Íslenskar landbúnaðarrannsóknir - 01.09.1981, Page 61

SEASONAL VARIATION IN TESTIS SIZE 59
luenced by ambient temperature fluct-
uations under Icelandic conditions, it se-
ems most plausible to consider daylight
relationships. Certainly, the effect of
photoperiod on the secretion of repr-
oductive hormones and on testis size of
rams has been well established (Ort-
avant, 1977; Sanford, Beaton, How-
land and Palmer, 1978; Schanbacher
and Ford, 1979). To quote an example,
Lunstra and Schanbacher (1976) rep-
orted the greatest testicular diameter in
October and the least in May in mature
Finn and Suffolk rams in Nebraska,
U.S.A., a pattern resembling that found in
the present study.
In considering further the seasonal
changes in testes diameter in relation to
daylength, there is evidence (FIGURE 1)
of the diameter starting to decrease before
the shortest day in late December and
increasing before the longest day in late
June. This ílnding appears to be in line
with the statement of Ortavant (1977)
that testis stimulation in rams occurs
ÍSLENSKT YFIRLIT
Arstíbabundinn breytileiki á eistnastœrð ís-
lenskra hrúta.
ÓlafurR. Dýrmundsson
Búnabarfélagi Islands,
Bmdahöllinni, Reykjavík,
Pétur Sigtryggsson
og
Stefán Sch. Thorsteinsson
Rannsóknaslofnun landbúnaðarins
Keldnaholti, Reykjavík.
Ritgerðin greinir frá niðurstöðum fyrstu
hérlendra rannsókna á samhenginu á milli
eistnastærðar fullorðinna hrúta og árstíða.
during long days and regression during
short days. Moreover, Lees (1969) has
demonstrated how scrotal measurements
ofadult rams in Aberystwyth, Wales (52°
26’N) began to increase before the longest
day and decrease before the shortest day.
His conclusion that scrotal size was
greatest some three months after the long-
est day and smallest some three months
after the shortest day may in fact be rec-
onciled with the pattern reported here for
Icelandic rams. However, the consider-
ably greater fluctuations in daylight
throughout the year due to the high lat-
itude of Iceland should be borne in mind
and certainly more detailed studies would
be needed to substantiate the above point.
ACKNOVVLEDGEMENTS
The authors thank Drs. Sean Quirke of the Agric-
ultural Institute, Ballinrobe, Ireland; John Conn-
olly of the Agricultural Institute, Statistics Dep-
artment, Dublin, Ireland and Jón Viðarjónmunds-
son of the Agricultuiral Society of Iceland for va-
luable assistance with the statistical analysis.
Þvermálsmælingar á eistum lífhrúta
reyndust hafa nána fylgni við eistnaþunga
þeirra eftir slátrun, en breytileiki á eistna-
þvermálinu var að mestu óháður þunga og
aldri þessara fullorðnu hrúta. Mælingar á
þvermáli eistna gefa því all nákvæma vís-
bendingu um stærð þeirra.
Niðurstöður þvermálsmælinga eistna
íjórum sinnum á ári, að hausti, vetri, vori
og sumri um tveggja ára skeið, sýndu
svipaðar árstíðabundnar breytingar bæði
árin. Haustmálin voru stærst, næst komu
vetrarmálin, þá sumarmálin, en þvermál
eistna var minnst á vorin. Samkvæmt