Rodents may be animals that can be reservoirs of zoonotic diseases that can be transmitted to humans by touch or bite of the animal but they have long served as the preferred species for biomedical research animal models. This is because of their genetic, physiological, and anatomical similarity to humans. Advantages of rodents in advancing biomedical research include their small size, abundant genetic resources, short life cycle, and ease of maintenance, according to Elizabeth C. Bryda, Ph.D. of the Rat Resource and Research Center.
Conserved cranial evolutionary allometry
These mammals come in different types, which can be distinguished by their differences in genetics and physical appearance, but their skulls correspond to one simple size-dependent shape. This is based on a new study co-led by scientists from the University of Queensland and Flinders University.
Their study, which appeared in The American Naturalist journal, focused on Australian rodents. It revealed that the skulls of rodents resemble each other in any given size, which means it takes little adaptation for them to survive in various habitats.
Vera Weisbecker from Flinders University, College of Science and Engineering told Science Daily that it seems intuitive that a group of animals will be more successful in evolution if they display a wide variety of shapes. Yet, the Australian rodents show that shape variety does not always mean success in evolution. The lack of change in skull shape and size is even the secret behind their survival.
Well-adapted skulls: the key to survival
Co-author Dr. Ariel Marcy also explained that it was around four million years ago when rodents first entered Australia and they quickly adapted to the diversity of habitats in their continent. It is known that the key to the survival of mammals is having well-adapted skulls, so the team was expecting to find plenty of locally adapted skull shapes. But what they found was the opposite as there was low variation in the skull shape of Australian rats. Invasive rodents, such as the Norway rat and house mouse, also share the same pattern. Furthermore, the link between skull size and shape is at least ten million years old.
Conservatism of shape
To better understand the patterns of adaptation, the researcher scanned the skulls of 38 rodent species using 3d surface scanners. These rodent skulls are obtained from museums. Then, they analyze the skull shape using geometric morphometrics, a statistical procedure of capturing morphologically distinct shape variables and is a part of a larger subfield in anthropology. Based on their findings, the researchers believe that the specialization of rodent jaws may play a role for such a surprising “conservatism of shape” such that their jaw allowed their skulls to be a “multi-purpose tool.”
Dr. Weisbecker added that rodent jaws and skulls have a complicated but highly versatile arrangement that enabled the species to work well in various conditions. At first, the authors thought it would discourage their evolutionary change but they noticed the unusual rodent skull shape only in extreme ecological adaptation, like in cases of rakali or water mouse. The rakali rodent is a top predator in saltwater and freshwater environments and is an unusual meat-eating predator.
The researchers pointed out that their findings can help answer the biggest questions in the field of evolutionary biologies, such as why some animal groups are more diverse compared to other groups.
How are rats counted?
World Atlas published that it is scientifically impossible to quantify rats because of their hibernating and nocturnal behavior. Even the attempt to use questionnaires to answer the question has not yielded results. Renowned institutions, like the FAO and WWF, have participated in the count. People could count them in dumpsites and sewers but since it is not easy to count them, rodent experts only provided for an estimate. Rodentologist Bobby Corrigan, for instance, said that the highest rat-infested cities in America are Boston, New York, and Philadelphia. What leads to the high population of rats is that they mature sexually between ages 4 and 5 weeks from birth and it only takes 21 to 23 days to bring forth an offspring.
A survey conducted by the National Pest Management Association (NPMA), which represents the interests of the professional pest management industry and pest control professionals in the US, also shows that 29% of Americans have had a mouse or a rat problem in their home. Of those who have had a problem, 45% said it happened in the fall or winter months. Half of the respondents said the rat problem occurred in the kitchen, 27% said it occurred in the basement, 25% said in the living room, 24% said in the garage and attic, 22% said in the bedroom(s), and 11% said in the bathroom(s).
Some rodent-proofing tips the NPMA recommends to homeowners is to inspect outside their home for easy access points. If there are narrow opening (crevice) or cracks, they should seal it. They should also pay attention to locations where utility pipes enter as mice can enter homes through holes as small as a dime. If there are larger gaps, fill it with pieces of steel wool as they are deterred by the roughness of steel fibers. Rodents, particularly, are unable to gnaw through steel wool.
NPMA also warned that with millions of people now tucked away in home isolation, it increases the amount of food we are consuming and the waste we are generating at home. These social distancing behaviors can, however, attract pests. There were even more than 303,000 online engagements in March 2020 on the topic of finding rats in the kitchen. To keep pests outside our home while we practice social distancing, NPMA suggests checking for damaged packages, discard boxes appropriately, inspect groceries, and keep children from squirreling away food in the living room, bedroom, or even in their playroom.
Meanwhile, database company Statista reports that as of June 2019, the global rodenticide market value was estimated to be US$4.7 billion and is forecast to reach $5.9 billion in 2025.
Rats may be the least favorite animal of many people and they can send shudder down our spine. Yet, despite their poor reputation, their species remain astonishingly successful. Some of their species are even one of the most widely distributed animals in the world.