Whisker growth dynamics of walrus under human care

Wildlife species that live in remote habitats often require significant investment for scientific study. As such, in lieu of direct observation, conservation research often relies on the analysis of biological samples that are easier to access (feces, hair, whiskers) to gain insight into a wild species' life history (Crawford et al., 2008). Using these samples, scientists can explore exposure to toxic elements, stress, foraging behavior, and even spatial habitat use without undue stress to the animal itself (Crawford et al., 2008). Whiskers (also known as vibrissae), and other keratinized tissues, can be analyzed via isotopic analysis for their content of elements, hormones, and isotopes that remain stable over time. These analyses provide longitudinal insight into an animal's stress, exposure to pollutants, diet, foraging habits, habitat use, and health that can be essential for wildlife managers and scientists alike (Ceia et al., 2018; Charapata et al., 2022; Crawford et al., 2008; Jones et al., 2020; Keogh et al., 2021; Kooyomjian et al., 2022). Whiskers can be obtained from both living and dead animals through minimally invasive sampling efforts, making them an ideal tool for tracking long-term biological processes and ecological effects on individuals. The behavior of semi-aquatic marine mammal species can be difficult to directly observe because they spend much of their time in the marine environment. Pacific walruses (Odobenus rosmarus divergens) are one such species. In combination with annual long-distance seasonal migrations up to 3000 km, dives as deep as 130 m, and haulouts on remote sea ice, Pacific walruses spend much of their time in isolated environments that require indirect methods of study when compared to direct observation of terrestrial-based mammals (Finley & Renaud, 1980; Kastelein, 2008). What is more, as suitable habitat continues to shift in the face of sea ice loss from a warming climate, and adaptive behaviors necessitate focus on new habitats and prey sources, understanding walrus life history has become paramount to understand conservation targets (Couch et al., 2022). Isotopic analysis of walrus whiskers has been proposed as one such tool to circumvent the above challenges while meeting conservation research objectives. However, while there is a range of information that can be determined from whiskers, much of the current research has lacked the ability to determine chronology because growth rates of walruses whiskers have not yet been estimated. Walrus whisker growth cannot be extrapolated from other pinniped species, as growth models vary among species, with some species' whiskers displaying linear growth, while others display asymptotic growth (McHuron et al., 2016). The purpose of this study was to fill this gap in knowledge for future study and conservation use by evaluating the dynamics of whisker growth of the walrus, using walruses under human care as the model system. Whisker measurements were collected by collaborating with animal keepers and veterinarians at two US zoos—the Indianapolis Zoo and SeaWorld San Diego under each institution's respective IACUC protocol. At these institutions, weekly measurements of whisker growth were obtained from a total of four animals, with two at each institution (Table 1). For measurement, each walrus was trained to perform a “target” behavior in which the animal holds its nose to a stationary object to allow husbandry staff to manually mark and measure their whiskers. For purposes of the study, 1–4 whiskers on each cheek were given a burn mark at a midpoint laterally along the length of the whisker using a high temperature woodburning tool. Whiskers were not marked at the cheek surface to avoid burning the animal during marking. Afterward, marked whiskers were straightened and measured by staff weekly for approximately 8 months. Measurements were recorded in millimeters and two measurements were collected per whisker at each time point—base to mark and mark to tip. The base of the whisker was used as a reference point, defined as the intial point that is visible on the surface of the skin—no pressure was applied to push the skin down to reach the root of the whisker. These measurements were taken to evaluate whisker growth rate (GROWTH—the change in length in millimeter/day from base to mark) and the attrition or wear rate at the tip of the whisker (ATTRITION—change in length in millimeter/day from mark to tip) to establish how fast walrus whiskers grow and how fast they lose length through attrition at the tip. These measurements were used to provide a timeline along which sections of the whisker can be compared. Once all measurements were collected, average GROWTH per day per whisker was estimated by subtracting the measurement from base to mark at one time point from the measurement (in millimeter) from base to mark at the subsequent time point, then dividing by the elapsed number of days between the two time points. The same procedure was repeated for whisker ATTRITION, instead using the measurements from mark to whisker tip (also in millimeter). Once all growth and attrition measurements were recorded, a whisker average was calculated for all GROWTH and ATTRITION measurements for each whisker from individual walruses using R statistical software (R Foundation, 2016). Due to small sample size, no age-specific or sex-specific analysis was conducted. Whisker measurements were obtained from four walruses from various demographic groups over an 8-month period (Table 1). Sample replicates included 1–4 whiskers per walrus and encompassed a total of 172 time points. Overall whisker length ranged from 27 to 99 mm, with an average length of 58.03 mm (SD ± 12.15). The period over which measurements were taken ranged from 7 to 33 consecutive weeks with a mean of 19.22 weeks of measurement across all whisker samples (Table 1). Four of the nine whiskers sampled contained gaps where measurements were not collected or only partial measurements were collected—gaps in measurement ranged from 1 week to four consecutive weeks. The partial data were dropped from the final analysis. All whiskers showed an average growth rate of between 0.18 and 0.47 mm gain in length per day per whisker (Figure 1; SE = 0.04–0.12). Whisker attrition due to wear was much more variable than growth and showed an average of between −0.08 and −1.39 mm loss per whisker per day (SE = 0.02–0.31). Whisker analysis is a relatively affordable and minimally invasive method for obtaining a range of information on an animal's life history. Until now, no such study has examined the growth of walrus whiskers to allow researchers to add temporal estimates to whisker-derived data. This study utilized measurements from walruses under human care to collect controlled data to analyze patterns of growth. The data presented here support an average fine-scale whisker growth rate (if taken from the base to the whisker middle) of 0.18 to 0.47 mm per day. The consistent growth recorded here mimics previous trends in other pinniped species, particularly otariids such as Steller sea lions (Eumetopias jubatus), Antarctic fur seals (Arctocephalus gazella), Subantarctic fur seals (Arctocephalus tropicalis), Australian fur seals (Arctocephalus pusillus), and California sea lions (Zalophus californianus) (Kernaléguen, Arnould, et al., 2015; Kernaléguen, Cherel, et al., 2015; McHuron et al., 2016; Rea et al., 2015). Evaluation of the whisker wear rate and overall length of the whisker samples presented by this study may also have husbandry implications. As part of a larger overarching study of gut microbial analysis of walrus conducted by Couch et al. (2022), intact cheek samples were obtained from a 2020 subsistence harvest in Alaska, and whisker lengths were measured. The mean whisker length of wild walrus from 19 cheek samples was 28.9 mm (n = 347 whiskers) compared to the 58.03 mm (n = 108 measurements of nine whiskers) of the walrus in this study under human care. This is consistent with prior research that compared whisker length between wild and captive walrus (Mohr, 1950). This difference is believed to occur due to the abrasion that occurs with natural foraging behaviors, which may not be replicated under human care (Fay, 1982). Thus, this difference may both highlight a future husbandry consideration and makes drawing extrapolations between wild and captive walruses for whisker attrition difficult. Based on study findings, only measurements from the base of whiskers would likely be consistent enough for temporal analysis. This study did not analyze age or sex differences due to sample size and only included Pacific walruses. Demographic differences are critical for understanding how a walrus' physiology can impact growth rates. It is also possible that growth rates may vary seasonally or across the lifespan. Increased sample size would allow for examination of differences in whisker growth between sexes, age groups, seasons, and dietary foraging strategy, which could lend additional information for future wildlife research and management. This study is currently ongoing and seeking to add additional historical whisker data from prior research, as well as to increase the sample size by partnering with new facilities in order to account for the range of variables that could impact growth rate. As the data in this study were collected by animal care staff in other states, the whiskers measured were at the discretion of the walrus caretakers. As such, it was not possible to accurately report the location of the utilized whiskers on the cheek. Based on the findings of McHuron et al. (2016) in their study of California sea lions (Zalophus californianus), it may be beneficial to control for whisker location in future studies. Given the potential impact of sex, sexual maturity, and location on growth rates, future studies should examine if demographic factors need to be considered for more precise growth estimates (Howard et al., 2024; Kernaléguen et al., 2012). Danielle Blackfield: Formal analysis; project administration; software; visualization; writing – original draft; writing – review and editing. Erika L. Allen: Data curation; writing – review and editing. Nick Northcraft: Data curation. Sydney E. Pitts: Data curation. Ashley Pratt Smith: Data curation. Mitzi Synnott: Data curation. Heather Broughton: Conceptualization; formal analysis; funding acquisition; investigation; methodology; project administration; software; visualization; writing – review and editing. Brianna R. Beechler: Formal analysis; project administration; resources; supervision; writing – review and editing. Shea Steingass: Conceptualization; data curation; formal analysis; funding acquisition; investigation; methodology; project administration; resources; supervision; validation; visualization; writing – review and editing. We would like to thank our partners at SeaWorld San Diego and the Indianapolis Zoo for making this project possible. Open access publishing facilitated by The University of Sydney, as part of the Wiley - The University of Sydney agreement via the Council of Australian University Librarians. National Geographic, Grant/Award Number: NGS-50280C-18; Point Defiance Zoo & Aquarium Dr. Holly Reed Fund. The author reports no conflicts of interest in this work.