Nov. 15, 1924 
Relation o f Sheep to Climate 
495 
was taken as a mean because the bulk of 
the lambs are born at that time (figs, la, 
lb, 2b, 3b). The rutting season was 
determined from the lambing season 
by counting back five months. The 
mean of the rutting season corresponds 
to the mean of the lambing season 
(figs, la, lb, 2c, 3c). 
The lambing and rutting seasons fall 
within comparatively narrow limits of 
temperature, rainfall, and humidity, 
and within narrower limits than does 
the composite graph for the whole year. 
The graphs made from sections which 
have less dense population than those 
from which the composite was made 
show some variations—the climate 
may be hotter in summer, colder in 
winter; it may have more or less rain¬ 
fall, a higher humidity, or a combina¬ 
tion of these factors. The lambing or 
rutting seasons may not fall within the 
limits of those in the composite. For 
example, a hythergraph of Cedar Lake, 
King County, Wash. (fig. 14), for 1919 
shows an excessive rainfall in the winter 
with 21J^j inches in January, and a dry 
summer with 0.5 inch rainfall in July. 
There were no sheep recorded in this 
county, as the rainfall is too great in 
winter and too little in summer. Yet 
the temperature conditions are favor¬ 
able, for the range is no greater than in 
some of the best sheep countries. 
CRITICAL PERIOD 
The periods in the life of a sheep 
during which it is more easily and more 
seriously affected by unfavorable con¬ 
ditions are the lambing and rutting 
seasons, which, as stated above, fall 
within narrower limits of temperature, 
rainfall, and humidity than the general 
conditions under which sheep live 
(figs, la, lb, 2b, 2c, 3b, 3c). The first 
few months of the life of a lamb and 
to a less degree the time of pregnancy 
are also critical periods (fig. 4). A 
successful lambing season is often in¬ 
terfered with by cold, wet, or snowy 
weather, or by a shortage of feed. The 
new-born lamb is not able to with¬ 
stand too rigorous climatic conditions. 
Likewise, the ewes, if exposed to exces¬ 
sive rainfall, or large amonts of snow, 
or if there is a shortage of feed, are 
liable to come to lambing time in un¬ 
thrifty condition and give birth to 
weak lambs. According to Hammond 
(8), if conditions are extremely un¬ 
favorable, atrophy of the fetus, due 
to undernutrition, may occur. (This 
is much less frequent in sheep than in 
swine.) 
The growth of the lamb during the 
first few months is readily retarded by 
hot, humid weather or by cold, wet 
weather. The extremely hot, humid 
weather also causes many lambs to 
lose weight (figs. 15 and 16). The 
late-born lambs are more severely 
affected than those born earlier. A 
breeder of purebred sheep in Illinois 
has found that clipping lambs in May 
has enabled them to withstand the 
summer conditions better. 
According to Hammond ( 8 ), the 
rutting season comes with a falling 
temperature. It may be delayed by 
hot weather, particularly by warm 
nights (4) • This becomes a serious 
problem in sections where the late 
summers and falls are hot, as it means 
that the ewes will not be bred until 
late in the fall, and the lambing season 
will be correspondingly late in the 
spring. Sections which have a warm 
fall usually have hot summers. In the 
West the ewes found unbred in the 
spring are gathered together and sent 
to the mountains, where a decreasing 
temperature is brought about by high 
elevation. Then they soon come in 
heat, and are bred for fall lambs. Con¬ 
ditions during the rutting season may 
have some effect upon the per cent of 
twins. According to Hammond (£), 
the number of ova produced by the 
ewes depends upon their condition at 
the rutting season. This is possibly 
the largest factor in the number of 
lambs born. (It is not true of swine, 
as many more ova are usually produced 
than can ever reach maturity.) 
In Tennessee, where climatic condi¬ 
tions are generally favorable for lamb¬ 
ing during the winter and early spring, 
the sheep men find it difficult to get their 
ewes bred early enough in the fall for 
early spring lambs. If they do not 
market the lambs before hot weather,, 
the lambs cease to make profitable 
gains. In Illinois, unfavorable weather 
conditions are often experienced at 
lambing time, but can usually be over¬ 
come by proper methods of housing 
(#0). Yet the lambs do not thrive 
quite so well during long spells of cold, 
rainy weather, even though adequately 
housed. When winters are extremely 
cold and the snow is deep, weak lambs 
and trouble at lambing time usually 
follow. The difficulty of getting the 
ewes bred early enough is quite fre¬ 
quent. The year 1915 was an ex¬ 
ception, as the summer was unusually 
cool (fig. 12). Ewes came in heat 
during all the summer months, and the 
first lambs came in November. The 
summer months are generally very 
trying on sheep, and the growth of 
lambs, particularly those born late in 
the spring, is retarded. In England it 
