160 
Table 4 
Estimates of periods of possible insect harassment 
of caribou calculated from Prudhoe Bay weather data. 
Number of days when 
Cumulative duration of 
harassment was 
Period nil mild 
1970 
June 21 4 
July 6 14 
August 21 6 
Summer Season 1970 48 24 
1971 
June 15 2 
July 8 13 
August 18 8 
Summer Season 1971 41 33 
1972 
June 23 7 
July 11 10 
August 4 8 
Summer Season 1972 38 25 
1973 
July 11 17 
August 5 7 
Summer Season 1973 16 24 
insect harassment (hr) 
severe mild severe 
0 - 
10 130(17) 12(2) 
0 48(17) 0(0) 
9 91(12) 101(14) 
2 53(18) 13( 4) 
11 
The index of insect harassment was estimated from temperature and wind speed records from 
the Prudhoe Bay weather station according to Fig. 4. Wind speed at 1.5 m from the ground (G, km 
hr! ) was used in the estimation and was calculated from the weather station wind speed records 
(W, km hr!) viz: G= 1.0 + 0.5 W. 
Value in parentheses is duration of harassment as a percentage of the total period (i.e., 744 hr 
in July, 288 hr in August). 
over nearly the whole study period from 25 
June to 10 August, whereas warble flies were 
noted in suitable weather between 23 July and 
15 August. 
(b) Effects of insect harassment on group 
size, composition, activity and movement. On 
cool, non-insect days in June, July, and August, 
caribou were typically segregated into female- 
yearling-calf groups and male groups (Table 5). 
These groups noted under insect-free conditions 
had a mean size of 22 individuals, with only four 
groups (3%) of over 100 (Table 6). However, 
under insect harassment, groups readily coa- 
lesced, resulting in both a significantly higher 
proportion of mixed groups (Table 5) and an 
increase in group size. The 92 insect-harassed 
Table 5 
Effect of insect harassment on segregation of caribou 
into female-yearling-calf and male groups. 
Group Structure 
Segregated Integrated 
No insects 59 Nez, 
Mild and severe 39 26 
harassment 
Chi-square test shows these differences be- 
tween level of harassment were significant (SZ 
= 5.18 at 1 d.f. P<0.05). 
Values shown are the number of groups in 
each category. 
