Splitting hairs: differentiating juvenile from adult deer (Odocoileus virginianus) by hair width

Ungulates are a main component in carnivore diets but determining consumption of juveniles is difficult. Past studies have used size of prey remains such as small hooves or bones to classify scat samples as containing content attributable to juveniles. Hair thickness and color may also be used, but seasonality could influence the coat of an adult by developing thinner hairs in summer that more closely resemble those from juveniles. Given this uncertainty, we aimed to quantitatively determine a hair diameter threshold to categorize the age-class of ungulate hair in scats. We obtained hair samples from captive (n = 133) and vehicle-killed (n = 5) white-tailed deer (Odocoileus virginianus) from Georgia and Virginia. We used microphotography image analysis to measure the width of hairs and their cuticular casts. We used a linear model to assess differences among body locations, age-classes, and locations along the hair strand. We also analyzed the change in hair width of juveniles as they aged. Hair diameter of adults, but not juveniles, differed significantly depending on body location, yet adult hairs were always significantly wider than those from juveniles. Juvenile hairs significantly increased in width after mid-September, when they molt into adult coats in our study area. We identified 104.2 µm measured at either 1/8 or 1/4 distance from the follicle as a threshold width to distinguish adult from juvenile hairs, with 95.3% accuracy. Our findings indicate that juvenile white-tailed deer can be distinguished from adults based on the width of hairs found in carnivore scats up until juveniles are 5 months old. More broadly, our results demonstrate that hair width may be used to classify juvenile versus adult prey remains in carnivore diet studies in other predator–prey systems. Los ungulados constituyen un componente principal en la dieta de los carnívoros silvestres; sin embargo, es difícil determinar el consumo de ungulados juveniles por carnívoros. Estudios previos han utilizado el tamaño de los restos de las presas en heces de carnívoros, tales como pezuñas o huesos pequeños, para identificar cuales contienen material de presas juveniles. El grosor y color del pelo también puede ser utilizado para este propósito, pero la estacionalidad puede influir la composición del pelaje de los adultos, produciendo pelos más delgados con apariencia de pelaje juvenil. Debido a esta incertidumbre, nuestro objetivo fue determinar un umbral cuantitativo de diámetro de pelo que permitiera categorizar la clase de edad de los remanentes de ungulados en heces. Colectamos muestras de pelo (n = 1,510) de venado de cola blanca (Odocoileus virginianus) provenientes de individuos en cautiverio o provenientes de colisiones vehiculares en los estados de Georgia y Virginia, EEUU. Usamos análisis de fotomicrografías para medir el diámetro del pelo y su impresión cuticular. Implementamos modelos lineales para evaluar diferencias entre ubicación corporal, clase de edad y tramos a lo largo del tallo piloso. También analizamos el cambio en el grosor del pelo de los juveniles en su transición a adultos. El diámetro del pelo en adultos, pero no en juveniles, fue significativamente diferente en función de la ubicación corporal, aunque los pelos de los adultos siempre fueron significativamente más gruesos que los pelos de los juveniles. Los pelos de los juveniles incrementaron significativamente en diámetro cuando mudaron a pelaje adulto, lo que ocurre después de mediados de septiembre en nuestra área de estudio. Determinamos que 104.2 µm a un 1/8 o un 1/4 del tallo piloso desde el folículo, es el diámetro umbral para distinguir adultos de juveniles con una exactitud del 95.3%. Estos hallazgos son aplicables en estudios de análisis de dieta de carnívoros cuando hay juveniles menores de 5 meses de edad en el paisaje, con el fin de establecer la proporción de heces de carnívoros que contienen restos de presas juveniles o adultas.

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