Maternal expenditure and the resolution of adult-offspring conflict in the South American guanaco

Sarno, Ronald
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Animal Ecology
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I examined patterns of pre-/post-birth maternal expenditure in the polygynous guanaco, Lama guanicoe, in Torres del Paine National Park, Chile from November 1990 to December 1993. Because of presumed greater variance in reproductive success of males of polygynous species, theoreticians have hypothesized that greater energy investment in sons might improve their competitive abilities and lead to increased reproductive success. Thus, it might be adaptive for parent(s) to invest more energy in sons than daughters. The period of parental care in guanacos is also punctuated by an interval of intense adult-offspring conflict. Adult males seasonally defend feeding territories and during spring become increasingly aggressive toward all juveniles and begin expelling them in October. In an apparent effort to appease territorial male aggression, juveniles display "submissive crouches" when being attacked or closely approached by them. It is not known if more submissive animals remain in family groups longest before expulsion, or if they are able to avoid being expelled. Additionally, juvenile males appear to be expelled before juvenile females. Thus, juvenile males might be under greater selective pressure to be more submissive, especially if this strategy serves to reduce territorial male aggression and delays expulsion;Contrary to our predictions concerning maternal expenditure, we found no apparent evidence of differential expenditure on sons or daughters. Birth weight of males and females was similar, and suckling times of males and females were also not significantly different;Submissive crouch frequency of juveniles that were expelled early, late, or not expelled increased with time in family groups before expulsion and were also significantly different. There was a significant positive correlation between the frequency of territorial male aggression and submissive crouches. Juvenile males generally displayed more SCs/hr and for longer duration than juvenile females in almost all months-seasons. Juvenile males generally moved among more groups. The more groups that juvenile females moved among, however, the earlier they were expelled; this trend was not evident for juvenile males. Although juvenile males were generally expelled before juvenile females, the proportion of juvenile males and females that were expelled was not significantly different.

Animal ecology, Ecology and evolutionary biology