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West African Crocodile Species

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Inadequate information remains one of the key stumbling blocks to effective wildlife management and conservation. This is especially true for the three West African crocodile species (Mecistops cataphractus, Crocodylus suchus and Osteolaemus tetraspis) which there is notable paucity of information on many aspects of their basic ecology and population status (Shirley, 2007). The situation is aggravated by the increasing anthropogenic related disturbances which have resulted in rapid population declines among all the three species.

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Crocodiles are recognised as keystone species in the aquatic ecosystem due to the irreplaceable roles they play (van der Ploeg et al., 2011; Ross, 1998). There is therefore the need to increase effort towards enhancing existing knowledge about these species as this will well position conservationists to respond to the increasing threats. In this study, an attempt has been made to expand existing knowledge on West Africa dwarf crocodile nesting ecology. Specifically, temperature dynamics in natural nests and dummy nest and the fecundity of West Africa dwarf crocodile as well as nest attendance and hatchling care were investigated. The results presented herein represents the first detailed quantitative data on the nesting ecology of the species in the natural setting.

The influence of temperature on crocodilian incubation is not new in the scientific arena with some of these studies dating back to over four decades ago (Deeming, 2004). It is an established fact that incubation temperature affects crocodiles in various ways including embryo sex determination, hatchling vigour, pigmentation pattern and many more (Pinal et al., 2003; Lang and Andrews, 1994; Webb et al., 1987; Ferguson, 1985; Ferguson and Joanen, 1982). This makes incubation temperature dynamics one of the critical aspect of crocodilian ecology to be investigated. Over the years, appreciable efforts have been made to breed crocodiles especially the threatened species in captivity most of which are for the purposes of reintroduction into the wild. Getting the true picture of temperature variations in the natural setting put scientists in better position to replicate it in laboratory for optimum outcomes. The result from this study indicates that temperature varies throughout the incubation period but this happens in only small quantum as there were small standard deviations for all the three nests (nest 3: 31.06 ±0.86; nest 4: 31.06 ±0.53; nest 2: 29.95 ±0.76°C). This finding provides strong background information for future captive breeding programmes on this threatened species. Furthermore, consistent with other studies on mound nesting crocodilians (Villamarín-jurado and Suárez, 2007; Waitkuwait, 1989), no correlation was detected between O. tetraspis incubation temperature variation and the ambient temperature (nest 2: r=0.09, p=1; nest 4: r=0.13, p=1) confirming earlier reports that mound nests temperature are mostly independent of ambient temperature. The dummy nest experiment did not prove good potential for serving as surrogate for O. tetraspis nest monitoring as temperature was found to be significantly and constantly lower than the nearby natural nest. However, caution needs to be exercised when invoking explanation for this outcome as the study fell short by lack of replicate for the dummy nest.

With regards to fecundity, O. tetraspis has long been reported as the least fecund among all the extant crocodilian species based on its small mean clutch size (Hara and Kikuchi, 1978; Greer 1975). This study found mean clutch size of 7.8±2.5 (range=5–13; n=6) which is consistent with what was reported by previous studies. For example Waitkuwait (1989) reported a mean clutch size of 10 ±4. Based on information in literature and the finding from this study, it can be said that African dwarf crocodile does indeed have the smallest clutch size among all crocodilians. The import of this is that dwarf crocodiles are at higher risk of extinction in situation of unsustainable exploitation. Bradshaw and McMahon (2008) stated that fecundity determines the number of new individuals that can be recruited into a population per given period. Therefore if small numbers are added per given time then the risk of extinction in event of overexploitation is high. It comes as no surprise that the species’ range is rapidly shrinking throughout West Africa where bushmeat is a delicacy. Like reported in other crocodilians studies (Platt et al, 2008; Thorbjarnarson, 1996), this work did not find any trade-off between clutch size and egg size when the confounding effect of female length was removed. Although the mean clutch size was small, the egg viability, nesting success and hatching rate were very high; a feature likely to balance the small clutch size of the species.

The level of commitment demonstrated by crocodilians towards nest attendance and protection is known to vary interspecifically and sometimes intraspecifically (Sicuro et al., 2013; Thorbjarnarson, 1989). While some nesting females spend most of their time around the nest vicinity to facilitate protection against predators, others show virtually no level of concern. The vulnerability of a nest to predation is largely influenced by nest defence and attendance. Species that dedicate more time towards nest protection mostly record low level of nest failures resulting from predation and vice versa. From this study, it appeared that West African dwarf crocodiles showed minimal care for nest as nesting females visited the nests at irregular intervals in the night and dawn and never during the daytime. The time of visits recorded during this study indicate that nests are at 100% risk of predation during the bright daytime since nesting female never visited during this period. One of the nests had all its eggs eaten by an unknown predator. It is highly suspected that these eggs might have been preyed upon by monitor lizard a notorious predator of crocodile eggs (Mazzotti et al., 2014; Somaweera et al., 2011) and the only potential predator confirmed so far on the site. It is not clear whether the consumption of the eggs took place within hours or couple of days. However, whatever the case may be it is likely that the nesting female was not around during the egg raiding.

Parental care in crocodilians is well reported in literature (Vergne et al., 2011; Vergne et al., 2009; Vergne and Mathevon, 2008; Senter, 2008; Herzog and Burghardt 1977; Campbell, 1973). Parental care has been found to decrease with age of hatchlings in some crocodilians (Chabert et al., 2015). The strong form of protection during the early days of hatchlings is believed to be a strategy of females to reduce the occurrence of predation. Although the study confirmed one form of parental which is transportation of hatchlings to water, it detected no response of nesting females to hatchling distress calls both during the day and in the night. The lack of response by the mothers to the distress calls of hatchlings could be explained in two folds. It is either West African dwarf crocodiles do not exhibit any form of aggression towards protecting of hatchlings from attack or their response depends on the species attacking. If the latter is the case, then our presence may have influenced the nesting female’s behaviour. However, if the former is true, then hatchlings will be more prone to predation and survival rate may be affected.

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