Reproductive competition among male Assamese macaques

Fights between malesThe number of conceiving females will determine the degree and form of competition among males, because they direct their efforts only to those females that conceive in a given year and ignore the females that do not conceive. The mating season is short and every female mates with more than 80% of the up to 12 males in the group and always with several males during her fertile phase which creates the potential for sperm competition. Thus, all males invest in sperm production and all males show elevated testosterone excretion in their feces independent of dominance rank (Ostner et al. 2011). In season with few females conceiving males showed also increased stress levels as evident from cortisol excretion in feces but the top ranking males were much more stressed than the low-ranking ones. Among top-ranking males, the more aggression a male received from other males the higher were his cortisol levels. So males also competed directly (Ostner et al. 2008). Direct male competition is also evident from a positive correlation between mal rank and the number of offspring a male sired as assessed from genetic paternity analyses (Schülke et al. 2010); male reproductive skew is relatively low, however with 33% alpha male paternity and males down to rank six siring offspring. The strategy that high ranking males use to secure some paternity in the absence of overt ovulation signals is extremely extended mate guarding. The top 2-3 males each follow one particular female for several weeks up to almost 2 months on a stretch, trying to fend off all mating attempts by other males. In other primates mate guarding is energetically very costly for males mainly due to reduced food intake. Male Assamese macaques do not have to pay these costs because they maintain at least equally long feeding times when in a consort compared to the days of the mating season (Schülke & Ostner 2010 talk). The patterning of long consorts follows the predictions from a priority of access model, i.e. if one female is being consorted it will be done by the alpha males if it is two females, alpha and beta and only if it is three females the gamma males will get a chance, and so on (Ostner et al. 2011).

Very few studies are available on genetically assessed male paternity success. In male Assamese macaques at PKWS direct competition is evident but appears to be reduced compared to other primates living in similarly sized groups. Interestingly, the most likely reasons for the reduced skew are female strategies of concealment and synchronization. But random demographic events can have important effects on male strategies, e.g. if fathers remain in the groups where their daughters mature they will not be able to reproduce with them and the subjective female group size may vary between males. Thus, more data are needed over longer periods and especially from different groups before final conclusions about the generality of the results can be drawn.