Cooperative Breeding in Groups of Synchronously Mating Females and Evolution of Large Testes to Avoid Sperm Depletion in African Striped Mice1

Abstract

Testis size has been related to the mating system of both vertebrates and invertebrates. These differences are regarded as adaptive responses to sperm competition in promiscuously mating species. However, not all variation in testis size can be explained by sperm competition. Here, we test the hypothesis that the evolution of large testes occurs when synchronously breeding females must be fertilized within a short period of time to avoid reproductive competition among the females. African striped mouse (Rhabdomys pumilio) males of a polygynous population with cooperative breeding and high risk of sperm depletion had testes and cauda epididymis twice as large as those of males of four different promiscuous populations with high risk of sperm competition. When paired with three females simultaneously in captivity, males of the polygynous population bred with three females within 8 days, leading to synchronous births in their harems, thereby potentially reducing the risk of infanticide. Males from the promiscuous population reproduced with only one or two females within 8 days, and births were not synchronous. We conclude that large testes are selected for in species with synchronously mating females, which is ultimately beneficial for the evolution of cooperative breeding.

Policing of reproduction by hidden threats in a cooperative mammal

“…1G). As in some other social mammals (27,36,37) and joint-nesting birds (38)(39)(40), female banded mongooses appear to kill offspring that are produced before, but not after, their own (26). Our experiments suggest that it is the presence or absence of reliable cues to maternity, rather than the pregnancy status of dominant females per se, that is the main determinant of this pattern.…”

“…However, the finding also suggests that reproductive coercion may extend well beyond the systems in which infanticide is common. Hidden threats of policing can deter subordinates from reproducing in the first place (44)(45)(46), or select for counterstrategies [such as synchronous birth (26,37), egg-laying (47), or egg-mimicry (48)] to render threats noncredible (i.e., unprofitable for a threatening individual to carry out, in the event that a transgression occurs). In both cases, threats of infanticide influence reproductive behavior even though no offspring are killed.…”

Extinction or Survival? Behavioral Flexibility in Response to Environmental Change in the African Striped Mouse Rhabdomys

“…Same-sex dyadic encounters comprising unrelated females were largely amicable [112] and triads of related females form long-term stable groups [113], indicating a level of tolerance contrary to their solitary lifestyles in nature. Thus, R. dilectus females might have the potential to form communally breeding groups, regardless of whether or not they are related.…”

“…Interestingly, in captivity, both R. dilectus subspecies display the potential for behavioral flexibility, including developing a bolder personality type [93], social tolerance among females [113] and males displaying paternal care [94]. Moreover, the flexible responses appear to reflect ancestral traits fully developed in the arid-adapted R. pumilio, elements of which have been retained in R. dilectus.…”

Reproductive competition and the evolution of extreme birth synchrony in a cooperative mammal

“…First, by giving birth in synchrony, females may limit the ability of cobreeders to commit infanticide, since infanticidal females risk killing their own young [3,4]. Evidence in support of this hypothesis comes from observations that females are less likely to commit infanticide after they have given birth themselves [5,6]. Second, females can minimize the negative effects of competition with cobreeders by ensuring their own offspring will not be outcompeted for resources [2].…”