Female Infanticide?

Based on interesting side comments by hollygolightly and schraf in one of the interminable abortion threads, I got curious about instances of infanticide in nature. I spent a bit of time digging around in the literature looking for an evolutionary explanation for for this behavior. Predictably, most of the articles dealt with the genetic and/or chemical basis for aggression. Most of the documented behavioral examples in mammals dealt, of course, with male infanticide. What is strikingly lacking is any instance of female infanticide of their own neonates that wasn't either pathological or induced by extreme environmental stress. Again, this is only in mammals - many other organisms make little or no distinction. (A fascinating but rather gruesome article discussed how male and female sand sharks (Carcharias taurus) for instance, begin to prey on their siblings before they're born!)The evolution of male infanticide is fairly straightforward. Although there are numerous proximate causes - from predation/cannibalism to inducing off-cycle estrus - the ultimate cause can be traced to increasing the male's reproductive success.Female infanticide, OTOH, appears to be limited primarily to non-offspring under normal conditions. Female wolverines (Gulo gulo), for example, will hunt down "foreign" infants in order to force the mother to abandon choice den sites or (in one study) limit future competition (although I have a problem with the idea of forward planning implicit in this explanation). In several Old World primate genera the dominant female will kill other females' offspring to reduce population pressure and provide more resources to her own. This latter may also be part of maintaining dominance. However, even in mousies and gerbils, the two examples given in the previous thread, female infanticide is ONLY associated with either severe stress OR virgin or inexperienced females.Does anyone have any documented examples of females of any species killing their own offspring (other than humans) under normal conditions? If so, does anyone have an evolutionary explanation for this exception to Hamilton's Rule?

I wasn't specifically looking for that, but there were a few cases. Although this may be a gross over-generalization, it appears that whether or not a father kills his own offspring is dependent on the reproductive strategy of the species. F'rinstance, in species with tight pair-bonding where the father assists in the rearing, paternal infanticide is apparently unheard of. One study discussed gerbils whose males are apparently murderous, blood-thirsty little sociopathic neonate serial killers - right up to the point where they father their first litter, after which they never kill any neonate again. At the other extreme, maximal "love 'em and leave 'em" species don't apparently make any distinctions. Male cheetahs will slaughter any neonate - or cub for that matter - they encounter. Territoriality, etc, also apparently plays a major role. An article you might find interesting on this score, and in light of the above is Schneider JS, Stone MK, Wynne-Edwards KE, Horton TH, Lydon J, O'Malley B, and Levine JE, 2003 "Progesterone receptors mediate male aggression toward infants", PNAS 100:2951-2956. Appears we males have been seriously maligned all these years. It ain't testosterone AFTER all.

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Neuroendocrine mechanisms that mediate male aggression toward infants are poorly understood. Although testosterone is known to enhance aggression in other social contexts, evidence that it modulates aggression toward infants is equivocal. We have found that male progesterone receptor knockout (PRKO) mice exhibit no infanticidal behavior and little aggression toward young. Male PRKO mice also display significantly enhanced parental behaviors. In wild-type mice, blockade of PR induces a behavioral phenotype similar to that of the PRKO males, whereas progesterone exacerbates aggressive tendencies toward infants. Aggressive behaviors directed toward adult males, by contrast, are unaffected by progesterone, PR antagonism, or PR gene deletion. Previously thought to be of diminished importance in male animals, PRs play a critical and specific role in modulating infant-directed behaviors in male mice.

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It's not our fault, I tell ya.

Reference?

Hey JohnI noted that about primates in my OP - however they apparently kill only OTHER mother's sons... I'm basically looking for references to support or refute hollygolightly, schraf and now Raha's claims to the existence of maternal infanticide (own offspring). Any help is appreciated.

Thanks for the replies, everyone. This is the kind of discussion I was looking for.Zhimbo: Thank you for the reference. I’m afraid my query wasn’t nearly as structured or formalized as it should have been. Remember, my curiosity was piqued by a side comment in another thread. I’m not really clear myself on what I’m looking for, although that’s firming up a bit as I research the issue. In the context of my original vague question, I’m not sure but that I’d consider food shortages to be abnormal conditions. I recognize, of course, that cyclical resource availability has a profound impact on population dynamics and evolution. However, even in that context, can you provide more info from the article you referenced (I don’t have access to Science articles that old on-line, and no access to a uni library)? Going only from the title, it appears that this isn’t necessarily a case of infanticide, but rather deliberate neglect — which has very clear evolutionary explanations. Even cheetahs, which have very few offspring and subsequently a huge genetic investment in each cub, will abandon young when food sources dwindle past a certain point.Permit me to expand on my initial question a bit: I think what I’m trying to get to (and it’s only now becoming clearer to me), is an evolutionary explanation for human infanticide. Whereas as several others have pointed out there are numerous instances in recorded human history where socio-economic or cultural factors render infanticide normative behavior (for the society in question), the behavior appears unique to humans. I think I’m trying to come at the question from the back end — seeing if a) there are satisfactory examples from nature where own-offspring infanticide is normative and b) there are reasonable adaptive or evolutionary explanations for it. So far, no joy. This is starting to appear like another example of where cultural evolution trumps biological evolution. In which case, tracing the biological/evolutionary component may be a fool’s game.Raha: I wasn’t trying to be facetious. (Well, only a little ). I’ve run across the hamster claim before, but the only references in the literature I’ve stumbled across thus far don’t describe the behavior. I was hoping you might know something off the top of your head, as it were.John: I know you’re not advocating the statements from the article. So just a couple of comments. This part from the article you referenced

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Women kill their babies in poor societies. Women sometimes contribute to the death of their sickly or weak children in situations of scarcity and extreme poverty. But these women do so in response to impossibly difficult situations. Some children were seriously neglected, "sacrificed", so that older children could live.

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is pretty consistent with the literature. See Zhimbo’s reference concerning the exact same behavior in wood rats. It’s a straightforward application of Hamilton’s Rule on kin selection.However, this part

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Women require more support because it takes so long for humans to mature sufficiently to provision themselves. That's why human infants are at greater risk from maternal abandonment. Human mothers are unusually sensitive to how much social support they are likely to have and explains why they are more prone to abandon babies and commit infanticide than other primates. Deliberate killing of her own infant has never been observed in wild primates. (emphasis added)

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while providing an explanation appears to beg the question of why — or more importantly, how and under what circumstances — the behavior evolved. If, as the emphasized portion indicates, this behavior is unique to humans and not manifested by our closest living relatives, then we have a serious evolutionary/causal disconnect. As I mentioned to Zhimbo, it may not be possible to trace the origins of maternal infanticide in humans by examining other organisms — culture trumping biology.

This isn't entirely accurate. Although it appears to hold for gerbils, it is inconsistent with the literature that suggests at best males will refrain from killing their own progeny, but there is no behavioral constraint in most species against slaughtering everyone else's neonates. In fact, the communal rearing strategy used by some rodents - where the females huddle in a bunch with their pups, and in some species will even suckle the pups of their sisters (consistent with kin selection) - is a defense reaction against the males. In this case, the males apparently "recognize" the nest-scent of the pups, and will refrain from eliminating them. They don't appear constrained about eliminating the pups without this nest-scent. Right. Which renders any counterexample - the supposed gerbil, hamster, and mouse examples used by schraf, et al - somewhat problematic. Which is why I got on this subject in the first place. Moreover, the quite plain human "violation" of Hamilton's rule through maternal infanticide needs an explanation. Again, right. However, almost any socio-economic reason for any behavior has a natural analog. I can think of few, if any, human group behaviors that don't - including religion . The analog is what I'm reaching for here. I'm actually surprised that the theists haven't jumped on this already. Aftr all, if it can be shown that maternal infanticide has no natural equivalent or adaptive explanation, then there's a tiny bit of additional weight to their argument that humans are "special" - not simply different. Admittedly a kind of gruesome argument in this case. Genetics ultimately provides the underpinnings of behavior, although as Ehrlich describes it: "Genes do not shout commands to us about our behavior. At the very most, they whisper suggestions..." (Ehrlich, P 2000 "Human Natures: Genes, Culture and the Human Prospect", Penguin Books, New York, pg 7). Culture vs biology, again.

Thanks to both of you for your replies and comments. Unfortunately, I suffer from a severe attention span deficit. The question in the OP has inevitably fallen off my personal radar screen over the past few years.On the other hand, welcome to EvCForum! From your quality of your replies, I very much look forward to reading your input on other topics.