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Keiper, R., & Houpt, K. (1984). Reproduction in feral horses: an eight-year study. Am J Vet Res, 45(5), 991–995.
Abstract: The reproductive rate and foal survival of the free-ranging ponies on Assateague Island National Seashore were studied for 8 years, 1975 to 1982. Most (52%) of the 86 foals were born in May, 13% were born in April, 22.6% in June, 10.4% in July, and less than 1% in August and September. The mean foaling rate was 57.1 +/- 3.9% and the survival rate was 88.3 +/- 3.6%. Forty-eight colts and 55 fillies were born (sex ratio 53% female). Mares less than 3 years old did not foal and the foaling rate of 3-year-old mares was only 23%, that of 4-year-old mares was 46%, that of 5-year-old mares was 53%, and 6-year-old mares was 69%. The relatively poor reproduction rate was believed to be a consequence of the stress of lactating while carrying a foal when forage quality on the island was low. The hypothesis was supported by the higher reproductive rate (74.4 +/- 2.4%) of the ponies in the Chincoteague National Wildlife Refuge on the southern part of the island. Their foals are weaned and sold in July each year. Despite the low reproductive rate on Assateague Island National Seashore , the number of ponies increased from 43 to 80, a 90% increase in the 8-year period or greater than 10%/yr. There were 24 deaths and 8 dispersals from the study area.
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Lingle, S., Rendall, D., & Pellis, S. M. (2007). Altruism and recognition in the antipredator defence of deer: 1. Species and individual variation in fawn distress calls. Anim. Behav., 73(5), 897–905.
Abstract: Mule deer, Odocoileus hemionus, females actively defend fawns against predators, including nonoffspring conspecific fawns and heterospecific white-tailed deer, O. virginianus, fawns. We hypothesized that the defence of nonoffspring fawns was due to a recognition error. During a predator attack, females may have to decide whether to defend a fawn with imperfect information on its identity obtained from hearing only a few distress calls. We examined fawn distress calls to determine whether calls made by the two species and by different individuals within each species were acoustically distinctive. The mean and maximum fundamental frequencies of mule deer fawns were nearly double those of white-tailed deer fawns, with no overlap, enabling us to classify 100% of calls to the correct species using a single trait. A large proportion of calls was also assigned to the correct individual using a multivariate analysis (66% and 70% of mule deer and white-tailed deer fawns, respectively, chance = 6% and 10%); however, there was considerable statistical uncertainty in the probability of correct classification. We observed fawns approach conspecific females in an attempt to nurse; females probed most offspring fawns with their noses before accepting them, and always probed nonoffspring fawns before rejecting them, suggesting that close contact and olfactory information were needed to unequivocally distinguish nonoffspring from offspring fawns. Taken together, these results suggest that acoustic variation alone would probably be sufficient to permit rapid and reliable species discrimination, but it may not be sufficient for mothers to unequivocally distinguish their own fawn from conspecific fawns.
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Lingle, S., Rendall, D., Wilson, W. F., DeYoung, R. W., & Pellis, S. M. (2007). Altruism and recognition in the antipredator defence of deer: 2. Why mule deer help nonoffspring fawns. Anim. Behav., 73(5), 907–916.
Abstract: Both white-tailed deer, Odocoileus virginianus, and mule deer, O. hemionus, females defend fawns against coyotes, Canis latrans, but only mule deer defend nonoffspring conspecific and heterospecific fawns. During a predator attack, females may have to decide whether to defend a fawn while having imperfect information on its identity obtained from hearing a few distress calls. Although imperfect recognition can influence altruistic behaviour, few empirical studies have considered this point when testing functional explanations for altruism. We designed a series of playback experiments with fawn distress calls to test alternative hypotheses (by-product of parental care, kin selection, reciprocal altruism) for the mule deer's defence of nonoffspring, specifically allowing for the possibility that females mistake these fawns for their own. White-tailed deer females approached the speaker only when distress calls of white-tailed deer fawns were played and when their own fawn was hidden, suggesting that fawn defence was strictly a matter of parental care in this species. In contrast, mule deer females responded similarly and strongly, regardless of the caller's identity, the female's reproductive state (mother or nonmother) or the presence of their own offspring. The failure of mule deer females to adjust their responses to these conditions suggests that they do not defend nonoffspring because they mistake them for their own fawns. The lack of behavioural discrimination also suggests that kin selection, reciprocal altruism and defence of the offspring's area are unlikely to explain the mule deer's defence of nonoffspring. We identify causal and functional questions that still need to be addressed to understand why mule deer defend fawns so indiscriminately.
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Taillon, J., & Cote, S. D. (2007). Social rank and winter forage quality affect aggressiveness in white-tailed deer fawns. Anim. Behav., 74(2), 265–275.
Abstract: Achieving a high social rank may be advantageous for individuals at high population densities, because dominance status may determine the priority of access to limited resources and reduce individual loss of body mass. The establishment of dominance relationships between individuals involves variable levels of aggressiveness that can be influenced by resource availability. The relationship between social rank and aggressiveness and the impacts of resource abundance on aggressiveness are, however, poorly understood, but may be relevant to understand the mechanisms determining dominance relationships between individuals. We experimentally simulated, in seminatural enclosures, a deterioration of winter forage quality induced by a high-density deer population and examined the effects of (1) social dominance and (2) diet quality on aggressiveness, forage intake and body mass loss of white-tailed deer, Odocoileus virginianus, fawns during two winters. Within diet-quality treatments, fawns were consistently organized into linear hierarchies and showed clear dominance relationships. Dominants initiated more interactions and showed higher aggressiveness than subordinates, but subordinates had higher forage intake than dominants throughout winter. Social rank did not influence cumulative body mass loss of fawns. During both winters, fawns fed the control diet maintained their aggressiveness level, whereas fawns fed the poor-quality diet decreased it. Our experimental approach revealed that white-tailed deer responded to a reduction in winter forage quality by modifying their aggressiveness, indicating that ungulates may show plasticity not only in their foraging behaviour in response to decreased resources but also in their social behaviour.
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