GROWTH RATE AND SHEDDING OF VIBRISSAE IN THE GRAY SEAL, HALICHOERUS GRYPUS: A CAUTIONARY NOTE FOR STABLE ISOTOPE DIET ANALYSIS

Abstract

Stable isotopes have become powerful tools for gathering information on food webs in marine ecosystems. The method is based on the concept that the ratio of Nitrogen‐14 to 15N (or Carbon‐12 to 13C) in the tissues of animals is directly related to the ratio found in their diet. Vibrissae provide a time series of stable isotope data as tissue is laid down sequentially over time. Here we examine the growth rate of 283 mystacial (muzzle) vibrissae of four gray seals, Halichoeruas grypus, over a five‐month period to investigate their applicability for stable isotope diet analysis. The individual vibrissae did not grow at a constant rate during the study, Fifty‐nine actively growing vibrissae were modeled to quantify the growth pattern using a three‐parameter von Bertalanffy curve, with the parameters estimated using non‐linear mixed‐effects models. This model incorporated the inherent serial correlation of these data. The growth rate was 0.024 cm/d (95% CI = 0.019–0.030), the asymptotic length differed significantly by location (F3,56=9.64, P < 0.001), but no significant trend was found with muzzle location (F3,56= 0.15, P= 0.93). The Δlength/Δtime between each measurement was calculated and most of these data fell at or near zero growth (median = 0.04 cm/d, range = 0–0.78). Individual vibrissae were shed asynchronously and without any seasonal growth trend. This has serious implications for researchers attempting to extrapolate diet data from vibrissae. Because the growth is neither continuous nor synchronous, it will be a challenge to accurately identify the dates when the isotopes were incorporated into the tissue.

Vibrissal growth parameters of southern elephant seals Mirounga leonina: obtaining fine-scale, time‑based stable isotope data

“…The vibrissae of otariids, such as Antarctic fur seals Arctocephalus gazella (Walters 2014), and Steller sea lions Eumetopias jubatus (Hirons et al 2001), grow linearly and are retained for multiple years (Hirons et al 2001;Cherel et al 2009;Kernaléguen et al 2012). In contrast, the vibrissae of phocids, such as grey seals Halichoerus grypus, and P. vitulina, grow asymptotically and are characterised by an asynchronous growth and replacement pattern (Greaves et al 2004;Hall-Aspland et al 2005;Beltran et al 2015). Furthermore, it is suggested that vibrissae of phocids have short retention times; for example, P. vitulina and spotted seals, Phoca largha, shed their vibrissae on an annual basis (Zhao & Schell 2004;McHuron et al 2016).…”

“…As a result, vibrissae are increasingly utilised to study the diet of pinnipeds (Greaves et al 2004;Hückstädt et al 2011Hückstädt et al , 2012Newland et al 2011;Hindell et al 2012;Walters et al 2014). Nevertheless, the vibrissal growth rates of several phocid species remain to be determined, despite the recent interest in the utilisation of vibrissae to increase the temporal resolution of the dietary data (e.g., Hirons et al 2001;Greaves et al 2004;Zhao & Schell 2004;Hall-Aspland et al 2005;Beltran et al 2015).…”

“…The vibrissae (whiskers) of pinnipeds (Ling 1966) are particularly useful for dietary studies when using SI analysis (e.g., Hirons et al 2001;Greaves et al 2004;Zhao & Schell 2004;Hall-Aspland et al 2005; Lewis et al 2006;Cherel et al 2009;Hindell et al 2012;Walters et al 2014;Rea et al 2015). Vibrissae can be sampled relatively non-invasively and archive ecological information over longer temporal scales at a finer resolution than other tissues, such as blood (Tieszen et al 1983).…”

“…The SI signature captured along the length of keratinous tissue, such as feathers (Grecian et al 2015), nails, hair or vibrissae, provides a time-series of the species' dietary history, if analysed chronologically (Hückstädt et al 2012;Robertson et al 2012;Walters et al 2014;Beltran et al 2015). Numerous studies focussed on ascertaining the exact, species-and tissue-specific growth rates of slow-growing, biologically inert keratinous tissue of various species in different habitats (e.g., Greaves et al 2004;Rohwer et al 2009;Beltran et al 2015).…”

“…The growth rate and retention period of vibrissae vary between otariid and phocid species (Greaves et al 2004). The vibrissae of otariids, such as Antarctic fur seals Arctocephalus gazella (Walters 2014), and Steller sea lions Eumetopias jubatus (Hirons et al 2001), grow linearly and are retained for multiple years (Hirons et al 2001;Cherel et al 2009;Kernaléguen et al 2012).…”