Male reproductive strategies

Sexually selected infanticide. In primate societies, it’s tough for males to hang on to their reproductive tenure for long. There is always a younger, stronger male lying in wait to take over the position as the dominant male. In species that have a high rate of male “takeovers” (i.e. a change in the dominant male), infanticide is a common male strategy for accelerating their ability to mate with the females in the group. Killing unweaned offspring that are not their own, males cause females with dependent offspring to quickly return to a fertile state. Males that produce offspring earlier in their tenure not only have a higher reproductive rate, but their infants are more likely to be weaned prior to another takeover. Therefore, infanticide is a real threat for pregnant and lactating females. Geladas provide a unique system for studying infanticide because we have observed a high rate of takeovers – combined with a high rate of infanticide – that likely has selected for one or more counterstrategies in females.

Social bonds between males and females. Yet, despite, the clear (and demonstrated) reproductive advantage that males reap from sexually selected infanticide, not all males commit infanticide. Why would males forego a strategy that accelerates their access to fertile females? Indeed, even in the most infanticidal species, males are not 100% infanticidal. Why do some males forego a strategy that would otherwise accelerate their access to fertile females? In geladas, one possibility is that some males take a alternative route – they invest in social relationships with the females in the group. For the males that adopt this alternative approach, it may be due to a particular social context, it may be due to some intrinsic characteristics to the males, and it may be entirely independent from whether a male commits infanticide or not.

Female reproductive strategies

Counter-strategies to infanticide.  In response to the threat of infanticide, pregnant female geladas may spontaneously abort (“Bruce effect”) and lactating female geladas may produce “deceptive swellings” to mimic the signs of fertility. Both appear to be strategies for conserving or protecting costly reproductive effort. However, these discoveries have raised more questions than they have answered with respect to the evolutionary endocrinology of geladas. Thus, we are exploring the idea that female reproductive suppression can be adaptive – particularly as it relates to sexual conflict.

Comparative approach to female counter-strategies. Some female counter-strategies to infanticide aim to prevent infanticide altogether while others aim to mitigate the costs. Prevention strategies typically involve behavioral tactics by females that act as either deterrents (e.g., enlisting protection) or manipulations (e.g., paternity confusion) in an effort to maintain the current fetus/infant. Mitigation strategies, by contrast, cause vulnerable females to terminate all additional investment in their current fetus/infant to allow investment in a future, and presumably more successful, infant (e.g., Bruce effect). Two likely factors that should determine which counter-strategies evolve are: (1) the likelihood of infanticide, and (2) the likelihood that a counter-strategy prevents “wasted investment” in a fetus/infant. For example, a Bruce effect should only evolve if there is some certainty that a female’s next offspring will be spared the same fate. We seek to identify social and ecological variables (and variation in these variables) that are associated with one strategy over another; and to further examine the success of these strategies. Remarkably, there is scant empirical evidence on the success of any female counter-strategies.