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12/20/06
Purposeful Infanticide?
Filed under: General, Birding & Outdoors, NM & SW US, Florida & SE US
Posted by: Ken @ 7:44 am

Coyote tracksAt Albuquerque’s Rio Grande Nature Center, the groups of school kids that I guided as an interpreter rarely sighted any coyotes, but their tracks were often evident. To help distinguish them from domestic dog tracks, I went through the “3 P’s.” First, the “Paws”: their robust shape and minimal nail imprint. Second, “Perfect-stepping”: when walking, coyotes place their hind feet precisely into the prints of their front feet (as do cats), producing a “two-legged” set of tracks. Finally, the tracks of coyotes are “Purposeful,” meaning that they exhibit intent, or are directed toward a goal or reward. They tend to go in straight lines, from this tree to the cover of that bush, to then intersect with the tracks of a bounding mouse or vole.

Wild creatures cannot afford to waste energy. In the snow of New Mexico’s Sandia mountains, I often noticed the difference between the “purposeful” stride of a coyote or Gray Fox, as opposed to a pet dog’s meanderings. Young coyotes (and young rabbits) appear to “waste’ much energy in play and seemingly mindless scurrying about. However, they are gaining survival skills with a long term payback.

Some behaviors, such as the territorial skirmishes of the Muscovy Ducks on our suburban lake seem easy to interpret. They are directed at passing on the toughest drake’s genes. When food is scarce, some birds of prey, such as eagles and owls, may produce two or more chicks, only to have the older and stronger siblings kill their nest-mates. Again, we see the value of this fratricidal behavior, as likelihood of survival of at least one of the progeny is increased.

Last January, 2005 one of the Muscovy Ducks that inhabit our South Florida lake, appeared at our back door with seven newly hatched ducklings, a bit early in the breeding season.  Two days later she was sitting on a neighbor’s lawn about 20 feet from the lake’s edge with the ducklings under her wings or milling close by.  Suddenly as if on signal, twenty or so adult ducks began flying or swimming rapidly towards her, mobbing her.

Thinking I saw a yellow ball of fuzz in one of the ducks’ mouth, I ran inside to get my binoculars, as the ducks were about 100 yards away.  Sure enough, the adults had captured and killed all seven ducklings.  They fought over the carcasses and attracted Ring-billed Gulls that stole most of them and flew off.

The ducks were at the height of courtship with lots of head-bobbing, tail-wagging, fights and copulation.  Perhaps this had something to do with this (presumably) unusual behavior.  The attackers/mobbers appeared to include both sexes.

It is difficult to interpret such behavior. Did it carry any survival value? Had the local duck population reached a critical mass that triggered some deep instinct to assure adequate resources, such as food and shelter? Infanticide may be analagous to fratricide– if the chicks were sired by a rival or non-relative of the the attackers, it might even be more effective, as it also eliminates competing genes.

See follow-up responses on avian infanticide here.

A FEW GLEANINGS FROM THE LITERATURE:

Eugenie C. Scott wrote in a review of  “Infanticide: Comparative and Evolutionary Perspectives,” that avian infanticide is not a “rare, pathological aberration brought about by unusual conditions. It is too common. The behavior has more recently been considered as an adaptive response at the population level to pressures such as overpopulation, or as an adaptive response by individuals.”

High breeding density appears to have increased the aggression of a female to the point of infanticide in the Spot-billed Duck (Anas poecilorhyncha) observed on a small pond at Yatsu tidal flat, central Japan, from May to June 2000.

“…(S)ix families of conspecific and one Gadwall A. strepera family, respectively, occupied territories. A female Spot-billed Duck with 11 ducklings attacked ducklings of all the other families killing eight conspecifics and three of the Gadwall. High breeding density (15.0 families/ha) may have increased the aggression of the female.” [A case of infanticide in the Spot-billed Duck in circumstances of high breeding density : Tetsuo Shimada, Kazuyuki Kuwabara, Saori Yamakoshi, Tomomi Shichi, Abstract]

Competition for nest sites, mates or food is implicated in this study of European Starlings:

“Avian adult infanticide (the killing of conspecific young by an adult) has been reported for several species (e.g.: Crook and Shields 1985, Loftin and Roberson 1983. Trail et al. 1981). Here we document one case of attempted infanticide and circumstantial evidence for 3 more cases of infanticide in the European Starling (Sturnus vulgaris).

“In all cases, the nature of the wound was the same: a peck mark near one of the auditory openings. Similar wounds have been reported by Crook and Shields (19 8 5) Shelley (1934), and Stacey and Edwards (1983) in the context of infanticide….  In addition, similar peck marks have been found around the head and neck of dead adults (n = 3) found in nest boxes, and on the faces of pairs of birds that we have captured fighting inside boxes (n = 8 pairs), suggesting that bill stabbing is not uncommon.

“In two of the four cases, a female was suspected to have committed an infanticidal act. We suspect that infanticide in our population is related to competition for nest sites, a limiting resource (e.g., see Hrdy 1979). Other reports of female infanticidal behavior have been reported by Loftin and Roberson (1983) and Picman (1977). In both cases, a limiting resource (such as nests, mates or food) has been suggested as a reason for infanticidal acts.

“The importance and frequency of avian infanticide is largely unknown primarily because the observation time necessary to document this phenomenon can be prohibitive. As a result, infanticide may have greater biological significance than is implied by the existing literature. We encourage all observers to be cognizant of the possibility
of infanticide in their study populations.”

 Intruding males and females may gain a reproductive advantage by killing the young of prospective mates [Prospective Infanticide and Protection of Genetic Paternity in Tropical House Wrens]:

“Sexually selected infanticide occurs when adults kill dependent offspring of potential mates and initiate new reproductive attempts with those mates more rapidly than if they had waited for the offspring to become independent.”

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