These Bat Teeth Were Made for Chompin’: An Evolutionary History of Bat Teeth
Featured Image Caption: Bats like the Honduran White Bat (Ectophylla alba) pictured above eat fruits, primarily figs, using their short jaws and small teeth, while close relatives are specialized to eating a completely different diet of insects or nectar with larger jaws and teeth. Recent research investigates how and why jaws and teeth are so diverse in this group of tropical bats. (Image source: “Honduran White Bat” by Jay Pruett, CC-BY-NC 4.0, via iNaturalist)
Source article: Sadier A, Anthwal N, Krause AL, Dessalles R, Lake M, Bentolila LA, Haase R, Nieves NA, Santana SE, Sears K. 2023. “Bat teeth illuminate the diversification of mammalian tooth classes.” Nature Communications 14(1): 4687. https://doi.org/10.1038/s41467-023-40158-4
You Are What You Eat – Or, You Adapt to It
Bats found across the American tropics eat a wide variety of foods from insects to fruits to blood. Across this group, bats have different jaw lengths and tooth types that allow them to specialize on their particular diet. Like humans, bats have four different types of teeth: incisors for cutting, canines for tearing, and both premolars and molars for chewing and grinding – though not all bats have the same number or size of these teeth. Surprisingly, many closely-related bats that have diverged in the last 25 million years exhibit different types of teeth and jaw shapes and sizes that would usually take hundreds of millions of years to differentiate. Little is known about how so many diverse jaw and tooth characteristics arose within this group in an evolutionarily short amount of time. A recent study investigated the tooth and jaw patterns found in this group of bats and the developmental pathways that lead to differences in bat teeth across evolutionary history.
Adaptive Radiation: Not the Radioactive Kind!
Multiple species evolving quickly from a single common ancestor is known as adaptive radiation, which often happens when a species encounters an environment with many different resources available (e.g. food types including nectar, fruit, and insects). A group of individuals can become specialized to eating certain foods if they have an advantage from natural variation in their physical traits; for example, a bat with a slightly longer tongue will have an advantage drinking nectar over a shorter-tongued bat. If the individuals that can more successfully and reliably get food reproduce together, their offspring will inherit these traits, allowing them to become more and more specialized to their food source over many generations. Ultimately, these groups become genetically isolated from others and diverge into new species.
Noctilionoid bats are a great example of this: there are over 200 species of bats within this group, primarily found in the American tropics. They are a closely related group that has evolved wildly different jaw lengths and tooth types to specialize on different foods, including insects, fruit, nectar, and even blood in the case of vampire bats. Bats with long thin snouts can reach into flowers to drink the nectar, while bats with short faces and wide jaws are well-positioned to eat small hard fruits such as wild figs. The size of the jaw determines how much space there is left for teeth, so specialization to a diet can ultimately increase the diversity of jaw and tooth types, from short pug-like faces with relatively few teeth, to long snouts like a greyhound with plenty of space for teeth and tongues.
Surprisingly, not much is known about how these different types of teeth have evolved in mammals – but using these bats as an example, researchers are uncovering how teeth and jaws have developed throughout evolutionary history.
Bat-ting 1000: How Bats Evolved to Take Advantage of Diverse Food Sources
Researchers compared over one hundred species of noctilionoid bats using physical traits and DNA similarity to determine how this diversity in tooth type might have come to exist. Looking specifically at the premolars and molars, researchers used micro-CT scans to capture the physical shapes and sizes of these teeth and the jaw. A micro-CT scan is essentially a 3D x-ray that creates a digital model of the bat jaw. The researchers then looked for correlations between these dental traits and diet type.
Results show very different jaw types and tooth arrangements across diets, but these changes in tooth size, number, and position follow consistent patterns. Bats that eat nectar or insects tend to have elongated jaws with space for three premolars and three molars, while others that eat fruit have shorter jaws, and typically have fewer teeth overall, eliminating the middle premolar or back molar (or sometimes both!). When these teeth are eliminated, it allows space for the remaining teeth to be a little larger.
The researchers compared these patterns to results from other studies and suggest that tooth development, particularly for premolars and molars, follow different sets of “instructions” in the DNA. Two independent segments of DNA inform how to assemble these teeth, and tiny changes to either of these areas could alter the bat’s smile. This result could help to explain how this group of bat species became so diverse so quickly – small changes at the molecular level have cascading effects to change tooth size and proportions, helping groups of bats specialize on different diets.
This isn’t just for bats either! Many of the key genetic “instructions” explored in this study are found across mammals. These results could even be a first glimpse into how other body parts following similar growth patterns could also vary in size and number, opening the door to exploring development across organs and species. One bat tooth at a time, scientists continue to explore how and why we see so much diversity of form and function all around us.
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