Offspring sex ratio in relation to body reserves in red deer

FLUCK, WT

Lemesos, Cyprus

Congreso; 25th Congress of the International Union of Game Biologists; 2001

Ministry of Interior

Red deer (Cervus elaphus) introduced to Patagonia have reached high densities in forest-steppe ecotones and have been monitored since 1992. Severe drought conditions prevailed during spring to autumn of 1998/99, followed by a spring-summer (2000) with above average moisture and temperatures, resulting in high primary productivity. At the same time and due to the previous drought, there were less deer and fewer females with calves, resulting in the highest recorded level of physical condition of females. I compared offspring sex ratios of red deer during an average period (1992-98), a drought year (1999), and an above-average year (2000).Adult females collected and examined during winter 1999 (n = 50) revealed a pregnancy rateof 56% as compared to 98% in 1992-98 (n = 88), or 100% in 2000 (n = 70). Body fat reserves in 1999 were severely depleted in most animals, irrespective of reproductive status; there were no signs of resorptions or abortions. The fetal sex ratio (n = 69) was 46% males during 1992-98, 71% males after the drought, and 39% males in 2000. Primiparous 2-year old females had 90% male and 100% female fetuses in 1999 and 2000, respectively. Mothers with female or male fetuses averaged 6,7 and 5,2 years, respectively (P = 0.14). The male-biased and female-biased offspring ratios in 1999 and 2000 contrast other reports that only females in best condition tend to have male-biased offspring. The temporal proximity of average years, a drought followed by an above-average year shows that changes in offspring sex ratios were immediate. In particular, due to the imminent chronology of events, the observed changes were unlikely related to changes within the female population, such as changes in age structure or dominance relationships. Rather, a conclusive relationship to energy reserves during breeding emerged as the underlying principle affecting the offspring sex ratios.