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ABSTRACTIn-air sounds of pinnipeds are important for interactions with conspecifics and threat displays. However, little is known about the in-air acoustic repertoire and associated behaviour of crabeater seals Lobodon carcinophaga. We investigated the in-air acoustic repertoire and associated behaviour of two male and one female wild juvenile crabeater seals that beached separately, and were rehabilitated in Durban and Gqeberha, South Africa. In-air sounds were visually identified and categorised into five classes validated via random forest model classification: brief, intermediate, and long moan calls, croaks, and hisses. Hisses were common (n = 25,105 sounds from 136 hrs of acoustic recordings) and detected during heightened arousal states and interactions with animal care staff and a conspecific. Furthermore, hisses were also recorded in low arousal states during exhalation. Moan calls (n = 241) were only detected from two of the three seals. During rehabilitation, the female seal ceased producing moan calls and later produced a series of croaks (n = 204). Acoustic characteristics of in-air moan calls differed from published underwater moan calls according to duration, minimum, and maximum frequencies, and pulse repetition rate. Our study shows that the in-air acoustic repertoire and associated behaviour of these Antarctic seals are dynamic, vary inter-individually and are context dependent.

The Cape gannet, Morus capensis, has long been considered a monomorphic species despite attempts at phenotypically separating the sexes. In a further effort we here recorded morphometric measurements from genetically sexed Cape gannets. Discriminant function analysis was used to classify sexes according to these morphometric measurements. Culmen and gular stripe lengths were selected as the best discriminators, correctly classifying 65% of birds. Male Cape gannets had significantly longer culmens and gular stripes than females. We thereby provide the first evidence suggesting that the Cape gannet demonstrates some level of dimorphism. Nonetheless, morphometric criteria used to separate sexes holds limited value due to overlap in measurements between sexes.

The Orange River is one of the longest rivers in southern Africa and one of the most regulated rivers in the world. Its estuary supports diverse marine and freshwater life, yet, ecosystem structure and functioning within the estuary are poorly understood. This study investigated trophic linkages and interactions among dominant fishes through carbon (δ13C) and nitrogen (δ15N) stable isotope (SI) ratios along a salinity gradient. The five dominant species utilizing the Orange River Estuary Continuum (OREC) included marine Chelon richardsonii, estuarine Gilchristella aestuaria, and freshwater species Labeobarbus aeneus, Pseudocrenilabrus philanderand Mesobola brevianalis. Variations in SI ratios were associated with changes in body size, seasonality and salinity. Variability in seasons and the salinity continuum influenced SI ratios. Chelon richardsonii, P. philanderand L. aeneusoccupied the widest isotopic niche in the lower OREC, an indication of generalist feeding behaviour in these species. Mesobola brevianalishad the narrowest isotopic niche, perhaps suggesting more specialized feeding on zooplankton. Isotopic similarity was observed among the five co-occurring species, suggesting trophic overlap, particularly during the high-flow season when environmental variables, productivity and food availability are predominantly freshwater driven and uniform throughout the river estuary continuum. This study is the first to utilize δ13C and δ15N analyses to assess trophic dynamics of dominant species co-occurring in the estuary and estuarine-freshwater ecotone of the lower OREC.