Deer are weirder than you think

 

One of the incredible suspended taxidermy displays at the Museo delle Scienze (MUSE), showing two male Central European or common red deer (Cervus elaphus hippelaphus) entangled in combat. In some rare instances interlocked males can no longer separate, and they usually both die.

As promised in the previous post, here's more on deer and why they're so fascinating.

For how mundane we may find them, deer (family Cervidae) are actually some of the weirdest mammals you can think of, for a variety of reasons, which include laryngeal modifications to enhance their calls during rut, ancient hybridizations, bipedal walking, carnivoran-like aggressive posturing and roaring, and much more (if you want to know more on this stuff, as you should, Darren Naish has previously blogged on all this here, here and here). But what deer are most renowned for, and arguably their weirdest specialization, is their headgear: instead of just evolving horns (keratinous structures with a bony core) each year most deer grow new and often elaborate bony outgrowths on their head, called antlers, used predominantly by males for display and intraspecific combat. These are covered in a specialized epithelium, called velvet, while growing, but this is then shed, leaving deer parading with what is essentially dead bone (Landete-Castillejos et al. 2019; Feleke et al. 2021; Heckeberg et al. 2022; Li et al. 2024). 

I say "most" deer because the water deer (Hydropotes inermis) is unique in lacking antlers, instead employing enlarged upper canines for intraspecific combat. All Miocene cervids had simple antlers and elongated upper canines, a condition still retained in modern muntiacins while most other deer reduced or lost canines in favour of more elaborate antlers, but because the water deer canines are of a different shape than those of early cervids and muntiacins, and because phylogenetic analyses place it close to roe deer, antler loss and canine elongation must have been secondary acquisitions (Heckeberg 2020).

A female southern red muntjac (Muntiacus muntjak) displayed at the Zoological Museum of Naples. Male muntiacins sport relatively simple antlers on long bony pedicles and enlarged upper canines which are similar to the condition found in Miocene cervids (see Heckeberg 2020). Also notice the large preorbital and frontal glands.

Antlers themselves are shed and regrown annually (in some tropical species the cycle is more irregular, see Geist 1999), and they're considered some of the fastest growing bones in the animal kingdom. As for why they shed them, it has been speculated that it could be a way of having a "fresh" set of antlers every breeding season, since dead bone can't repair itself from breakage, while a complex branched structure made of living bone may have been too complex to repair, so they just cast and regrow the whole structure instead (Landete-Castillejos et al. 2019). The entire process puts deer under tremendous physiological stress, and part of the minerals necessary for antler formation are borrowed from the rest of the skeleton (Li et al. 2024). This extreme example of secondary sexual characteristic is made even weirder when one considers that mammals otherwise display very limited capacity of appendage regeneration, and genetic studies have found that antler growth is actually more akin to a controlled form of bone cancer growth than normal bone growth (Landete-Castillejos et al. 2019; Feleke et al. 2021; Heckeberg et al. 2022; Li et al. 2024).

A male specimen of giant deer (Megaloceros giganteus) displayed at the Natural History Museum Vienna.

The largest known antlers belong of course to the extinct giant deer or Irish elk (Megaloceros giganteus), which could reach 1.7 m each. Based on the size of the species and on the gestation times in modern deer living in northern regions, Geist (1999) estimated that for giant deer the height of rut would have been in the second week of August, the velvet would have been shed in late July, and the antlers themselves would have been shed in early March. Growing such huge antlers would have taken a heavy toll on the males, and it has been suggested that the preponderance of male specimens compared to females in some lake deposits can partly be explained by a tendency of males to seek calcium-rich water vegetation, thus being more exposed to drowning in places with the right conditions for fossilisation (see Geist 1999, also both Darren Naish and Mark Witton have previously blogged on this, so check them out).

For all the majestic skeletal mounts of male giant deer showing their famously enormous antlers, here's the partial skull of a specimen that had just lost them, at the Museo Paleontologico di Montevarchi. It was found in Figline Valdarno (Tuscany, Italy). You can see well the pedicles, the permanent bony protuberances on top of which antlers grow each year.

While surely huge, Megaloceros antlers were not absurdly so, being within cervid allometry (Tsuboi et al. 2024). Not only that, they were also in agreement with its general lifestyle, but more on this in the next article.

References

• Feleke M., Bennett S., Chen J., Hu X., Williams D, Xu J. (2021). New physiological insights into the phenomena of deer antler: a unique model for skeletal tissue regeneration J. Orthop. Trans., 27 (2021), pp. 57-66, 10.1016/j.jot.2020.10.012
• Geist, V. (1999). Deer of the World. Swan Hill Press, Shrewsbury.
• Heckeberg, N. S. (2020). The systematics of the Cervidae: A total evidence approach. PeerJ, 8, e8114.
• Heckeberg N. S., Zachos F. E., Kierdorf U. Antler tine homologies and cervid systematics: A review of past and present controversies with special emphasis on Elaphurus davidianus. The Anatomical Record. 2023;306(1):5-28. DOI: 10.1002/ar.24956
• Landete-Castillejos, T. et al. Antlers - Evolution, development, structure, composition, and biomechanics of an outstanding type of bone. Bone 128, 115046 (2019).
• Li, C., Wang, W., Zhang, G. et al. Bone metabolism associated with annual antler regeneration: a deer insight into osteoporosis reversal. Biol Direct 19, 123 (2024). https://doi.org/10.1186/s13062-024-00561-3
• Tsuboi, M., Kopperud, B.T., Matschiner, M. et al. Antler Allometry, the Irish Elk and Gould Revisited. Evol Biol 51, 149–165 (2024). https://doi.org/10.1007/s11692-023-09624-1