Abstract
The distribution of mammals is determined by a suite of endogenous and exogenous factors. In territorial, polygynous species like tigers (Panthera tigris), males often center their space-use around female territories, repelling competitors from these areas. Competition among males for females leads to increased mortality of both sexes and infanticide of unrelated cubs, which can lead to population declines. We hypothesized that increased territorial overlap among adult male tigers and elevated levels of inter and intra-sex competition would be manifest in populations with male-biased adult sex ratios (ASR). We also assessed whether inter-sex variation in adult survival or degree of habitat connectivity resulted in skewed ASR. We evaluated these hypotheses using camera trap data from three tiger populations occupying habitat patches with varying levels of connectivity and ASRs. Data were analyzed using multi-state occupancy models, where states were defined as habitat use by one or more male tigers in sites with and without female use. As predicted, in populations with male-biased or even ASR we found evidence for increased spatial overlap between male tigers, particularly pronounced in areas adjacent to female territories. Given parity in adult survival, habitat fragmentation likely caused male-biased ASR. Our results suggest that the persistence of small tiger populations in habitat patches with male-biased ASR may be significantly compromised by behavior-mediated endogenous demographic processes that are often overlooked. In habitat fragments with pronounced male biased ASR, population recovery of territorial carnivores may require timely supplementation of individuals to compensate for population losses from intraspecific competition.
Highlights
Adult sex ratio (ASR, male: female) is an important demographic parameter that influences both individual behavior and population dynamics (Caswell, 2001; Haridas et al, 2014; Le Galliard et al, 2005; Székely et al, 2014)
Apparent survival for adult male and female tigers were similar in the three sites, suggesting that differential survival of adult male and female tigers is unlikely to be the key factor underlying variation in adult sex ratios (ASR) in these sites
Our study casts light on how space use of a territorial carnivore is influenced by the species territorial behavior and intra-specific interactions
Summary
Adult sex ratio (ASR, male: female) is an important demographic parameter that influences both individual behavior and population dynamics (Caswell, 2001; Haridas et al, 2014; Le Galliard et al, 2005; Székely et al, 2014). Intraspecific competition and aggression, especially in areas with male-biased ASR can increase the extinction risks of small populations (Barlow et al, 2009). We predicted high competition among males to secure access to female territories would be manifest in the following space-use patterns: (a) high male-use of tigers in locations with no female use ( along the margins of female territories; hypotheses 2 and 4, Table 1); and (b) high probability of multiple rather than single male-use in locations with female-use (hypotheses 3 and 5, Table 1) This is expected because harem sizes are smaller in areas with male-biased ASR, and multiple males are expected to compete intensively for access to each female (Table 1). When adult survival is similar for both sexes, or when female survival > male survival -- male-biased ASR in populations can be an artifact of high rates of female emigration, and/or differential male immigration from surrounding populations (Smith, 1993)
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