Fine-scale whisker growth measurements can reveal temporal foraging patterns from stable isotope signatures

Abstract

Stable isotope analysis of slow-growing, metabolically inert tissues is a common method for investigating foraging ecology in migratory animals, as direct observations of feeding are often not possible. Using tissue growth dynamics to interpret foraging timelines can maximize the utility of foraging data; however, applying inappropriate growth models is problematic, and high-resolution growth measurements are seldom made. We used photogrammetry to repeatedly measure the length of whiskers in 93 follicles over 670 d in a trained, captive northern elephant seal Mirounga angustirostris. We developed and optimized a follicle-specific growth model to describe the 18 000 whisker length measurements. Whiskers from the captive seal exhibited asymptotic growth that was described by the von Bertalanffy growth function. Applying the growth model to serially sampled whiskers from 4 free-ranging adult female northern elephant seals resulted in alignment of peaks in δ 15 N along the length of whiskers with the breeding haulout period, when seals are not feeding. Our study provides a high-resolution whisker growth model and is the first to use whisker growth dynamics to improve temporal interpretation of isotopic ratios.

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

“…The ability to obtain fine-scale, temporal integrated biogeochemical data was demonstrated in a similar study conducted on a single, captive adult female northern elephant seal, Mirounga angustirostris (Beltran et al 2015). However, ascertaining the starting date of the vibrissal growth remains a prerequisite to assign dates to dietary data (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). 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).…”

“…However, such a combined approach requires higher resolution SI data (Beltran et al 2015; also see Banks et al 2014). 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).…”

“…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).…”