By D. Rex Mitchell

Each year, thousands of wild animals across the world are rescued. These can include young animals saved from abandonment, injury, disease, or the death of their parents. Just like humans, these animals love to munch on foods that are easy to eat. Captive animals can be fed delicious snacks that are processed, pureed, diced, or pre-portioned in some way. Examples include minced meat for meat eaters, fruit smoothies for fruit eaters, or small pellets and peeled or diced fruit for plant eaters. But these kinds of foods are often very different to what these animals would find to eat in the wild. Wild foods will likely require more effort to obtain and chew, and in many cases be harder or tougher to bite into. We wanted to know what it could mean for development of the skull if baby animals are reared exclusively on foods that are easy to eat.

Luckily, we had a large collection of computed-tomography (CT) scans collected back in 2012 for different research. Forty rats had been fed different diets from when they finished suckling all the way to adulthood. For that previous research, some rats were fed hard pellets that required plenty of biting and then chewing. Others were fed a soft diet of ground up pellets that required almost no biting or chewing. Two other groups were switched halfway as juveniles. Using these scans, we created three-dimensional digital models of each rat skull and carried out bite simulations to compare the amount of stress each skull model experienced when biting. Greater stress would indicate thinner bones.

Oftentimes, bones are thought of as simple, hard objects. But bone is actually a complex living tissue that is constantly adapting. Every time a bone is used to perform an action it bends a little bit. Thanks to specialized cells that deposit bone, the more often a bone bends over time, the thicker the bone can get – especially when lifting or moving heavier things. We expected rats that hardly had to bite their foods growing up to have less bone deposited into their skulls and that this would make them weaker than the skulls of the rats fed harder foods.

Our simulations showed that the skulls of rats that never had to bite very much had the most amount of stress during biting. This supports the idea that softer foods lessen the amount of bone that is deposited in the skull, leading to thinner bones. But we also found that some parts of the skull were weakest in the group that switched as juveniles from hard food to soft food. So how to explain this? Well, previous studies have shown that the number of cells that deposit bone decreases when bones experience sudden and prolonged disuse. We suggest that if this happens during development, that fewer bone cells available may cause bones to finish growing to adult size weaker than they should.

What does this mean for the rehabilitation and release of baby animals? Well, it’s similar to sports and exercise. When a person goes running to practice for a race, they aren’t just preparing their bones and muscles to run faster and more efficiently, but they’re also reducing the risk of injuries. Ultimately, it’s about conditioning the body to be able to better perform the tasks that are expected. If rescued baby animals are raised on diets that are overly reliant on softer, processed, pre-peeled, cut, blended, or portioned foods, much of the effort has already been done for them. This means the bones and muscles of their skulls won’t be as conditioned for the more difficult foods they may need to eat in the wild. They could therefore be at a disadvantage when released, possibly less able to immediately access some of the tougher foods available – and there is also a greater risk of injury if they try. This is especially relevant to juvenile animal rescues. Animals that may have started their lives on tougher wild foods, and then are switched to a captive diet of foods that are much easier to eat, might have even weaker skulls when returned to the wild as adults.

Ultimately, all kinds of animals have all kinds of diets and feeding behaviors. So, these findings simply serve as an additional consideration for all those wonderful animal carers out there committing their time and efforts to rearing sick and injured wildlife. Such efforts are widely recognized as a great contribution to conservation efforts.

Rex received his PhD in Zoology from the University of New England (Armidale, AU) in 2019 and is currently at Flinders University as a postdoc researcher. He specialises in comparative anatomy and digital morphology, with a focus on animal skull shape and feeding biomechanics. Rex is also an industry-qualified chef, a veteran cocktail bartender, and Speculative Biology geek. You are welcome to contact him via drexmitch311@gmail.com or find him on Twitter @DRexMitchell