Maggot Generated Heat

Maggots are gregarious and tens of thousands may inhabit a carcass simultaneously. Their collective metabolism generates enough heat to raise their temperatures and that of parts of the carcass to 45-52°C in sheep, dogs, pigs and humans, even in cold weather (Anderson and VanLaerhoven 1996; Deonier 1940; Marchenko 1988; O'Flynn 1983; Payne 1965; Reed 1958; Richards and Goff 1997; Waterhouse DF 1947). This may be 25°C above the type of ambient air temperatures measured at weather stations, and even above the temperature of a standard black body in full sun (Fig. 7.4), and represents a significant measurement bias. Because the rate of development of ectothermic larvae is correlated with the microenvironmental temperatures they experience, the influence of maggot-generate heat may need careful consideration when estimating a PMI .

Several important points about the bias caused by maggot-generated heat are illustrated in Fig. 7.4. On the temporal scale of the life cycle, it becomes increasingly

Date

Shade Temperature -Black Bulb Temperature Cat Core Temperature

Fig. 7.4 Graph illustrating heat generated by maggots of Chrysomya chloropyga in the carcass of a ginger cat in Grahamstown, South Africa, that died on 29 September. The dotted line is the temperature in the core of the abdominal cavity; the upper continuous black line is the temperature of air inside a standard black body (a hollow copper sphere, 150 mm in diameter, spray-painted matt black) placed in full sun beside the carcass, 0.3 m above ground; and the lower continuous grey line is the air temperature in shade 1 m from the carcass, 1 m above ground. Temperatures were recorded every 30 min using a BAT 12 digital data logger and three calibrated copper/con-stantan thermocouples that were fixed in place. The heat generated by maggots in the abdomen warmed it 12°C above the temperature of the black body by Day 5, by 20°C on the night of Day 6; and 24°C warmer than shade temperature on Days 6 and 7. On Day 9 the carcass temperature reached 50°C. Cooling of the carcass lagged behind air temperatures by about 5 h, even on Days 1-4 when the maggots were small or not active around the thermocouple important as larvae grow (Hanski 1976), especially once the volume of aggregations of maggots exceeds 20 cm3 (Slone and Gruner 2007). It is also not consistent over the temporal scale of a day because solar radiation makes a contribution to elevating metabolism beyond the effect of maggot-generated heat alone. Solar heating may dissipate only slowly at night (Fig. 7.4) due to the thermal inertia of corpses. Nor is maggot-generated heat spatially uniformly distributed in a corpse; it is focussed where the maggots are currently feeding. To compound the problem, maggots ther-moregulate to some degree (see days 7 and 9 in Fig. 7.4) by moving around (and probably also by evaporative cooling using saliva and urine), and almost certainly do not spend their entire larval life in the hottest part of the carcass (Slone and Gruner 2007). In fact, maggots fare best at temperatures nearer 25 - 30°C (Hanski 1976; Richards et al. 2008), and may be physiologically compromised by living at higher temperatures for long periods (e.g. (Richards et al. 2008)), so they can be expected to be 'sensible' about thermoregulating. Chrysomya albiceps (Wiedemann)

Fig. 7.5 Segregation of larvae of Chrysomya albiceps (dark maggots on left; temperature 33.2°C) and C. marginalis (whitish maggots on right; temperature 40.8°C) on a warthog carcass. Similar differentiation has been reported between C. rufifacies and C. megacephala (Greenberg and Kunich 2002) (photograph by C.S. Richards)

Fig. 7.5 Segregation of larvae of Chrysomya albiceps (dark maggots on left; temperature 33.2°C) and C. marginalis (whitish maggots on right; temperature 40.8°C) on a warthog carcass. Similar differentiation has been reported between C. rufifacies and C. megacephala (Greenberg and Kunich 2002) (photograph by C.S. Richards)

characteristically inhabits the periphery of carrion, while Chrysomya marginalis (Wiedemann) inhabits the core (Fig. 7.5), apparently because the latter has a higher tolerance for maggot-generated heat (Richards et al. 2009a). In early instars and small clumps of more mature maggots, maggot-generated heat may be a trivial issue, and it seems unlikely to affect beetle larvae significantly, but its true prevalence needs refined quantification (Slone and Gruner 2007). Wandering larvae are not affected by this problem, and buried pupae may be at cooler and more stable temperatures than ambient conditions.

The magnitude of the biasing effect of maggot-generated heat is potentially large. In Australia, an excess temperature of about 27°C has been recorded in dead sheep (Deonier 1940; Waterhouse DF 1947), and this is not unusual (Fig. 7.4). If one was making a hypothetical estimate of PMImin using a routine thermal accumulation model (Higley and Haskell 2001), and assuming a (conceptually fictional) lower developmental threshold (D0, also called the base temperature, Tb) of about 10°C (a rough compromise between the values characteristic of Calliphora and Chrysomya: (Higley and Haskell 2001)), average ambient temperatures of 20°C (which is generously high for many areas) and an excess temperature of 20°C (i.e. a carcass temperature of only 40°C), the calculations would fall short by 720/240 h °C or 300% a day. This worst-case scenario is restricted to the later part of the larval phase, and no consideration has been given to thermoregulation; (Marchenko 1988) has suggested that in practice the estimate might fall short of the true development time by 100%.

Ideally, the temperatures of maggot aggregations should be measured before the corpse is removed from the site of discovery and compared to ambient and black-body temperatures to account for maggot-generated heat and solar radiation in an investigation. The temperature of a maggot mass is a biased estimate of maggot-generated heat because is includes solar radiation (Fig. 7.4). One may then design a correction to retrospective weather station data (see Fig. 7.4), or assume that the maximum temperature routinely reached 45°C (cf. Anderson and VanLaerhoven 1996; Deonier 1940; Marchenko 1988; O'Flynn 1983; Payne 1965; Reed 1958; Richards and Goff 1997; Waterhouse DF 1947). However, while the maggots found near the warmest parts of the corpse may have grown fastest, they may also be stunted by competition and thermal stress, which makes their size harder to interpret. However, thermoregulation (by migration or evaporative cooling) may be practically impossible to account for. Obvious ways around this are to sample larvae from smaller aggregations that generate less heat, and to focus on species like C. albiceps (Fig. 7.4) that inhabit thermal niches on the corpse that are more similar to ambient conditions. One might also use the development of species that inhabit different thermal niches to cross-validate estimates. This emphasises the value of the recommended standard procedure of recording the location of samples on a corpse (Amendt et al. 2007).

Even in thermostat-regulated incubators, maggots may not be at the set temperature (Nabity et al. 2007) because of maggot-generated heat. It is recommended that maggots' temperatures should be measured directly even in such controlled environments to improve the measurement precision of models.

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