Elephant Seal Harem Mating

Overview

Northern elephant seals (Mirounga angustirostris) breed on beaches along the California coast. Males compete fiercely for access to females, with the top-ranking alpha male monopolizing the majority of matings. This creates a classic polygynous mating system with extreme sexual dimorphism—males can outweigh females by a factor of four or more.

The result is one of the most dramatic examples of reproductive skew in any mammal. A handful of dominant bulls do nearly all the breeding, while the majority of males never reproduce at all. This simulation puts you on the beach to document it firsthand.

What You'll Do

Observe a simulated beach colony during the breeding season. Record behavioral states of males—resting, patrolling, displaying, fighting, and mating. Track which males copulate with which females over multiple observation sessions. Build an interaction matrix from dominance encounters. Count and categorize agonistic events and mating events to quantify each male's reproductive output.

You'll use both focal animal sampling (following one individual continuously) and scan sampling (recording the state of every visible animal at fixed intervals) to build a comprehensive behavioral dataset.

Learning Objectives

1. Construct a behavioral ethogram from systematic observation of a breeding colony
2. Quantify reproductive skew and relate it to male dominance rank
3. Test whether mating success follows a predicted distribution (e.g., Poisson or geometric)
4. Practice focal animal and scan sampling techniques used in real field studies

Background

Sexual selection theory, first articulated by Darwin and later formalized by Trivers, predicts that when one sex invests more heavily in offspring, the other sex competes for mating access. In elephant seals, females carry and nurse pups for months. Males invest nothing beyond sperm. The result is intense male-male competition.

Adult males can weigh up to 2,300 kg. They arrive at breeding beaches weeks before females and fight to establish dominance hierarchies. Once females haul out, the alpha male positions himself in the center of the harem and aggressively repels challengers. Field studies at Año Nuevo State Reserve in California have shown that a single dominant male can sire more than 80% of the pups born in a given season.

This extreme polygyny makes elephant seals a textbook system for studying sexual selection, dominance hierarchies, and the relationship between fighting ability and fitness. It's also a system where behavioral observation skills really matter—the difference between a successful and unsuccessful study comes down to careful, systematic data collection.