1. Transgenerational plasticity (TGP) occurs when the environment encountered by one generation (F0) alters the phenotypes of one or more future generations (e.g. F1 and F2). Sex selective TGP, via specific lineages or to only male or female descendants, has been underexplored in natural systems, and may be adaptive if it allows past generations to fine-tune the phenotypes of future generations in response to sex-specific life history strategies. 

2. We sought to understand if exposing males to predation risk can influence grandoffspring via sperm in threespined stickleback (Gasterosteus aculeatus). We specifically tested the hypothesis that grandparental effects are transmitted in a sex-specific way down the male lineage, from paternal grandfathers to F2 males.  

3. We reared F1 offspring of unexposed and predator-exposed F0 males under ‘control’ conditions and used them to generate F2s with control grandfathers, a predator-exposed maternal grandfather (i.e., predator-exposed F0 males to F1 daughters to F2 offspring), a predator-exposed paternal grandfather (i.e., predator-exposed F0 males to F1 sons to F2 offspring), or two predator-exposed grandfathers. We then assayed male and female F2s for a variety of traits related to antipredator defense. 

4. We found little evidence that transgenerational effects were mediated to only male descendants via the paternal lineage. Instead, grandpaternal effects depended on lineage and were mediated largely across sexes, from F1 males to F2 females and from F1 females to F2 males. When their paternal grandfather was exposed to predation risk, female F2s were heavier and showed a reduced change in behavior in response to a simulated predator attack relative to offspring of control, unexposed grandparents. In contrast, male F2s showed reduced antipredator behavior when their maternal grandfather was exposed to predation risk. However, these patterns were only evident when one grandfather, but not both grandfathers, was exposed to predation risk, suggesting the potential for non-additive interactions across lineages.

5. If sex-specific and lineage effects are common, then grandparental effects are likely underestimated in the literature. These results draw attention to the importance of sex-selective inheritance of environmental effects and raise new questions about the proximate and ultimate causes of selective transmission across generations.

All behavioral and morphological data were collected live on paper and transcribed electronically. Cortisol data were calculated from OD scores.