BEACON Researchers at Work: Carnivore Skull Evolution

This week’s BEACON Researchers at Work blog post is by MSU graduate student Nikki Cavalieri.


Why do tree frogs stick to glass but toads don’t? Why are baby skinks tail’s blue but adult’s not? Why are puppies and kittens born with their eyes closed but calves and foals born with their eyes open? Why did a girl from Cloudy, Oklahoma become a scientist? I was a child with many questions, which no one could answer. I grew up, went to college and still no one could answer all of my questions. So I became a scientist to find out WHY?

The essential question of my dissertation research is “Why don’t all carnivorans have a strong skull with formidable teeth?” Carnivorans are members of the mammalian Order Carnivora. They are some of the most recognizable animals on earth: cats, dogs, bears, seals and many others. They range in size from the least weasel (100 g) to the polar bear (800 Kg). They live on all major landmasses and occur in every ocean. Carnivoran diets range from completely herbivorous to completely carnivorous and include every combination in between.

Nikki showing the full range of carnivore skull sizes

Nikki showing the full range of carnivore skull sizes

The ancestral features of this group are a robust cranium and jaw with large canines and sharp cheek teeth. Many modern carnivorans have more delicate features. Why lose a good thing? We know that species evolve via natural selection. That is, individuals that perform better, through morphological, physiological, and behavioral traits, survive and have more opportunities to produce offspring. Those offspring inherit the traits of their parents. This differential survival and reproduction of individuals with different traits is what determines how species function and make a living.

We would expect that species would evolve only traits that perform best. However, this is not what we observe. So why? We know that there is a limited amount of energy available in the environment. In order to reproduce and pass traits to the next generation an individual must grow and survive first. Individuals must balance energy demands between traits for survival, growth and reproduction. These traits are competing. When traits compete we expect to see trade-offs. A trade-off is a compromise between conflicting selection pressures on a trait. This can result in the phenotype being suboptimal for one or more traits. To further complicate matters, selection pressures on a trait may be different during different times in life.

Not only do we expect trade-offs between what traits species have, but also when those traits develop. There is no advantage to developing secondary sexual characteristics to attract mates if reproductive organs are not fully developed. Thus, the evolution of timing of life history events (i.e. weaning, independence, age at first reproduction) is influenced by trade-offs between competing traits for survival, growth and reproduction. I believe that understanding the interaction and timing of competing traits will help us decipher why there so much variation in the carnivoran cranium and jaw.

Since energy is limited, natural selection should act strongly on traits for obtaining energy. The main structure for obtaining energy for carnivorans is the skull. The Carnivora skull is a multipurpose structure that serves as a feeding apparatus plus houses and protects the brain and sensory organs. The carnivoran skull is not completely developed at birth and must go through extensive post-natal growth before reaching morphological maturity. It has to meet demands at each life stage and to develop between life stages (e.g. morphology for nursing versus procuring food). In its role as a feeding apparatus, the skull must procure and process food. Many studies have found a strong relationship between diet and morphology of the carnivoran skull.

Specifically I study the patterns of interspecific variation in growth and development of the carnivoran skull. I focus on two questions: (1) whether interspecific differences in the timing of morphological maturity are reflected in life history schedules and (2) whether timing of morphological maturity is influenced by diet.

CNCavalieri2015_setupI hypothesize that delayed morphological maturity shifts reproduction to later in life. To test this I get to travel to natural history collections and photograph specimens ranging from one day to several years of age. From these photographs I construct ontogenetic skull series. Shape and size are quantified for the skull series using geometric morphometrics. I calculate age at morphological maturity for skull shape and skull size, for each species. I examine changes of ontogenetic trajectories, allometric trajectories, and disparity. I compare the timing of morphological maturity of the skull relative to life history schedules across the Order Carnivora, taking into account body size and longevity. I plan to examine some of the behavioral correlates (e.g. level of maternal care) that accompany these shifts.

My second hypothesis is that there is a trade-off between dietary challenge (i.e., difficulty in procurement and processing) and timing of skull development, such that species with more challenging diets reach morphological landmarks (e.g., age at adult skull morphology) later in their life cycle than species with less challenging diets. I measure dietary challenge with two components: 1) procuring difficulty and 2) processing difficulty. Procuring difficulty is quantified using a score that takes into account the physical and behavioral aspects of the predator and food item. A higher score indicates a more difficult diet to procure. Processing difficulty is quantified using empirical measurements of food items. I get to use engineering machinery, usually used to test material properties of steel beams, to measure the toughness, tensile strength, and hardness of food items.

One of the cool analyses I will be able to do is to map dietary challenge and timing of skull maturity on a phylogeny of the Order Carnivora. By using Bayesian inference to estimate ancestral character states, I can explore transitions between high and low dietary challenge and early or late skull maturity through evolution. This is exciting because it can allow me to understand why and when variation in carnivorans evolved.

As of right now I still don’t know “Why all carnivorans don’t have a strong skull with formidable teeth?”. I have collected enough data to know that it is going to be interesting but not enough to see the whole picture. I will just have to wait a little longer to find out WHY?

For more information about Nikki’s work, you can contact her at cavali13 at msu dot edu.

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