Populations of Saltwater Crocodiles (Crocodylus porosus) have been monitored in the Northern Territory since the species' protection. This monitoring relies on standardized spotlight surveys in which crocodiles are counted and classified by estimated total length (TL). Although the accurate estimation of TL is important for analyses, often crocodiles are submerged and their TL is estimated from their heads, which may be visible above the water surface. With some variation, it is generally thought that TL of a crocodile is seven times the length of the animal's head. This study examined the ratio of TL to head length (HL) from morphological measurements of 2,755 Saltwater Crocodiles caught in various locations in the Northern Territory. The results showed that the generic ratio for crocodiles ranging from 38 to 503 cm was 7.01. The ratio increased logistically from 6.7 to 7.1 for crocodiles in the 120–420-cm size range in TL in 30-cm intervals. We suggest that TL can be reliably estimated from HL at least for crocodiles in 120–420 cm in the wild, with the use of the generic and size-specific ratios calculated for the various size classes at every 30 cm. The size-specific estimation for animals <120 cm or >420 cm was not possible because of limited sample sizes in these crocodile size ranges, although a few reliable records suggest that a ratio increasingly becomes larger and 1:8 should be used for crocodiles >510 cm in TL.

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