Dystocia in free-ranging sika deer Cervus nippon under food limitation

Abstract

Graduate School of Agriculture, Hokkaido University, Sapporo 060-0809, JapanIn understanding the mechanism of population dynamicsof large mammals, it is essential to clarify the mortalityfactor. For ruminants, birth size is a major factor of neo-natal survival because smaller neonates are more suscep-tible to exposure (Haughey 1980; English and Mullery1992) and vulnerable to predators (Singer et al. 1997).Birth size is commonly related to maternal body sizewithin and across species (Clutton-Brock 1991). Thefetus/neonate-mother proportion may be a consequenceof developmental or morphological constraints (MaynardSmith et al. 1985). For example, it is well known thatfetomaternal disproportion (FMD) causes dystocia, oftenresulting in high fetal/neonatal mortality in domesticruminants (cattle, Johnson et al. 1988; Schwabe and Hall1989; Noakes 1997; West 1997: sheep, McSporran andFielden 1979; Thomas 1990; Dwyer 2003). However,dystocia in wild ruminants has been scarcely reported indetail. This is partly because dystocia may rarely occuras a consequence of natural selection toward optimalbirth size (mass) for mother and offspring (Clutton-Brock 1991), and/or dystocia may be simply difficult todetect in the field. In fact, neonatal mortality in prong-horn Antilocapra americana was caused partly bydystocia (1.8%) but mostly by trauma (61.8%) due topredation (Dunbar et al. 1999). Also, dystocia causedmortality of adult females (<1%) in Swedish roe deerCapreolus capreolus, but the major causes were trau-matic injuries, starvation and pneumonia (Aguirre et al.1999). However, in animals whose growth and maturesize are restricted by environmental factors such as foodlimitation (e.g. Fowler 1987), FMD may occur fre-quently and be a cause of mortality. In this paper, weexamine the occurrence of dystocia in sika deer Cervusnippon under severe food limitation and searched theincidence of FMD.