Reproductive trade-offs in the burying beetle Nicrophorus marginatus: does parental competitive environment influence offspring sex ratio or brood size dynamics?

Burying beetles (Silphidae: Nicrophorus) are known to modulate brood size through selective infanticide. The reproductive tradeoff hypothesis of life history theory predicts that when parental adults experience austere conditions (low food encounter rates, high competition; C), lifetime reproductive success (LRS) is increased by producing fewer but larger offspring, with a female sex ratio bias. Conversely, parents experiencing favorable conditions are thought to increase LRS by producing a brood of smaller offspring with a male sex ratio bias. The reproductive strategies of these Silver Spoon (SS) parents are thought to be driven by the gamble that offspring will favor similar low competition conditions. In seeking to understand reproductive strategies in Nicrophorus marginatus, a species exhibiting hypervariable size variation in wild populations, we investigated the following questions: 1) is variation in size explained by heritability, 2) does the social environment of parental burying beetles influence the average brood size? 3) does the social environment of parental burying beetles influence the average individual size of brood members? 4) does the social environment of parental burying beetles influence the sex ratio of broods? We found low heritability for size in the species, and in breeding experiments in which we experimentally manipulated parental social environment we found that SS (female) x SS (male) and SS x C pairings produced larger broods of smaller offspring. Our findings support the hypothesis that the large size variation observed within N. marginatus populations is likely a result of reproductive decisions influenced by stochastic competitive environments.