Interspecific Competition

Maggots predigest their food with secretions (Mackerras and Freney 1933), and this process is more effective when more maggots are involved (dos Reis et al. 1999). Maggots grow faster in groups, probably due to maggot-generated heat and improved pre-digestion, which explains their gregariousness. However, escalating numbers lead to intraspecific competition, increased mortality, earlier emigration and pupariation and smaller-bodied adults (dos Reis et al. 1999; Ireland and Turner 2005; Saunders and Bee 1995; Shiao and Yeh 2008; Smith and Wall 1997). Interspecific competition has similar effects (dos Reis et al. 1999; Shiao and Yeh 2008; Smith and Wall 1997). These conditions lead to the anomaly that the PMI

' J min will be underestimated if based on the size of the dwarfed larvae, but overestimated if based on the onset of emigration.

Competition, especially amongst conspecifics, is pervasive. It sets in at low densities and increases fairly linearly with increasing maggot densities (dos Reis et al. 1999; Ireland and Turner 2005; Shiao and Yeh 2008). Because the assimilation efficiency of maggot may vary between tissues (Ireland and Turner 2005), competition will be exacerbated on certain parts of a corpse or carcass.

Maggots in their third instar pass through a brief phase of obligate feeding and then a much longer phase of facultative feeding (Denlinger and Zdárek 1994), which allows feeding to stop well before the larvae are fully grown. Larval development can end about 25 h early in Chrysomya megacephala (Fabricius) and 34 h early in Chrysomya rufifacies (Macquart) under interspecific competition at high densities; when these species compete with one another, development can end about 54 h early (Shiao and Yeh 2008). Similar results were found by (Kheirallah et al. 2007) using L. sericata and C. albiceps. Generally, the effects of interspecific competition in carrion are reported to be more extreme than those of intraspecific competition. The effect of competition on size has generally been quantified as the mass of the dried adult (e.g. (Ireland and Turner 2005; Shiao and Yeh 2008)), so that the magnitude of the effect on larval size, and therefore on the PMI , has not been estab-

min lished, but is expected to not exceed a few millimetres of length, which translates into a day or two's growth. It is not known how competition affects the duration of the pupal stage.

This effect is one of the elephants in the room for estimating PMImin: until recently, it was recognised as a problem but not discussed in practical terms. One might manage it by comparing estimates of PMImin based on size and on onset of wandering, and averaging the results, which will have opposing biases, although it is not known whether they have comparable magnitudes. Unfortunately, this solution is not possible if the specimens have already started to pupariate or are preserved, because it is not possible to rear them to a developmental milestone.

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