Sex ratio allocation supports the Trivers-Willard hypothesis in Cervinae

Adaptive secondary sex ratio allocation represents one of the most studied topics of behavioural ecology. Among many theories explaining this phenomenon, two are the most influential: Trivers-Willard hypothesis (TWH) and Local Resource Competition Model (LRCM). Despite many studies examining this topic, only a few of them focused on more than a single species. As natural history and social organisation among various species differ, different species might adapt different strategies in sex ratio adjustment. Therefore, we tested these hypotheses using data in five deer species born in six zoos - Père David's deer (Elaphurus davidianus, N = 377), elk (Cervus canadensis, N = 252), sika deer (Cervus nippon, N = 167), chital (Axis axis, N = 188) and reindeer (Rangifer tarandus, N = 371). We found that the overall secondary sex ratio did not differ from 1:1. In Cervinae, we found that the secondary sex ratio increased with increasing age of the mother corresponding with the TWH. On the other hand, in reindeer, we revealed the opposite results in line with LRCM. In addition, we found that in Père David's deer, and Vietnamese sika deer males were born earlier during the peak of the birth season than females. This result lends support to TWH as males born earlier could receive higher investment from mothers than those born later and this might affect their reproductive success in the future. Our study represents one of a few including several closely related mammalian species on this topic.