Trigger of twin-fights in captive common marmosets

Prior research has shown that common marmosets typically produce twins that live within cohesive family units. These groups consist of parents and several sets of offspring born at different times. Adolescence often marks the period when these siblings begin to exhibit aggressive behaviors toward one another. Scientists previously hypothesized that internal biological shifts, such as the onset of puberty, drove these early conflicts. However, the influence of external social pressures remained poorly understood due to overlapping developmental milestones. No prior work had resolved whether social changes or physiological maturation served as the primary catalyst for these interactions. This uncertainty drove the need for a long-term observational analysis of captive colonies. Investigating these factors requires separating events that usually occur in tandem during the life cycle. This gap motivated the current study to clarify the specific triggers of these adolescent twin-fights.

The study aimed to determine the proximate causes that trigger twin-fights in captive common marmosets. Researchers sought to resolve whether these conflicts depend primarily on internal events or external social changes. Previous hypotheses suggested that the onset of puberty was the main driver for such aggressive behavior. However, the researchers questioned if the birth of younger siblings and group behavioral shifts were more influential. The team recognized that these developmental and social events usually coincide in natural settings. To address this, they employed birth control methods to separate the timing of these occurrences. This experimental approach allowed for a clearer examination of the factors influencing adolescent conflict. The investigation focused on identifying the predominant trigger for these interactions within a captive colony.

The researchers conducted a retrospective analysis of behavioral records collected over twelve years. This review approach focused on identifying specific triggers for agonistic interactions between twin pairs. The team compared groups that experienced natural birth cycles against those subjected to birth control. Prostaglandin administration served as the primary method for manipulating ovulation and interbirth-intervals. This technique successfully decoupled the timing of puberty from the arrival of new infants. The investigators recorded the exact day of conflict onset for each observed twin pair. They also calculated the frequency of these fights across different social conditions. Statistical comparisons determined whether delayed births influenced the timing of the first aggressive encounters.

The strongest finding indicates that external social events serve as the primary triggers for adolescent twin-fights. The researchers observed that the timing of conflict onset was significantly delayed when the birth of younger siblings was postponed. This delay occurred even as the twins continued to mature physiologically under the birth-controlled condition. The data revealed that these fights are driven by a combination of internal and external factors. External social changes remain the predominant influence on the initiation of these aggressive behaviors. Higher rates of conflict were consistently recorded between same-sex twins compared to mixed-sex pairs. These results align with previous observations regarding directed aggression in callitrichine primates. The study confirms that social environmental shifts are essential for triggering the observed adolescent behavioral changes.

The authors conclude that twin-fights result from a complex interplay between internal maturation and external social shifts. External events act as the predominant drivers for the initiation of these aggressive encounters. The researchers propose that the arrival of younger siblings significantly alters the group environment. This change in social structure likely forces the older twins to compete or adjust their status. The timing of conflict onset shifts when the birth of new siblings is experimentally delayed. These findings suggest that developmental maturity creates a vulnerability to social triggers. The team notes that same-sex twins exhibit higher rates of aggression than mixed-sex pairs. This pattern reflects established behavioral traits observed across various callitrichine species.