Säde, S. 2004: Twinning rate and foetal sex ratio of moose Alces alces in Estonia during low-density and increasing population phases in relation to maternal age. - Wildl. Biol. 10: 295-300.
My study was carried out to evaluate hypotheses about the effects of maternal age and population growth rate on twinning rate and offspring sex ratio in moose Alces alces. Reproductive tracts from 128 female moose of known age and with embryos present were collected across Estonia during the hunting seasons (1 October-15 November) of 1993-1999. Maternal age was treated as an indicator of condition, with younger mothers assumed to be in poorer condition than older mothers. The sex of embryos was determined using cytological methods. As indexed by sex ratio, there was a clear increase in maternal investment as maternal age increased. The sex ratio in younger mothers was nearly one male to three females, whereas in older mothers it was nearly equal. If the age of the mother is a good index of condition, this can be explained by the Trivers-Willard theory of condition-dependant sex ratios. Yet, because the Trivers-Willard theory predicts the sex ratio only of an individual, older females might compensate for the excess of female offspring among subadults according to Fischer’s equal allocation theory. Population phases were distinguished using data on fecundity. In the increase phase there was strong agreement with the Trivers-Willard theory, but there was no clear agreement in the low-density phase. This was probably a result of the greater participation of young females in reproduction during the increase phase. Foetal sex ratio did not differ between different population growth phases.
Key words: Alces alces, moose, population dynamics, reproduction, sex ratio, twinning rate
Sander Säde, Institute of Zoology and Hydrobiology, University of Tartu, 46 Vanemuise Street, EE-51014 Tartu, Estonia - e-mail: sanders@ut.ee
Received 25 October 2002, accepted 2 April 2004
Associate Editor: John P. Ball