Highlights d Wasps learn about new individuals by observation alone d Wasps watched fights, then interacted with the same or a different fighter d Wasps were less aggressive to individuals they observed fight well

In many cooperatively breeding animals, subordinate group members have lower reproductive capacity than dominant group members. Theory suggests subordinates may downregulate their reproductive capacity because dominants punish subordinates who maintain high fertility. However, there is little direct experimental evidence that dominants cause physiological suppression in subordinates. Here, we experimentally test how social interactions influence subordinate reproductive hormones in paper wasps. queens commonly found nests in cooperative groups where the dominant queen is more fertile than the subordinate queen. In this study, we randomly assigned wasps to cooperative groups, assessed dominance behaviour during group formation, then measured levels of juvenile hormone (JH), a hormone that mediates fertility. Within three hours, lowest ranking subordinates had less JH than dominants or solitary controls, indicating that group formation caused rapid JH reduction in low-ranking subordinates. In a second experiment, we measured the behavioural consequences of experimentally increasing subordinate JH. Subordinates with high JH-titres received significantly more aggression than control subordinates or subordinates from groups where the dominant's JH was increased. These results suggest that dominants aggressively punished subordinates who attempted to maintain high fertility. Low-ranked subordinates may rapidly downregulate reproductive capacity to reduce costly social interactions with dominants. Rapid modulation of subordinate reproductive physiology may be an important adaptation to facilitate the formation of stable, cooperative groups.

ABSTRAcrNinety-six Romney ewe hoggets from a group of 236 hoggets of known history were treated with intra-vaginal sponges containing 20 mg or 60 mg methyl-acetoxy-progesterone (MAP) for 14 days near the onset of the breeding season. Observations were made for oestrus in all hoggets and for ovulations in a sample of animals from each group. After treatment the amount of MAP "absorbed" from each sponge was chemically determined.The effects of sire, birth rank of hoggets, dose level of MAP, and occurrence of oestrus on the amount of MAP "absorbed" were estimated. The incidence of oestrus within seven days of sponge withdrawal was 62%, and its occurrence was similar for both dose levels of MAP. The numbers of ovulations in treated hoggets were normal.

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