, having more nutrient or lipid reserves, etc.) and vice versa.

However, there are other rules that come into play, such as Bergmann’s rule, which states that insects tend to get larger as the temperature gets colder. This is because larger body sizes in endothermic species often correspond to a smaller surface area to volume ratio and reduced heat dispersion in colder climates. Additionally, there is the rule of sexual size dimorphism, which states that female insects are typically larger than males.

So, how does all of this play out on our windy island? The good news is that the elevation (and thus climate) gradient is responsible for many trait differences among the beetles. However, it turns out that coloration does not seem to be a major factor in determining the performance of the beetles. Bergmann’s rule does appear to be at play, as elevation does seem to boost beetle body size (although at the very top of the mountain, the extremely harsh conditions have made them smaller again). Sexual dimorphism is also present, with females having more lipid and sugar reserves (which is pretty clever of them, if you ask me).

Body size as a function of altitude for dark (blue) versus light (green) male (dashed) versus female (full line) beetles. Elevation clearly affects body size, but with virtually no difference between the two colorations or even genders.

However, a so-called ‘functional hypervolume analysis’ did not show clear niche partitioning of the insects along the studied altitudinal gradient, and we had to conclude that niche partitioning was happening for body sizes rather than coloration types.

In conclusion, the mystery of the dark and light beetles on Crozet Island remains unsolved. The absence of clear patterns in the relationship between temperature and size highlights the complexity of the interactions between insects and their environment. Factors such as low temperatures and limited resources at higher altitudes likely play a role in shaping the beetles’ functional traits. But hey, isn’t that what science is all about? Asking questions and trying to find answers, even if they’re not always easy to come by. So let’s just say, the adventure continues!

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