Assessing rake mark ontogeny to investigate social aggression in common dolphins (Delphinus delphis) in Aotearoa New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University, Albany, New Zealand
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2024
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Massey University
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Aggressive competition can impact resource allocation, social structure, and reproductive success in social species. Rake marks, semi-permanent parallel scars from teeth raking across the dermis of another individual, indicate delphinid social aggression. When examined alongside ontogeny, demographic traits, and position, rake marks can inform social dynamics, and potentially health, in delphinids. The majority of existing rake mark studies have been field based and often lack sex, age, total body length (TBL), body condition and reproductive context. Here, I analysed rake marks on common dolphins (Delphinus delphis) examined postmortem between 2006-2024 (n = 102) in Aotearoa New Zealand. To account for skin loss and surface area variation, a zero-inflated generalised linear mixed model assessed rake mark occurrence (probability of rake density > 0) and prevalence (rake density, rm/dmĀ² ) across the body (n = 89; dorsal/ventral, cranial/caudal, left/right), inferring potential confrontation, avoidance, and lateralisation. Zero-inflated generalised linear model was used to assess rake mark occurrence and prevalence with sex, age, TBL, sexual maturity, and body condition (n = 89), as well as female reproductive status (n = 51). Females demonstrated higher rake mark prevalence, but lower occurrence than males, possibly due to infrequent but severe aggressive sexual coercion. Males may engage in frequent intrasexual competition for mates. Resting mature females exhibited higher rake mark prevalence and occurrence than immature and pregnant and/or lactating females, likely attributed to reproductive availability and heightened sexual coercion. Rake mark occurrence and prevalence increased with greater TBL, possibly due to the increased surface area available to receive rake marks. Larger male body size, with a pronounced post-anal hump, may signal dominance, resulting in frequent severe aggression. Sexual maturity, age, and body condition displayed no relationship with rake occurrence or prevalence. However, biotic and abiotic factors may influence dolphin skin healing and rake mark longevity, which potentially inhibited the detection of existing relationships. These results provide first insights to how ontogeny may influence rake mark scarring in common dolphins, and the inferred context of the interactions that led to these rake marks.