Seasonal reproduction in the mountain hare : ecological and physiological constraints
During a two-year field study, the ecology and reproduction of mountain hares was investigated to determine which factors are important determinants of the season of production of young. Two approaches were adopted. 1) A live-trapped, marked population of hares was maintained to provide information on the seasonal appearance, growth and survival of leverets. 2) Throughout December 1983 to March 1985 a sample of carcases was obtained in each month to provide information on reproductive performance. Blood samples were obtained from carcases and from the live population to provide information on seasonal changes in hormone levels. A rapid method of ageing mountain hare carcases was developed; this involved polishing mandibular sections by a geological method and counting periosteal growth lines.The photoperiodic control of seasonal reproduction acting through the pineal organ and its secretion of melatonin, was confirmed for the mountain hare. A radio-immunoassay for this hormone has been validated for mountain hares and samples from wild animals indicate that there is a marked circadian pattern of plasma melatonin concentration. This pattern varied with season in a manner consistent with changes in photoperiod and was confirmed by two sets of serial blood samples from captive animals under two widely different natural photoperiods.In 1984 females produced up to 3 litters. Larger females produced more offspring in total because they were successful at the first as well as the second and third litters. Early production of offspring resulted in diminished production in the final litter of the year. Small females were less likely to produce early offspring. Females in their first breeding year suffered heavier pre-natal losses of ova or embryos, especially during the first litter period. Male leverets, especially those born late in the breeding season, grew at faster rates than females. Despite this faster growth rate they reached lower asympototic skeletal sizes and body weights by ceasing growth sooner. Current theory suggests that fast growth rate and longer duration of growth produce increased juvenile mortality. There is no clear sex bias in mortality in mountain hares and it is suggested that the hypothesised mortality-producing agents cancel each other, the former acting on males and the latter on females. Late-born young reached smalled asymptotic body sizes than those born earlier and they were more likely to die after weaning. It is of considerable advantage for mountain hares to successfully breed early rather than late. Early born young are of higher quality than late-born; they reach greater body sizes which are associated with greater post-weaning survival and in females a greater reproductive success.Chemical and botanical analyses of stomach contents showed a clear seasonal switch in utilisation of the two major food types; heather (Calluna vulgaris) and grasses. On average most grasses were ingested in the summer months of April-August but some animals continued to eat them until October. Changes in plant selection could not be explained by selection for protein or against acid detergent fibre. The seasonal pattern of plant selection is commensurate with avoidance of phenolic secondary plant compounds present in heather particularly during the summer. Reproductively active females and to a lesser extent growing leverets, avoided ingesting plants containing these compounds. The season of production of young may be restricted by the availability of forage with low levels of secondary compounds.