A biased adult sex ratio (ASR) can influence cooperative breeding behavior if the bias limits mating opportunities for the more abundant sex. We tested predictions associated with the ASR-cooperation hypothesis in the brown-headed nuthatch (Sitta pusilla). We manipulated ASR by cross-fostering known-sex nestlings within 2 large (≥100 ha) experimental plots for 5 years using a crossover design where each plot received an opposing male- or female-biased treatment for 2 consecutive years. A year with no manipulations followed before the bias was reversed on each plot for 2 additional years. Variation in ASR (adult males/total adults) was pronounced compared to background proportions (0.55) and ranged from a female bias in female-biased plots (0.47) to a strong male bias in male-biased plots (0.71). Sex ratios during the postbreeding period ranged more broadly (0.33 in female-biased plots vs. 0.74 in male-biased plots). Territory densities did not change significantly and allowed 6 predictions to be assessed. Consistent with predictions, the prevalence of cooperative breeding groups doubled under male-biased treatments and large cooperative groups appeared (≥2 male helpers vs. the single male helper most common prior to the experiment). These changes occurred despite increased dispersal of cross-fostered males in male-biased plots. Most juvenile females dispersed, but, consistent with predictions, the prevalence of female helpers increased under female-biased treatments. Manipulations did not alter the sex of nestlings produced nor extend the time that males served as helpers. Taken collectively, results support the ASR-cooperation hypothesis and the role that mate limitations play in cooperative breeding behavior.

Interspecific aggression, similar to intergroup conspecific aggression, has been observed in a variety of taxa. The dominant group or individual is determined by multiple aggressive events and can be influenced by the size, age, or group size of the participating individuals. Interspecific aggression between Atlantic bottlenose (Tursiops truncatus) and spotted (Stenella frontalis) dolphins, both resident and sympatric to Little Bahama Bank, the Bahamas has been consistently observed for over two decades. However, it is unclear whether one species is more dominant and little is known about the factors that influence the progression of aggression. For this study, underwater video recordings of 32 aggressive encounters composed of 451 aggressive behavioural events were analysed over a 12-yr period (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004). These were used to describe the interspecific aggression observed and quantify which factors (the species and age class of the participants or the group size and behaviour of spotted dolphin groups) had the strongest impact on the progression and outcome of aggression. Over the long term, interspecific aggression was bidirectional with neither species being more dominant. During a single encounter, spotted dolphin group synchrony had the strongest impact on the dynamic of aggression, specifically impacting which group (1) initiated aggression, (2) the direction of aggression and (3) the occurrence of dynamic shifts or dominance reversals. This is the first study to quantify the dynamic of aggression for this population, to document bidirectional aggression and dynamic shifts during long-term interspecific aggression in free-ranging delphinids, and this study quantifies the role of synchrony during interspecific aggression using underwater observations.

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