A unique roe deer photographed

The spectacular photograph was taken by the Dutch amateur photographer corne_wildlife_fotografie. He titled it “Black is beautiful”, something irrefutable. It is an adult male specimen sporting beautiful tall, pearled and symmetrical antlers, which was interrupted by the camera while feeding in a cereal sowing.

Black roe deer, documented melanism

A rare genetic phenomenon, concentrated in a few enclaves on the continent, is beginning to reveal its molecular secrets. In the oak and beech forests of northwestern Germany, between 10% and 25% of the roe deer that cross the paths do not display the usual reddish or cinnamon-brown coat, but an intense black, almost jet-black, which makes them stand out like living shadows among the undergrowth. They are not a different species or an isolated rarity: they are melanistic specimens of Capreolus capreolus, the European roe deer, and constitute one of the best-documented yet most enigmatic cases of melanism among the continent’s entire ungulate fauna.

What melanism is and why it attracts so much attention in a cervid

Melanism is the opposite phenomenon to albinism: instead of a lack of pigment, there is an overproduction of eumelanin, the compound responsible for dark shades in skin and hair. In mammals, coat colour depends on the balance between two pigments, eumelanin (black-brown) and pheomelanin (reddish-yellow); when the scales tip completely towards the former, the result is a uniformly black specimen, with the skin also darkened.

What is striking about the case of the roe deer is that, according to the scientific literature, it is the only species of cervid that naturally and stably maintains a numerically significant melanistic population. In other deer species, melanism exists, but as a one-off anecdote; in the roe deer, however, there are entire districts where the “black roe deer” is not an exception, but a common variant of the landscape.

Where we can find black-coloured roe deer

The geography of melanism in the roe deer draws a very particular map, with two major historical focuses. The first is in northwestern Germany, especially Lower Saxony and neighbouring areas of North Rhine-Westphalia, where hunting and naturalist tradition has documented the presence of black roe deer for several centuries, even with the explicit interest of former royal houses in this variant. The second is in adjoining strips of the Netherlands, where the forest continuity with northwestern Germany has allowed the variant to persist in border populations.

Outside that central European core, Austria and other areas of central Europe also record appearances of melanistic specimens, although more scattered and with a much lower prevalence.

The Spanish focus: the Las Batuecas Regional Hunting Reserve

In the Iberian Peninsula the phenomenon is practically anecdotal and, when it appears, it is usually linked to historical introductions of roe deer from Germany: this is the case of repopulated populations between southern Salamanca and northern Cáceres, as well as some isolated records in areas such as La Alcarria, which suggest that the trait may persist latently in the gene pool even far from its area of origin.

Within that reduced Iberian map of melanism, there is one enclave that concentrates a good part of the documented sightings in Spain: the Las Batuecas Regional Hunting Reserve, located in the south of the province of Salamanca, bordering the Cáceres district of Las Hurdes. It is precisely in this area that roe deer from Lower Saxony, the German core where melanism is most frequent, would have been introduced in the mid-20th century, which would explain why the trait has managed to become fixed here and not in other Iberian populations.

This highly localised distribution —specific focuses with high prevalence surrounded by vast areas where melanism is practically non-existent— is in itself a relevant fact: it suggests that we are not dealing with a mutation that has spread homogeneously throughout the species, but with a trait that has become fixed or favoured in specific populations, probably through processes of genetic drift, reproductive isolation or population bottlenecks.

The genetic cause: a switch in the ASIP gene

For a long time, although melanism in the roe deer was a phenomenon known and documented for centuries, the exact mutation that caused it was unknown. That gap began to close with a study published in the journal Genes in 2020, by a team led by Monika Reissmann (Humboldt University of Berlin), together with researchers from the Leibniz Institute for Zoo and Wildlife Research and the University of Kiel.

The team focused its search on two key genes in pigment production in mammals: MC1R (the melanocortin 1 receptor) and ASIP (the agouti signalling protein). These two genes act as a switch system: under normal conditions, the ASIP protein locally blocks the MC1R receptor, allowing mainly pheomelanin, the reddish pigment, to be synthesised. When that blocking fails, the receptor is free to activate the production of eumelanin, the dark pigment, throughout the hair follicle.

That inheritance pattern is key to understanding why melanism behaves as it does on the ground: as it is a recessive trait, a roe deer needs to inherit the mutated copy from both parents to display the black coat. A single carrier specimen —heterozygous— retains the usual chestnut colour, but can pass the mutation on to its offspring without showing it itself. This explains why melanism can remain “hidden” for generations in a population and re-emerge more visibly when conditions, such as genetic isolation or mating between close relatives, increase the likelihood of two carriers breeding with each other.

Why is melanism maintained? The functional hypotheses

The fact that a mutation persists and is concentrated in certain populations raises a fundamental evolutionary question: does the black coat provide any advantage, or does it simply survive by genetic chance? Research into melanism in other species —from insects to reptiles— has proposed several functional hypotheses that specialists also consider as possible explanations, not yet conclusively confirmed, for the case of the roe deer:

• Protection against ultraviolet radiation: eumelanin absorbs UV radiation better than pheomelanin, so a darker coat could offer some additional protection against sun damage to the skin.
• Thermoregulation: the so-called “thermal melanism hypothesis” maintains that dark-coloured individuals absorb more solar radiation and reach higher body temperatures in cold, low-wind conditions, which could provide an advantage in certain forest microclimates of northern Europe.
• Resistance to pathogens: in several species, an association has been observed between greater melanic pigmentation and more robust immune responses, since the biochemical pathways that produce melanin share components with certain defence mechanisms of the organism.
• Genetic drift and bottlenecks: in contrast to adaptive explanations, there is also the possibility that melanism confers no particular advantage and has simply become fixed by chance in small and relatively isolated populations, a well-known evolutionary mechanism in other colour variants that remain stable in very specific locations, as also occurs with albinism.

A phenomenon that continues to raise questions

Despite the progress made by identifying the mutation in the ASIP gene, the authors of the 2020 study themselves acknowledged that there is still some way to go: it has not been resolved with certainty whether the genetic variant confers any real selective advantage in the natural environment, nor why the phenomenon has remained so strong in such a delimited area of northwestern Germany for centuries, while it has barely spread to the rest of the roe deer’s vast distribution area, which extends from the Iberian Peninsula to the Urals.