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Theropod infanticide -- speculations



Using the remarkable Coelophysis Ghost Ranch specimens as a fulcrum, one can advance possible ideas about the taxon. Undoubtedly, there was pressure on dominant females (egg production, parental care = stronger, more adaptable biochemistry than males, plus larger size than males for nest protections) to form colonies directly linked to their selections of hunting habitats. Mate choice, hence, would be an equally important component. Thierry Boulinier (among others) has hypothesized among extant, flying dinosaurs the "performance-based conspecific attraction hypothesis": selection of habitat for hunting + conspecific breeding success = formation of colonies (cf. Gyps fulvus, and the studies conducted by Sarrazin, for an example of theropod coloniality).  Other students of avialian behaviour (Richard H. Wagner's papers on the razorbill theropods) hypothesize theropod colonies arise from "hidden leks": a monogomous! ! taxon, using display leks to gather mates, in turn perpetrate breeding colonies. A synthesis of these hypotheses would see theropod colonies as a secondary result of habitat selection and choice of mates, and the question becomes: why are colonies so rare among living dinosaurs? Cecile Rolland in 1998, using sea avialians, believes colonies arose in avialian history before these taxa adapted marine habitats for food supplies.
One can, with a minimum of effort, advance a scenario for Coelophysis (the only theropod found in sufficient numbers to qualify as a colony,  in one location, to enable age-growth interpolations). It has been speculated that, in times of environment stress (e.g., a drought), infanticide was common among these taxa: needed nutrition for starving, larger individuals, including females who  would obtain new nesting sites in the process of eating the hatchlings (and possibly killing and eating smaller breeding females).  These gains -- discovered among mammalian taxa by Sarah B. Hrdy as a possible path of enhanced survivability (i.e., "fitness") -- may be a reason for the number of small Coelophysis at Ghost Ranch. Sarah Hrdy, in her 1999 Mother nature: a history of mothers, infants, & natural selection, outlines 5 reasons for infanticide among animals: "pathological" responses to inner/outer stress; a way for mothers to reduce number of! ! offspring by cannibalizing "defective" young; males killing offspring to obtain a wanted female; males/females killing offspring to alleviate pressure for scarce food supplies (fewer young = less pressure on hunting); offspring killed because they are an available food source.
Jerry Wolff, in his studies of vertebrate infanticide, has found the costs for the behavior among both females and males to be increased energy expenditure in searching for, then eradicating conspecific nestlings, possible injury (even death) if the nest is successfully defending by another adult parent. In turn, e.g., if an animal leaves its young to kill others, may return to find its young killed for the same reasons by another adult.  Thus, among Coelophysis, it could very well be that females had multiple mating strategies: confused paternity > migratory delays, failure to obtain mates > suppression of breeding activity if the females could not  have both control of their nesting sites and be the dominant mate (S.K. Wasser and D.P. Barash's 1983 paper on mammalian sexual suppression is still quite useful and applicable to our speculations here). Colonies of Coelophysis resulted not because BBC film crews from the Discovery Channel were ne! ! arby and needing an easy shot, rather because communal breeding systems meant successful surviving of hatchlings coupled with heightened vigilance behaviour. R.A. Ims has aptly termed this "reproductive synchrony as a predator-swamping strategy", the predators being female/male taxa who could perpetrate conspecific infanticide.  
Moreover, one can ask if field notes on the Ghost Ranch Coelophysis can show distribution of females within the colony, the colony itself being a theropod breeding strategy (females suppressing their own breeding to co-parent offspring of dominant females).  Multiple mating > confusion among males as to paternity of altricial nestlings, as pregnant female theropods would mate with "strange" conspecifics. Could it be female Coelophysis had multiple matings when infanticide was a real possibility, the MMs decreasing if infanticide was a remote possiblity?  This leaves a possible question: the Ghost Ranch ensemble of several animals may be evidence of a large infanticide intrusion vs. accidental death of so many theropods.