LIFE HISTORIES 



We here choose four species to show Hfe- 

 history adjustments to the habitat and community: 

 two mammals for the grassland, a bird for the forest- 

 edge, and a millipede for the forest. 



Voles (Microtus, Pedomys) 



The meadow vole, M. pcnnsylvanicus. and 

 prairie vole, P. ochrogaster, are small, dark gray or 

 brownish, compactly built mice with short legs and 

 tail, small eyes, and partly hidden ears. They spend 

 most of their time in an elaborate system of tunnels, 

 partly underground and partly as almost hidden gal- 

 leries in dense grass. The food of these species con- 

 sists mostly of grasses but also includes legumes, 

 composites, fruits, and occasionally insects. Grass 

 stems grow close together. The voles thrust them 

 aside to form paths on the ground surface. These 

 runways are heavily trafficked networks, and feeding 

 is done in them. Runways formed through clover or 

 alfalfa are less permanent. Underground passages 

 lead to nests and chambers where food is stored. 

 Nest cavities are round, lie 7 to 45 cm below the sur- 

 face, and often have two tunnels leading up to the 

 surface. The cavities are lined with dried grass and 

 leaves (Jameson 1947, Martin 1956). 



The voles may be active at any hour of day or 

 night throughout the year, but the periods of greatest 

 activity come in early morning and evening. The 

 mean monthly home range of an individual is very 

 small, commonly about 364 m^ (.09 acre), although 

 males wander somewhat farther than do females. 

 There is no defense of territory. Because of their 

 small home ranges and high rates of reproduction, 

 vole populations in years of abundance may reach 366 

 or more per hectare (146 per acre). The level of 

 population fluctuates through the years, however, in 

 response to rainfall and competition, and there is 

 some evidence of a four year cycle. Populations regu- 

 larly decline each winter and increase to an annual 

 peak in the autumn. Predators on the two voles are 

 numerous and varied, and include hawks, owls, crows, 

 weasels, foxes, coyotes, badgers, and snakes (Hamil- 

 ton 1937, 1940, Martin 1956). 



During the peak of a population cycle, breeding 

 may continue throughout the year, but it is ordinarily 

 curtailed during the winter and periods of summer 

 drought. The number of young in a litter increases 

 with the age of the female but commonly varies be- 

 tween three and five. The duration of the oestrus 

 cycle in the female, if such a cycle actually occurs, is 

 not known, but is likely only a few days long. Voles 

 are promiscuous, and the female may accept a male 



within a few minutes or hours after the birth of her 

 young and be capable of ovulating and conceiving a 

 new litter within five or six days. The gestation 

 period is short (about 21 days) and it is estimated 

 that 8 to 10 Microtus litters may be produced in a 

 favorable year (Hamilton 1941). P. ochrogaster is 

 less prolific. 



The young are born pink-skinned, hairless, blind, 

 and with ear pinnae closed. They soon attach them- 

 selves to the teats of the mother, who may even drag 

 them along as she forages. They weigh two or three 

 grams at first but grow rapidly, gaining one-half to 

 one gram per day. Meadow voles at sexual maturity 

 weigh 25 to 30 grams ; when fully adult, 40 to 50 

 grams. The backs of the young voles are covered 

 with soft velvety hair on the fourth or fifth day after 

 birth, the incisors erupt on the si.xth or seventh day, 

 and the eyes open and the pinnae unfold on the 

 eighth or ninth day. With their eyes open they be- 

 come more active and may take short trips away from 

 the nest to nibble on succulent vegetation. The young 

 voles are weaned at two to three weeks, but may re- 

 main with their mother for several days longer. 

 Males may become se.xually mature when five weeks 

 old, and females may mate successfully when only 

 four weeks old. The mortality rate in young mice is 

 high, but the normal life span of adult meadow voles 

 under natural conditions is 10 to 16 months. Prairie 

 voles appear not to live as long (Hamilton 1941, 

 Fitch 1957). 



Bobuhite (Colinus virginianus) 



The bobwhite is found over the eastern part of 

 the ITnited States and south through Central America 

 (Stoddard 1931). It occupies open woodlands, 

 shrubby fields, fence rows, and forest-edges border- 

 ing on grassy fields or farmlands. Nests are usually 

 located within 16 meters of roads, paths, or culti- 

 vated fields. This vegetation serves as cover against 

 both weather and enemies. The bird feeds primarily 

 on seeds, occasionally on fruit and herbage, and, dur- 

 ing the summer and autumn, on insects. It also in- 

 gests a considerable quantity of mineral matter in 

 the form of grit. The young chicks feed largely on 

 insects the first three weeks after hatching, but then 

 become gramnivorous like the adults. With ample 

 cover and food, the species may reach maximum pop- 

 ulation of 2.5 per hectare (1 per acre), but popula- 

 tions of one bird to 2-5 hectares are more common. 

 In the South, populations are fairly stable year after 

 year, but in the North they fluctuate widely (Ken- 

 deigh 1944). 



The birds have a number of call notes for com- 

 munication between individuals of a pair or covey. 

 These notes serve to attract mates, express alarm or 



142 Habitats, communities, 



succession 



