Tugging contests reveal feeding hierarchies in naked mole-rat colonies

Naked mole-rats, Heterocephalus glaber (Rodentia: Bathyergidae), are eusocial mammals that inhabit arid regions in northeastern Africa (Jarvis 1981; Jarvis et al. 1994). Colonies contain 70–80 individuals on average (range: 2–295+; Braude 1991; Brett 1991), among which only one female (the queen) and one to three males breed. Colony members are extremely closely related, apparently due to natal philopatry and inbreeding (Faulkes et al. 1990a; Reeve et al. 1990). Non-breeders (workers) cooperate to forage for subterranean bulbs and tubers, build nests, maintain tunnel systems, and defend against predatory snakes and foreign colonies (Jarvis et al. 1991; Lacey & Sherman 1991). Not all aspects of naked mole-rat colony life are harmonious, however. For example, the queen repeatedly attacks and shoves non-breeders of both sexes, thereby stimulating work activities (Reeve & Sherman 1991; Reeve 1992) and suppressing their reproduction (Faulkes et al. 1990b, 1991). Breeders and non-breeders also exhibit a variety of agonistic interactions including pushing, shoving, tugging each other by the skin, and head deflections (Lacey et al. 1991). Rymond (1991) recorded these four agonistic behaviour patterns among three captive colonies in Ann Arbor, Michigan; her data suggested that dominance hierarchies exist, with breeders dominating non-breeders and heavier workers dominating lighter ones. Recently, we noticed another manifestation of intra-colony conflict: tugging contests. These occurred when two hungry colony mates bit into the same piece of food and pulled in opposite directions. During a contest one or both tuggers usually gave ‘loud chirp’ calls, which are indicative of mild aggression (see pp. 258–261 and Figure 9-13 in Pepper et al. 1991). Contests lasted 1–180 s, and ended when one individual gained control of the food item and left to consume it. We used tugging contests as an experimental bioassay to investigate the relationships between social and reproductive dominance and between size, age and dominance. Two captive mole-rat colonies, designated K (N=18 individuals: nine males and nine females) and TT (N=29: 16 males and 13 females), were studied from January to November 1991. Founders of both colonies were captured in southeastern Kenya (K in June 1980, TT in October 1977); they were shipped to Cornell University in March 1981. Each colony was housed in a separate room, within its own artificial tunnel system made of clear Plexiglas tubes and boxes (for details see Lacey & Sherman 1991). To simulate the mole-rats’ natural environment, colony rooms were maintained in dim red light at 26–28)C and 50–65% relative humidity. Each mole-rat was tattooed uniquely for individual identification. The animals were weighed once (&0·1 g) during this study, on 21 July 1991. Reproductives were identified from observations of matings. We conducted observations 24–28 h after a colony was last fed. We attempted to equalize hunger levels among colony members by removing all uneaten food 6 h after a feeding and adding no food for the next 18 h. Because there was always uneaten food left at 6 h, we believe that individuals were satiated at that point. Our analyses assume that colony members were equally hungry after 18 h of food deprivation, or at least hungry enough to display true dominance relationships. To start an observation, we placed one cube of apple or yam (0·88&0·1 cm) in the food hamper nearest (<1 m) the nestbox. The food piece was small enough that two mole-rats could not eat it simultaneously. We then observed all interactions between individuals and the food cube until it was consumed. When a tugging contest occurred, we recorded the identity of the animal that had the *Present address: 691 Old Jonas Hill Road, Lafayette, CA 94549, U.S.A. †To whom all correspondence should be addressed.