Increased mass of reticulo-rumen tissue and contents with advancing age in Svalbard reindeer V. Veiberg 1,2 , A. Mysterud 2 , R. J. Irvine 3 , W. Sørmo 4 & R. Langvatn 5 1 Department of Arctic Biology, University Centre in Svalbard, Longyearbyen, Norway 2 Department of Biology, Centre for Ecological and Evolutionary Synthesis, University of Oslo, Oslo, Norway 3 The Macaulay Institute, Craigiebuckler, Aberdeen, Scotland, UK 4 Nesna University College, Nesna, Norway 5 Norwegian Institute for Nature Research, Trondheim, Norway Keywords foraging ecology; life history; digestion; Rangifer tarandus platyrhynchus; ungulate; reticulo-rumen; particle size. Correspondence Atle Mysterud, Department of Biology, Centre for Ecological and Evolutionary Synthesis, University of Oslo, PO Box 1066 Blindern, NO 0316 Oslo, Norway. Tel: +47 22 85 40 45; Fax: +47 22 85 47 26 Email: atle.mysterud@bio.uio.no Editor: Andrew Kitchener Received 11 September 2008; revised 3 November 2008; accepted 6 November 2008 doi:10.1111/j.1469-7998.2008.00538.x Abstract Little is known about the effect of age on digestive processes in wild cervids. One potentially important mechanism is that tooth wear alters the occlusal surface topography, causing gradual loss of functionality. Mastication efficiency is crucial to digestion processes among ruminants, as a larger particle size is associated with longer retention times and potentially reduced digestion efficiency. Using data from 49 adult Svalbard reindeer Rangifer tarandus platyrhynchus females, we investigated whether the mass of reticulo-rumen (RR) contents and tissue and the amount of back fat showed age-related changes. Older animals had higher RR content mass than younger individuals. This is consistent with the prediction that altered particle size due to decreased mastication efficiency led to increased rumen retention time either through increased RR capacity or filling. Additional data on RR particle size distribution were available for a subset (n = 30). The correlation between the proportion of small particles (o1.0 mm) and RR contents was much weaker than the correlation between age and RR contents. This suggests that additional factors apart from particle size contribute to the age-related pattern in RR content mass. The mass of RR tissue in older animals was higher than that in younger animals. This possibly reflects an adaptation to the higher organ fill. The amount of back fat decreased with age, suggesting that alterations in digestion- related processes are not sufficient to compensate for reduced mastication efficiency in Svalbard reindeer. Our results present one possible link between foraging, digestive processes and life-history patterns. Introduction Ruminants are especially well adapted to digest plant carbohydrates such as cellulose and hemicellulose. This is essentially achieved through the action of symbiotic mi- crobes in the rumen (e.g. Van Soest, 1994, chapter 12). The microorganisms are, on the other hand, inefficient in redu- cing forage particle size (Spalinger & Robbins, 1992). Mechanical degradation through mastication processes is therefore crucial to facilitate microbial decomposition and hence digestion efficiency. Wear of dental tissue is an inevitable consequence of this process, and has led to the evolution of lesser or larger degrees of hypsodonty and molar complexity (in terms of enamel–dentin infoldings and infundibuli) among all ungulate species (Janis, 1988; Janis & Fortelius, 1988). Despite these adaptations, wear has been found to cause a gradual reduction in chewing efficiency (Lanyon & Sanson, 1986b; Skogland, 1988; Kojola et al., 1998; Perez-Barberia & Gordon, 1998; Veiberg et al., 2007). However, there have been few empirical examinations of the relationship between age and reticulo- rumen (RR) function, which may be affected if mastication efficiency or other factors related to digestive efficiency change with age. An exception is in koalas Phascolarctos cinereus, where it has been shown that individuals with more worn teeth used a longer time to feed, chewed more and had larger particles in the stomach compared with individuals with better teeth (Lanyon & Sanson, 1986a,b; Logan & Sanson, 2002a,b). The mechanisms involved in selective retention of RR particles are a combination of passive processes related to filtering and particle properties, and active turnover of RR fill through rumen wall contractions (Lechner-Doll, Kaske & Engelhardt, 1991; Van Soest, 1994). The reticulo-omasal orifice represents the bottleneck for flow of digesta from the RR to the omasum. For a long time, the reticulo-omasal orifice was considered the single mechanism for selective retention, but this has been repeatedly disproved (e.g. Lechner-Doll et al., 1991). The fully opened reticulo-omasal orifice diameter is recognized to far exceed the threshold size Journal of Zoology Journal of Zoology 278 (2009) 15–23 c  2009 The Authors. Journal compilation c  2009 The Zoological Society of London 15 Journal of Zoology. Print ISSN 0952-8369