Necrophilia in crows: Occasional contacts, which take a variety of aggressive and sexual forms, may result from an inability to mediate conflicting stimuli

Occurrence and variability of tactile interactions between wild American crows and dead conspecifics. Kaeli Swift, John M. Marzluff. Philosophical Transactions of the Royal Society B: Biological Sciences, September 5 2018, Volume 373, issue 1754, 10.1098/rstb.2017.0259

Abstract: Observations of some mammals and birds touching their dead provoke questions about the motivation and adaptive value of this potentially risky behaviour. Here, we use controlled experiments to determine if tactile interactions are characteristic of wild American crow responses to dead crows, and what the prevalence and nature of tactile interactions suggests about their motivations. In Experiment 1, we test if food or information acquisition motivates contact by presenting crows with taxidermy-prepared dead crows, and two species crows are known to scavenge: dead pigeons and dead squirrels. In Experiment 2, we test if territoriality motivates tactile interactions by presenting crows with taxidermy crows prepared to look either dead or upright and life-like. In Experiment 1, we find that crows are significantly less likely to make contact but more likely to alarm call and recruit other birds in response to dead crows than to dead pigeons and squirrels. In addition, we find that aggressive and sexual encounters with dead crows are seasonally biased. These findings are inconsistent with feeding or information acquisition-based motivation. In Experiment 2, we find that crows rarely dive-bomb and more often alarm call and recruit other crows to dead than to life-like crows, behaviours inconsistent with responses given to live intruders. Consistent with a danger response hypothesis, our results show that alarm calling and neighbour recruitment occur more frequently in response to dead crows than other stimuli, and that touching dead crows is atypical. Occasional contacts, which take a variety of aggressive and sexual forms, may result from an inability to mediate conflicting stimuli.

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Across both experiments, we observed n = 11 attempts to mate with a crow in dead posture (4.7% of N ¼ 234 trials), 90% of which were coupled with scolding and all of which took place before the end of May. Sexual behaviours around dead conspecifics are rare, but not unique to crows. Hetero and homosexual necrophilia have been observed across a wide variety of taxa. Sexual arousal in response to dead conspecifics has been documented in bottle nosed dolphins [4] and humpback whales (Magaptera novaeangliae; [34]). Mating attempts with dead conspecifics have been observed in Richardson's ground squirrel (Citellus richardsoni), mallards (Anas platyrhynchos), sand martins (Riparia riparia) cururu toads (Rhinella steuvax) and great ameivas (Ameiva ameiva; [35.39]). The copulation posture typical of dead birds has been proposed as the releasing factor for such inappropriate attempts to mate, particularly among monomorphic birds [37]. In Experiment 2, however, we show that crows attempted to mate both with a life-like crow in neutral standing posture and a dead crow with the wings tucked close to the body.  These observations call into question the validity of posture as the primary releasing factor for copulation events between crows and dead crows, and warrant further investigation.

Given the prevalence of scolding before, during or immediately following copulation events with dead, but not life-like, crows, alarm induced arousal, rather than reproductive attempts, might better explain copulation with dead crows. Increased sexual behaviour following alarm or excitement has been observed in the zebra finch (Taeniopygia guttata; [40]), vermillion flycatcher (Pyrocephalus rubinus, [41]) and pied avocet (Recuruirostra avosetta; [42]). Following the death of a group member, sexual behaviour occurring outside the breeding season was observed in rhesus macaques [32]. Likewise, we observed mating attempts between presumed pairs following discovery of a dead crow. It is possible in this context that distress induces arousal resulting in copulation attempts between mates if possible, but in the immediate absence of the mate results in displacementmounting.  In rooks (Corvus frugilegus), sexual displays by males sometimes stimulate reverse mounting by females [43]. In our study, females witnessing male precopulatory behaviour prior to mounting the stimulus may be responsible for the two possible instances of reverse mounting.

In addition to the multiple mating attempts with the dead and life-like crows, we also observed one attempted copulation with the dead pigeon. Attempts to mate with live heterospecifics have been observed in a variety of species including seals and non-human primates [44,45]. Although these events are rare enough that determining causal factors remains difficult, restricted access to conspecific females has been commonly observed in these cases. Such information about the crow involved in this case is not known.