Field-Based Age Estimation of Juvenile Galapagos Sea Lions (<I>Zalophus wollebaeki</I>) Using Morphometric Measurements

Abstract

Information about the age of juvenile pinnipeds is necessary for an understanding of ontogeny-specific patterns and strategies. Exact age determination of juvenile cohorts from wild populations is best achieved through birth observations and subsequent marking, but this involves a considerable time lag during which juveniles mature. A combination of body and teeth measurements of knownage Galapagos sea lion juveniles taken during brief routine captures in the field was used to create age prediction models. Several general linear models (GLMs) produced reliable age estimates for male and female juveniles up to an age of 2 y. Teeth measurements were important predictors of age: male age was best estimated using upper canine length (CL), mass, and girth, while the best predictors for female age were CL, canine width (CW), body length (SL), body mass, and an interaction between CL and CW. The presented method of aging wild unmarked juveniles in the field is applicable during routine captures, requires little equipment, and yields a considerable increase of information for studies involving brief sampling periods in the field. We suggest its adjustment, testing, and application in studies of juveniles of other species.

Applying the tools of ecological immunology to conservation: a test case in the Galapagos sea lion

“…We captured sea lions using hoop nets (Fuhrman Diversified, Seabrook, TX, USA) and briefly restrained them in a prone position without the use of chemical immobilization following Jeglinski et al . (). We sampled 30 6‐month‐old juveniles in each colony during April 2009 and marked them with the tagging method of a long‐running Galapagos sea lion study (Wolf & Trillmich, ).…”

Same size – same niche? Foraging niche separation between sympatric juvenile Galapagos sea lions and adult Galapagos fur seals

“…Body mass was recorded using a digital scale hanging from a tripod (Kern HUS 300 K 100; Kern & Son GmbH, Balingen Germany; precision 0·2 kg, max 100 kg), and morphometric measurements of standard length (SL), fore‐flipper length and width (FL, FW) and axillary girth were taken (see details in Jeglinski et al . , ).…”

“…Adult animals were marked with passive transponders and flipper tags (adult sea lions) or individual fur clippings (adult fur seals). Body mass was recorded using a digital scale hanging from a tripod (Kern HUS 300 K 100; Kern & Son GmbH, Balingen Germany; precision 0Á2 kg, max 100 kg), and morphometric measurements of standard length (SL), fore-flipper length and width (FL, FW) and axillary girth were taken (see details in Jeglinski et al 2010Jeglinski et al , 2012.…”

Age, body mass and environmental variation shape the foraging ontogeny of Galapagos sea lions

“…Six additional juveniles that had just moulted into their first juvenile pelage (age about 4 to 6 mo) were captured and marked with passive transponder and plastic tags to increase the sample size. A second cohort of known age was established by capturing juveniles in October 2008 and calculating their age by using species-specific age estimation equations based on morphometric and teeth measurements (Jeglinski et al 2010 (April 2009 andApril 2010). All adult females suckled either newborn pups or 6M juveniles in the cold or warm season, respectively.…”

“…Body mass was recorded with a digital scale hanging from a tripod (Kern HUS 300K 100, precision 0.2 kg, maximum capacity 100 kg), and morphometric measurements of standard length (SL), fore-flipper length and width (FL, FW) and axillary girth were taken (Jeglinski et al 2010). Instruments, glued onto neoprene and attached to a piece of black mesh with cable ties, were attached to the dorsal fur on the midline of the back of the study sea lions just below the shoulder blades by using fast setting epoxy (Araldite 2012, Huntsmann Advanced Materials).…”